• Topics: Environmental Horticulture  | Nursery Irrigation  | Leatherleaf Ferns  | Cut Foliage (florists' greens)  | Cold Protection and Chilling Damage for Nursery Production  | Stamps, Robert H  | Irrigation Water Quality 

Irrigation and Nutrient Management Practices for Commercial Leatherleaf Fern Production in Florida¹

This publication offers improved irrigation and nutrient management practices for use during the commercial production of leatherleaf fern. These management practices are designed to reduce production costs and improve crop quality while at the same time protection ground water quality.

Introduction

Plant Water Use

Water from the soil enters the plant through the roots and moves through the stem (rhizome) to the leaves (fronds) and then out of the leaves into the air surrounding the leaves. This evaporative loss of water from plants is called transpiration. During transpiration, water is pulled through the soil-plant-atmosphere system by differences in water potentials (pressures) in various parts of the system. Water moves from areas of higher water potential to areas with lower water potentials. Water potential is usually expressed as bars, centibars, megapascals, or kilopascals (1 bar = 100 centibars = 0.1 megapascal = 100 kilopascals). Sandy soils saturated with water have water potentials of 0 and, after excess water has drained due to gravity, potentials decrease to about -0.1 bar/-10 centibars (-0.01 megapascal, -10 kilopascals). Water potentials decrease further due to

transpirational water loss and evaporation of water directly from the soil. This water loss directly from the soil to the air is a minor component of water loss in an established fernery. The combination of transpiration + evaporation is called evapotranspiration. This process of evapotranspiration depletes the soil water reservoir (see Soil Water Budget)

Figure 1. 

Water and Leatherleaf Fern Production

Leatherleaf fern [Rumohra adiantiformis (Forst.) Ching] is an herbaceous perennial crop that is grown predominantly on well-drained (Figure 1), mostly sandy soils having low water- and nutrient-holding capacities. The high leaching potential of these soils places water resources in leatherleaf fern production areas at risk of contamination unless appropriate management practices are followed. Although annual precipitation in leatherleaf fern-producing areas is around 50 inches (127 centimeters) per year, it occurs unevenly (Figure 2). Fern growth and productivity are markedly reduced under water stress and, therefore, supplemental irrigation is required for commercial production. In addition, irrigation water is also used commercially to deliver pesticides and nutrients to the crop.

Figure 2. 

Figure 3. 

Water permeability rates of soils typically used for leatherleaf fern production in Florida

Evapotranspiration is driven mainly by radiation--with wind, temperature and humidity (vapor pressure differences between water in the leaf and that of the surrounding air) also having an effect. Since all commercial leatherleaf fern production occurs under artificial or natural shade--radiation, wind and temperatures are reduced compared to those occurring out in the open. Because of these environmental differences, annual water use during leatherleaf fern production (water transpired by the crop + a small amount of water evaporated from the soil) is only about 20 inches (50 centimeters). This water use rate is around 60 to 85% less than rates for turfgrasses and perennial agronomic crops grown in full sun. Additionally, 20 inches is less water than most annual agronomic row crops use during their 120-150 day growing period.

Figure 4. 

Average monthly precipitation (in inches) at Daytona Beach, FL, for the years from 1960 through 1989.

Nutrients

Of the many elements known to be essential for plant growth and development, three--carbon (C), hydrogen (H), and oxygen (O) (which the plants obtain from the air and water)--typically account for 92% or more of plant dry weights. The other elements, all of which may be supplied to plants using fertilizers, account for only a small percentage of the plant dry weight. Even so, a deficiency of even one of these essential elements can lead to reduced plant growth and/or quality.

The primary nutrient that is most often deficient and growth limiting in soils in leatherleaf fern producing areas is nitrogen (N). The N sources used in commercial fertilizers consist of urea (CO[NH₂ ]₂ ), ammonium (NH₄⁺ ), and nitrate (NO₃^- ). The first two forms are quickly converted to the NO₃^- form in the warm, moist, aerated soils in leatherleaf ferneries. Most N taken up by plants is in the NO₃^- form. Since the NO₃^- ion and soil are negatively charged, the NO₃^- ion is not bound to the soil. Therefore, NO₃^-N moves freely with the soil solution and is readily leachable. This leachability is a cause for concern, since N can end up in drinking water supplies where it may become a health hazard.

The primary nutrient phosphorus (P), unlike NO₃^-, is not very leachable and has a low solubility at the pHs at which leatherleaf fern is normally grown (5.5-6.5). Phosphorus content of plants is often around one-tenth that of N or potassium. High soil P can cause micronutrient deficiencies, especially of iron (Fe), to occur. Both P and N are highly mobile inside plants, and deficiency symptoms first appear on the older tissues from which these elements have translocated to the younger, more actively growing, tissues.

The third primary macronutrient, potassium (K), is usually present in ornamental plants in about the same quantities as N. Potassium occurs in the soil solution as the positively charged ion (cation)--K⁺. Most Florida soils contain low amounts of K; due to the low cation exchange capacities (CEC) of Florida's sandy soils and high rainfall, K is prone to leaching. Since K is also very mobile in plants, deficiency symptoms appear in older tissues first.

Two secondary nutrients, calcium (Ca) and magnesium (Mg), occur in the soil solution as divalent cations (Ca₂⁺, Mg₂⁺). Magnesium content of Florida's sandy soils is usually low and Mg₂⁺ is leachable so Mg deficiency is a common nutritional problem. Magnesium is an essential component of chlorophyll, the pigment that gives leaves their green color. Therefore, a deficiency in Mg can lead to light green or even yellow (chlorotic) leaves. Calcium is a component of liming materials that are used to correct soil acidity problems. Both Ca and Mg are contained in the liming materials dolomite and dolomitic limestone. Since Ca₂⁺, Mg₂⁺, and K⁺ all compete for the same exchange sites in the soil, too much of any one of these elements can cause plant deficiencies of the other two.

Sulfur (S) is rarely deficient in ornamental plant production because it is commonly a component of fertilizer sources used to supply other elements. In addition, S may be supplied by rain water and is sometimes applied to lower soil pH. Sulfur occurs in the soil solution mainly as the negatively charged sulfate (SO₄^2-) ion and is subject to leaching like NO₃^-.

Micronutrients are required in relatively small quantities, typically less than 5 pounds/acre/year (6 kilograms/hectare^-1/year^-1). Micronutrient deficiencies are not uncommon on Florida's highly leached sandy soils, especially when the soils have been heavily treated with fertilizers not containing micronutrients. Boron (B) occurs as H₃BO₃ in the soil solution and is prone to leaching on sandy soils. Plant uptake of B decreases with increasing pH over the pH range from 5.0 to 7.0 and deficiency symptoms occur most commonly on alkaline soils with high calcium content. Copper (Cu) deficiency is uncommon in ornamental plants in Florida. In fact, excess Cu in soils that were previously in citrus production can sometimes be a problem since Cu occurs in the soil as both monovalent (Cu[OH]⁺) and divalent (Cu²⁺) cations like iron (Fe[OH]₂⁺, Fe[OH]⁺, Fe²⁺) and can induce iron deficiency. Maintenance of higher soil pHs (6.0 to 6.5) can reduce this problem because Cu becomes less available with increasing pH. Iron (Fe) deficiency is fairly common in Florida and can be due to excesses of other heavy metals (Cu, manganese, zinc) relative to Fe. Iron deficiency can also be due to Fe in the soil being unavailable for plant use because of high soil pH and/or high P levels. Leaf yellowing due to Fe deficiency is fairly common where this element is not being supplied to the crop through fertilization. Manganese (Mn), like Cu, Fe and zinc, occurs in soil in a divalent cation form (Mn²⁺) and competes with those other ions for binding locations and root uptake. As with Fe and Mg, Mn deficiency can cause yellowing of foliage. Molybdenum (Mo) is required in very small amounts in plants and is rarely deficient when soil pH is above 5.2. Zinc (Zn) deficiency on soils high in P is fairly common. However, Zn is fairly immobile in soils and deficiencies are uncommon in commercial production where Zn-containing fungicides and fertilizers are used. Additional elements, such as chlorine (Cl), cobalt (Co), selenium (Se) and silicon (Si), have been shown to affect the growth of some plants but little is known about their role in ornamental plant growth and development.

Sources of Crop Nutrients

Nutrients necessary for crop production can be supplied from minerals in the soil, decomposition of soil organic matter, rain and irrigation water, deposition of airborne soil particles, pesticides, and fertilizers. Of these sources, the soil and fertilizers supply the majority of the nutrients. Fertilizers are used to make up the difference between the amount of nutrients available from the soil and the total amount necessary to produce optimum (economic) yield. Once a soil test determines available

soil nutrient levels, fertilization amounts can be determined.

Nutrients and Leatherleaf Fern Production

Even though leatherleaf fern can grow epiphytically (on other plants, such as trees) and on rocks, production of commercially acceptable fronds in economically viable numbers requires the application of nutrients to the low nutrient-containing soils in which it is grown. Two of the primary macronutrients, N and K, each constitute about 2 to 3% of the dry weight of commercially produced leatherleaf fern fronds and must be supplied through fertilization, usually in about equal amounts. Determining how much N and K to apply involves knowing how much is available in the soil.

Phosphorus (P), the other primary macronutrient, is usually available in sufficient quantity in the soils to supply most of the P needed for commercial production of leatherleaf fern. However, applying P in the winter may result in a crop response. Secondary macronutrients and micronutrients are often deficient in Florida's sandy soils and must be applied to prevent yield or quality reductions when producing leatherleaf fern. However, application of excessive and/or arbitrary amounts of these nutrients can cause toxicity to leatherleaf fern.

Irrigation Management

Proper irrigation management consists of many components:

• proper irrigation system design, installation, calibration and maintenance

• accurate irrigation scheduling

• sensible timing of irrigation in relation to chemigation needs and rainfall

• determination of correct amounts of irrigation water to be applied

• precise use of water for cold protection.

Proper irrigation management helps maximize yields while minimizing production costs (fuel, equipment repair, fertilizer, pesticides). At the same time, energy and water are conserved, pollution is reduced, and washoff and leaching of nutrients and pesticides is minimized. Growers who do not practice good irrigation management may find it impossible to produce commercially acceptable leatherleaf fern without violating ground water quality standards.

Irrigation System Design and Installation

Essentially all commercial leatherleaf ferneries have permanent solid set sprinkler irrigation systems (pipes and sprinklers set in regular patterns to irrigate the entire crop at one time). These systems are used because the crop completely covers the area within the fernery (excluding roadways) and must be protected, using water, when freezing temperatures occur. It is very important that these irrigation systems be designed and installed to provide water uniformly over the crop production area. Systems should have uniform pressures throughout and sprinklers should have the appropriate diameter and pattern of water application for the riser spacing used. Sprinklers should rewet the foliage frequently (every 30 seconds or more often, i.e., two or more revolutions per minute) to maximize the effectiveness of water for cold protection. Design of irrigation systems should be done by knowledgeable professionals and the systems should be installed as designed. After installation, calibration should be done to assure proper functioning and uniformity of coverage.

Irrigation System Calibration

Irrigation system calibration can be done in several ways depending on the purpose and precision needed. A simple way to determine the water application rate of an irrigation system is to run the system equipped with a calibrated, correctly functioning flow meter for a given amount of time. Then, knowing the number of gallons of water applied and the size of the irrigated area, the water application rate (in inches per hour) can be calculated as:

Figure 5. 

Alternatively, if the irrigation system is run at a constant pressure and the flow meter indicates the flow rate in gallons per minute (gpm), then the water application rate (in inches per hour) can be calculated as:

Figure 6. 

Another way to calibrate an irrigation system is to place containers (such as coffee cans) randomly throughout the fernery and run the system for a known period of time. The containers should be straight-sided, lipless, all the same size, and located where fern foliage will not interfere with the direct flow of water from the sprinklers into the containers. The containers may need to be placed on supports so that the tops of the containers are just above the fern canopy. These tests should be run when there is no wind and maximum uniformity is obtainable--the same conditions that should prevail when the irrigation system is normally used for purposes other than cold protection. Measure the depth of the collected water in each container and average those numbers. The water application rate (in inches per hour) is calculated as:

Figure 7. 

This method does not require a flow meter. In addition, if enough containers are used and especially if they are placed in the fernery systematically, the results can be used to determine the uniformity (or nonuniformity) of the water distribution pattern in addition to the water application rate. Uniform water distribution is extremely important during the production of leatherleaf fern since water, fertilizers, and pesticides are all typically applied using the irrigation systems.

If sprinkler spacing, spacing pattern, sprinkler nozzle orifice size, and water pressure at the nozzle are known, irrigation water application rates can be approximated using tables and equations (see Reference 22). Water pressure at the nozzle should be measured throughout the system using a pitot tube and pressure gauge. These readings will give an

indication of the pressure uniformity throughout the system--an indication of the quality of the design and installation of the system. Sprinkler manufacturers' literature can then be consulted to determine the water flow rate based on the pressure and orifice size. Nozzle orifice size can increase with time due to abrasion from water and debris in the water. Therefore, rates taken from the manufacturers' specifications may underestimate water application rates of older nozzles unless the actual orifice size is measured. A more direct method of determining water discharge rates would be to collect all the water flowing from individual sprinklers operating at a given pressure for a given time. Flexible tubing can be used to divert the water from the sprinklers into containers on the ground. These flow rates, in gallons per minute (gpm) per sprinkler, can be averaged, and using the tables mentioned previously, water application rates can be determined. If the sprinklers are spaced in rectangular, square or triangular patterns, the water application rate (in inches per hour) can be calculated as:

Figure 8. 

where gpm = gallons per minute

S = sprinkler spacing along the laterals

and L = spacing between the laterals

Irrigation Scheduling

Proper irrigation system management requires knowing when irrigation is needed. Leatherleaf fern should not be irrigated on a calendar-based schedule. Fortunately, several alternative irrigation scheduling methods are available for use during leatherleaf fern production. These methods use direct measurements of soil water potentials or indirect methods to estimate changes in the soil water reservoir.

Monitoring Soil Water Status

Tensiometers

Tensiometers are the most commonly used method to monitor soil water status in Florida's sandy soils. They are inexpensive and easy to use. Tensiometers are sometimes referred to as "mechanical roots." Each one is composed of a water-filled tube with a porous ceramic tip at one end and a vacuum gauge at the other end (Figure 3). As soil water is removed due to evapotranspiration, the soil water potential is reduced and water moves from the tube through the ceramic tip and into the soil, creating a partial vacuum inside the tensiometer. The vacuum gauge registers these changes, indicating the energy necessary to extract water from the soil. As water is added to the soil from rainfall or irrigation, soil water potential increases and water moves from the soil back into the tensiometer, resulting in lower readings on the vacuum gauge. Because the water must penetrate the soil and move down through the soil profile and into the tensiometer, there is a lag period before equilibration is complete. Therefore, growers should experiment to determine at what point the irrigation system can be shut off and still have the vacuum gauges eventually indicate a value near zero.

Proper placement of tensiometers is essential since they indicate the soil water status only in the area directly around the porous tip. They should be installed in areas that are representative of fernery conditions in general, not unusually dry or wet areas. In addition, they should be placed in the middle of the fern root zone at two or more sites per fernery. Using more than one tensiometer makes it possible to detect a malfunctioning one by comparing the readings from all the tensiometers. Since leatherleaf fern root zones in Florida's sandy soils are generally very shallow (3 to 6 inches [8 to 15 centimeters]), vertically oriented tensiometers are usually installed so that the ends of the 2¼ inch- (6 centimeter-) long ceramic tips are 4 to 6 inches (10 to 15 centimeters) below the soil surface. If the tip of a tensiometer extends below the root zone (where available water removal does not occur), inaccurate readings (for irrigation purposes) will occur. In fact, the vacuum gauge readings may rarely vary from zero. Irrigation setpoints of around -18 and -12 centibars (-18 and -12 kilopascals) for the 4-inch (10-centimeter) and 6-inch (15-centimeter) installation depths, respectively, have been used successfully for leatherleaf fern growing on well-drained sandy soils. Growers can adjust their setpoints depending on soil type, root zone depth, and other cultural and management factors.

Figure 9. 

Tensiometers measure soil water status and are useful tools for scheduling irrigation

Some tensiometers are designed to be installed horizontally and can, therefore, be used underground where they are protected from mechanical damage during harvesting or freeze damage (ice forming in the tube and breaking it) during cold weather. Disadvantages of installing tensiometers horizontally are the cost and time involved in preventing the soil from caving in (usually by installing a valve cover box or similar device) and in removing the valve box cover in order to read the vacuum gauge. As with vertically installed tensiometers, care should be taken so that the porous ceramic tip ends up in firm contact with the soil, and is in the middle of the root zone.

Tensiometers will not provide accurate soil water potential measurements if the soil is saline and/or the irrigation water is high in salts, both very rare conditions in commercial leatherleaf fern production areas of Florida. Readings can also be erroneous if the tip is not in good contact with the soil, the vacuum gauge is inaccurate, there is air in the tube, or there is an air leak in the system. Tensiometers are accurate for use in irrigation scheduling over a range of about 0.1 to 0.7 bar (10 to 70 centibars) [0.01 to 0.07 megapascal (10 to 70 kilopascals)], a range suitable for leatherleaf fern production. See reference 15 for more information about installation, maintenance, and use of tensiometers. Finally, the rainfall distribution inside of shadehouses is uneven due to the sagging of the shade cloth and channeling of rain from higher areas near supports to lower ones before the water passes through the shade cloth. This factor may at times have an effect, so tensiometers should be placed in areas under, but not directly under, shade cloth supports where the amount of rain reaching the soil is the least.

Tensiometers indicate only when to initiate irrigation, and not the amount of water to apply. Growers can experiment with irrigation system run-times to determine the minimum run-time necessary to bring the soil around the tensiometers back close to field capacity. If a moisture characteristic curve (Figure 4) is available for the fernery soil, the amount of water to apply can be determined from it.

Additional Methods for Measuring Soil Water Status

Resistance blocks, soil psychrometers, freezing point depression apparatus and moisture probes are included here for completeness; however, these methods are not well-suited for use with leatherleaf fern growing in well-drained sandy soils. Resistance blocks are made of fiberglass, gypsum or other porous materials that absorb water in proportion to the moisture content of the surrounding soil. The blocks contain electrodes that measure electrical resistance and that resistance depends on the amount and salinity of the water in the blocks. Like tensiometers, resistance blocks only gauge soil moisture conditions in the area directly surrounding them. Resistance blocks are accurate only at soil water potentials outside the range needed to schedule irrigation for leatherleaf fern growing in sandy soils. Likewise, soil psychrometers and freezing point depression apparatus are not useful in the soil moisture ranges necessary for irrigation scheduling of leatherleaf fern. In addition, these latter two methods require specialized equipment. Neutron, time domain reflectometry and other moisture probes can indirectly measure soil water status in mineral soils. These probes measure changes in neutron scattering and dielectric constants of soil, both characteristics which are greatly affected by water content. However, these moisture probes are expensive, are less convenient than tensiometers, require calibration, and do not work well near the soil surface where the root system of leatherleaf fern occurs.

Figure 10. 

Characteristic soil water capacity curve for a fine sand soil in Florida.

Soil Water Budget

A soil water budget (balance) is an accounting procedure that tracks water inputs and withdrawals from the soil to determine soil water content changes with time. These inputs and withdrawals are illustrated in Figure 5. When the soil moisture content drops to a predetermined level, irrigation water is applied to bring the soil moisture content back up to field capacity (or a slightly lower moisture content to allow for rainfall storage).

Figure 11. 

Fernery water balance components.

Estimating Soil Water Withdrawals (Evapotranspiration)

Knowing the rate at which water is removed from the soil and the available water-holding capacity of the soil, irrigation can be scheduled using the water budget method (see page 9). There are several ways to estimate the rate that water is depleted from the soil due to evapotranspiration.

Evaporation Pans

Evaporation pans can be used to estimate the amount of water that is used by the crop. The rate at which water evaporates from these open pans of water (E_pan) is determined by the same climatic factors that affect evapotranspiration (ET_crop). Therefore, the rate of water loss from evaporation pans is proportional to the rate of crop water use when soil water is readily available, as is the case in commercial leatherleaf fern production. The relationship between E_pan and ET_crop is termed crop coefficient, is defined as ET_crop /E_pan, and is symbolized as K_crop. K_crop for leatherleaf fern can vary depending on season (see Table 1).

United States Weather Service Class A evaporation pans are 47½ inches (121 centimeters) inside diameter by 10 inches (25 centimeters) inside depth cylindrical tanks made of galvanized steel, stainless steel, or monel (nickel-alloy) metals (Figure 6). These pans are placed on level wooden pallets that raise them 6 inches (15 centimeters) off the ground. Pans are usually placed in open areas such as a field covered with mowed bahiagrass. Placement of pans inside the fernery is not recommended since applications of fertilizers and pesticides could contaminate the water in the pan. This contamination could cause corrosion of the pan and be a potential health hazard. Regardless of the location of the evaporation pan, it should be open to unimpeded light, rain and wind. The use of fencing and/or bird netting may be necessary to protect the water from being used by humans and/or wildlife. The water level in the pan is measured periodically and this information is used along with the K_crop listed above (Table 1) to estimate crop water use. Evaporation pans can be used to determine both when and how much to irrigate. Further information about setting up, maintaining and using evaporation pans is given in Reference 17.

Figure 12. 

United States Weather Service Class A evaporation pans are used to make standardized measurements of evaporation.

Predictive Evapotranspiration Models

Predictive evapotranspiration models utilize weather station data to calculate reference (potential) evapotranspiration (ET_o). The ET_o values are then adjusted using crop coefficients (K_crop), as was done earlier to convert E_pan values (see above), to estimate ET_plant . The Penman equation (Reference 7) is thought to be the most accurate model for estimating daily water use under Florida conditions, but requires considerable amounts of micrometeorological input data and is fairly complex. Seasonal K_crop relating weather station based ET_o (Penman) to ET_actual have been determined for leatherleaf fern growing in shadehouses (Table 2).

Table 2. 

Seasonal monthly crop coefficients (K_crop = ET_crop / ET_o) for leatherleaf fern growing in shadehouses covered with polypropylene shade fabric designed to provide 70% shade. ET_o was determined using the Penman equation and weather station data collected in a bahiagrass field outside the shadehouse.

Winter (Nov. - Feb.)	Spring/Fall (Mar., Apr., Oct.)	Summer (May - Sep.)
0.49	0.37	0.47

Generalized Evapotranspiration Rates

Generalized evapotranspiration rates (Table 3) can be used to estimate water removal rates from the soil by the leatherleaf fern if no better method is available. These generalized rates, of course, do not reflect localized or yearly variations in evapotranspiration (ET_crop). In addition, these generalized values are currently based on only two years data. Regardless, these values should provide better irrigation scheduling than using no method at all.

Table 3. 

Generalized seasonal daily crop water use (evapotranspiration, ET_crop) for leatherleaf fern (in inches [cm]) growing in shadehouses covered with polypropylene shade fabric designed to provide 70% shade.

Winter (Nov. - Feb.)	Spring/Fall (Mar., Apr., Oct.)	Summer (May - Sep.)
0.03 [0.08]	0.05 [0.11]	0.08 [0.19]

Calculating Soil Water Budgets

The water budget for irrigation scheduling (on a daily basis) is calculated as:

Figure 13. 

The soil water budget is started with the soil reservoir full, that is at field capacity, as it would be about 24 hours after a heavy rain or irrigation. Daily rainfall is added to and measurements or estimates of evapotranspiration are subtracted from this value until the soil water has been reduced by the allowable depletion amount. Drainage + runoff is calculated as the depth of rain in excess of the amount that could be stored in the root zone following each rain event.

For example, consider (established) leatherleaf fern with a 6-inch root zone growing in Astatula fine sand with an available water-holding capacity in the upper 6 inches, as determined by the University of Florida for the Soil Conservation Service (now known as the Natural Resources Conservation Service ), of 0.07 inch of water per inch of soil. It is very important for growers to find out what the available water-holding capacity of their soils are since these values are the basis for irrigation scheduling. (Appendix A: Characteristics of Florida Soils Used for Leatherleaf Fern Production lists available water-holding capacity ranges for various soils by county. Private analytical laboratories can determine more precise site-specific available water-holding capacity values).

Figure 14. 

The grower decides to limit the allowable soil water depletion to one-half the total available water--0.21 inch (0.5 centimeter). Assuming it is winter so the average daily evapotranspiration rate is about 0.03 inch (0.08 centimeter) (Table 3) and rain does not occur, the number of days that can elapse before irrigation is necessary is calculated as:

Figure 15. 

Therefore, it would be about a week before the soil available water reached the critical level and irrigation was required.

Table 9 is an example of a filled-in soil water budget worksheet. Table 10 is a blank worksheet form that can be used for scheduling irrigation using the water budget method. As was done in the preceding example, entries start after a rain or irrigation event has returned soil moisture to field capacity (day 0). Reductions (evaporation, transpiration) and increases (rain, irrigation) in soil water are tabulated periodically (daily or slightly less frequently, depending on weather conditions) until the allowable water depletion limit (.0) is reached. Irrigation water is then applied to replace all, or nearly all, of the available water the soil can hold. If the gauge used to monitor rainfall amounts is placed outside the shadehouse, it should be located where there are no obstructions that can interfere with the rainfall pattern. Since shade cloth disrupts the evenness of rain distribution inside the fernery, rain gauges inside shadehouses should be placed under, but not directly under, the shade cloth supports where rain penetration is typically lowest. Locations under

supports (high points in the shadehouse roof) may receive only one-half the precipitation of locations between supports where the shade fabric sags (low points in the shadehouse roof). The gauge should also be elevated so that irrigation water does not enter the gauge.

Determining Water Application Amounts

The amount of irrigation water that should be applied is generally the amount needed to eliminate the soil water deficit (restore soil moisture nearly to field capacity). Applying more water can lead to leaching of nutrients and pesticides beyond the root zone of the plant. Irrigating too often can encourage shallow rooting which is also undesirable.

Several methods of determining how much water is needed to return the soil to (or near) field capacity have been suggested earlier in this publication. Trial and error manipulation of irrigation run-times to return tensiometer readings to values close to zero have been suggested. The reason for the trial and error approach is that, due to the time required for the water movement in the soil and into the tensiometer (as mentioned previously under Tensiometers), irrigating until the vacuum gauge registers zero can result in overirrigation. An advantage of this method is that it automatically adjusts for evaporative water losses during the irrigation process.

Calculation of the soil water budget does not correct for irrigation water losses due to evaporation, wind drift and percolation of water below the crop root zone. Therefore, these factors must be considered when determining the amount of water to apply. Runoff of water during irrigation of leatherleaf fern is uncommon due to the combination of well-drained soils and irrigation water application rates. Nonuniform water application is likely the major factor reducing application efficiencies during irrigation of leatherleaf fern. Sprinkler irrigation system water application efficiencies for field-grown crops are typically 70% (30% loss) if the water is applied during the day and 85% if the water is applied at night. Irrigation application efficiencies in leatherleaf ferneries where low angle sprinklers are used and evaporation is greatly reduced compared to outside, are not known. However, based on the fact that daily E_pan inside ferneries averages about 25% of that outside, it is probable that irrigation water application losses due to evaporation inside ferneries are relatively small. However, recent tests of irrigation systems have shown that the distribution of water in many leatherleaf ferneries is not very uniform. This lack of uniformity is a cause for concern since it affects water, as well as fertilizer and pesticide application amounts. Knowing, or estimating, the water application efficiency of the irrigation system, the amount of irrigation water required can be calculated (in inches) as:

Figure 16. 

Using the allowable soil water depletion calculated previously of 0.21 inch and an irrigation application efficiency of 80%, the irrigation requirement is calculated as:

Figure 17. 

A survey of leatherleaf fern growers (Reference 21) indicates that some growers use application amounts much greater than the one calculated above. Those growers can save energy (fuel) and money by reducing the amount of water they apply to the amount that is actually needed by the crop. Water passing below the root zone is generally unavailable to the crop in Florida's sandy soils, where upward movement of water is extremely limited.

Additional Factors to Consider

When planning irrigation events, factors such as the likelihood of rain and the need to fertigate and/or chemigate should be considered. These additional uses of the irrigation system should be integrated into the irrigation scheduling process to optimize the use of irrigation water. Additionally, irrigation water should be applied when it will have the least potential either to be lost due to evaporation or to extend the period of foliar

wetting that can enhance spore germination and subsequent disease development. Early morning applications generally do not extend the period of foliar wetting since the foliage is already wet with dew. This is also the preferred time to apply fertilizer and pesticides since temperatures are usually at their daily low and this tends to minimize crop damage. In addition, insects like caterpillars tend to do more active feeding during the cooler part of the day and are thus more exposed and vulnerable to contact pesticides applied at that time of day. Growers should be aware that the application of fertilizer and some pesticides (for example, herbicides) at the same time can increase the potential for crop damage.

Irrigation Water Quality

The majority of the irrigation water used for leatherleaf fern production comes from the Floridan aquifer and is of high quality (low salinity, low in toxic ions). However, high dissolved bicarbonates can occur in water from this aquifer. In addition, a few ferneries are located where there may be water salinity problems, such as along the St. Johns River. Other growers are using surface water sources to reduce the need to use groundwater, especially for cold protection. Regardless of the source of water--ponds and lakes, surficial aquifers, or deep aquifers--growers should have their water tested periodically to determine if there are changes occurring and/or problems with this resource. Soil testing should also be done regularly to monitor pH trends and to help determine fertilization requirements.

Salinity

The salt tolerance of leatherleaf fern is unknown. Although leatherleaf fern grows on beaches and sand dunes, high soluble salts or salinity in beds reportedly can reduce productivity by damaging the roots. Growers using water with electrical conductivity above 0.75 deciSiemens/meter (mmhos/centimeter) or total dissolved solids concentrations above 480 parts per million (milligrams/liter^-1) may have to use special management techniques (see Reference 9).

Bicarbonates

Water containing high bicarbonate levels can cause soil pH levels to increase to unacceptable levels for leatherleaf fern production. The more this water is applied, the worse the problem; therefore, overirrigation should be avoided (see Calculating Soil Water Budgets; Determining Water Application Amounts). Acids and acid-forming fertilizers can be used to compensate for this problem (see Reference 8).

Irrigation for Cold Protection

Essentially all commercial irrigation of leatherleaf fern is done using overhead irrigation systems that use impact sprinklers because these systems can be used to cold protect this subtropical crop during freezes. There are several methods and techniques that can be used to minimize the amount of water necessary for cold protecting this crop. See Reference 23 for additional cold protection information not covered below.

Sprinklers

Frost protection impact sprinklers having faster rotation rates (2 to 3+ revolutions per minute [rpm]) than conventional impact sprinklers (1 rpm) have been shown to provide equivalent cold protection using about 50% less water than when using conventional sprinklers (Reference 24).

Dual Irrigation Systems

Shadehouses equipped with two irrigation systems, one to apply water to the shade cloth and one to apply water to the crop, can be used to decrease the amount of water needed to cold protect leatherleaf fern during freeze events and to reduce the amount of crop damage during advective freezes. The over-the-shade cloth irrigation system is run just long enough to wet the cloth sufficiently so that ice can form and seal the openings. Icing of the shade cloth reduces advective and radiational heat losses.

Determining When to Start/Stop Irrigation

During mild radiation freezes where temperatures drop slowly over the course of the night, growers can watch for the onset of frost formation on the crop and start irrigating when frost first starts to develop. An additional technique growers can employ is to monitor immature fronds and start irrigating when the tender exposed fronds at the top of the crop canopy first begin to stiffen up, but before ice forms that causes plant damage. Fronds located in the coldest parts of the fernery should be monitored. (See Additional Factors to Consider for another water-saving technique to use during mild, calm freeze events.)

During more severe freezes, irrigation water applications for cold protection should start when wet bulb temperatures in the shadehouse or hammock reach 34°F (1°C) and stop when wet bulb temperatures rise to that same temperature, or slightly higher if it is windy.

Additional Factors to Consider

During moderate to severe freezes, irrigation water is usually applied continuously to the crop; however, intermittent water application can successfully be used to cold protect leatherleaf fern during mild radiation freezes when ambient temperatures stay in the upper 20s (above -3°C). Careful monitoring of leaf temperatures and/or leaf surfaces for unfrozen water can be used to determine when to apply additional irrigation water. As long as there is a significant amount of water on the foliage in the liquid state, additional water application is unnecessary. As the water turns to ice, heat energy is released. When the supply of liquid water on the foliage gets low due to ice formation, additional water is applied. This technique is most practical for growers with one fernery or only a few ferneries located near one another.

Windbreaks and shelterbelts, when used in conjunction with irrigation water, can be beneficial in reducing cold damage during windy (advective) freezes by reducing air movement. However, temperatures inside shadehouses (prior to being cold protected) are often colder than temperatures outside shadehouses during radiation freezes. Most freeze events in Florida are the radiation type where lack of air mixing is the problem. Under these calm conditions, windbreaks and shelterbelts can make the temperature inversions caused by the stagnant air movement worse. Therefore, windbreaks that can be opened and closed are preferred so that they can be left open as long as possible during radiation freezes and closed up prior to advective freezes.

Nutrient Management

Proper nutrient management requires the integration of irrigation, liming, pest control, and fertilization practices. Correct irrigation management is essential to being able to fertilize leatherleaf fern efficiently. Excessive or poorly scheduled irrigation events waste fuel, leach nutrients, and may increase the potential for disease development. Nutrient contributions from water, soil, and pesticides should be determined to aid in planning fertilization programs. Amounts of nutrient losses and the causes for those losses should also be considered.

Nutrient Losses

The major avenues for nutrient losses from leatherleaf ferneries are movement in water past the root zone (leaching) and removal of the nutrients in the harvested fern fronds. In addition, under certain conditions, nitrogen can be converted to gaseous forms that are lost to the atmosphere (denitrification and ammonia volatilization). Knowledge of these processes can enhance a grower's ability to manage nutrients and reduce the risk of ground water pollution.

Nutrient Leaching

Nutrient leaching can occur readily from the soils used to produce leatherleaf fern. Nitrogen, in the negatively charged nitrate (anionic) form NO₃^-, is by far the most abundant soluble nutrient in soil water. As mentioned in the introduction, soils are also negatively charged so NO₃^- moves freely with soil water. The United States Environmental Protection Agency (EPA) has set a maximum contamination level (MCL) for NO₃^-N in drinking water of 10 parts per million (10 milligrams/liter^-1) and this standard has been adopted in Florida as an enforceable ground water quality standard. Therefore, one of the management goals for this nutrient is to keep its concentration in the aquifer below the MCL. Overirrigating and the resultant need to overapply N is a situation that all growers should avoid.

Even positively charged nutrient ions (cations) can easily be leached from most of the sandy soils used for leatherleaf fern production since these soils have low cation exchange capacities (CECs of 1-5 meq/100 g of soil) because they contain relatively little clay or organic matter. These soils can generally be improved, both in nutrient- and water-holding capacities, by the addition of organic matter. However, the addition of organic matter in quantities sufficient to significantly improve these sandy soils is not always economically feasible. Regardless, frequent applications of small amounts of nutrients can be most effective and efficient. This applies to anions as well as cations. A labor-saving alternative to applying small amounts of nutrients frequently is to use long-term controlled/slow-release nutrient sources. Both strategies can optimize nutrient availability and minimize fertilizer leaching potentials.

Nutrient Removal Due to Harvesting

Nutrient removal due to harvesting must be considered when determining nutrient application rates. Frond production and nutrient content can be useful in predicting nutrient requirements needed to reach specific production levels. For example, a grower might have quite different yield expectations/goals for a hammock producing fern for the domestic market and a shadehouse producing fern for export to Europe. The amount of nutrients required in the latter case might be two or three times greater than for the hammock because of the differences in frond size, N content, and frond numbers (see Table 4).

Gaseous Nitrogen Losses

Gaseous nitrogen losses can occur under certain conditions. Denitrification, the change of nitrate to N gases (N₂, N₂O) by bacteria, usually occurs in poorly aerated soils. Denitrification can occur very rapidly in warm, slightly acidic soils that are low in oxygen (as when waterlogged due to heavy rains and/or the presence of clay hardpans). Avoiding over-irrigation and installing drainage tile can help prevent denitrification from occurring. Ammonia volatilization, the release of ammonia (NH₃) to the atmosphere, can occur if NH_4-N or urea are placed on the surface of alkaline (high carbonate content) soils. Avoiding the use of NH_4- and urea-containing fertilizers directly following the application of liming materials and irrigating NH_4-N and urea in with about one-quarter inch of water after application will prevent this potential loss of nitrogen.

Nutrient Sources and Availability

Nutrients are present in soil and water; however, most are not present in sufficient quantities to supply all the requirements for commercial production of leatherleaf fern. Regardless, the amounts available from these sources should be considered when determining fertilizer application rates.

Table 4. 

Examples of annual nutrient removal due to harvesting of leatherleaf fern fronds.¹

	Low range - 8 cases/acre/week, frond fresh weight of 0.4 oz.		High range - 12 cases/acre/week, frond fresh weight of 0.6 oz.
	Frond Nutrient Content (dry wt. basis)	Nutrient Removal (lb/acre/year)	Frond Nutrient (dry wt. basis)	Nutrient Removal (lb/acre/year)
Primary Nutrients (%)
N	2.0%	52.0	2.8%	163.8
P	0.2%	5.2	0.4%	23.4
K	2.3%	59.8	3.4%	198.9
Secondary Nutrients (%)
Ca	0.30%	7.8	0.7%	41.0
Mg	0.20%	5.2	0.4%	23.4
Micronutrients (ppm [mg/kg-1])
B	25	0.07	75	0.44
Cu	10	0.03	30	0.18
Fe	100	0.26	400	2.34
Mn	400	0.10	150	0.88
Zn	30	0.08	150	0.88
1 Assuming frond water content of 75%.

Soils

Sampling

The first step in getting a useful soil test is to collect a representative composite sample for the fernery that excludes unusual areas (such as where burn piles were located). Ten to fifteen uniform soil cores taken from the soil surface down to the bottom of the effective root zone--about 4 to 6 inches (10-15 centimeters) for leatherleaf fern--are enough for ferneries up to 40 acres (16 hectares) in size. Samples should be taken at least twice a year so that trends can be detected and action taken before major problems occur.

Samples should be analyzed by a competent soil testing laboratory as soon as possible after they are taken. Since testing methodologies vary from lab to lab, it is important to find a good lab and use them routinely so that results from sampling to sampling can be easily compared.

pH

Although leatherleaf fern is tolerant of a wide range of soil pHs (from below 4 to above 7), soil pH should usually be maintained in a range between 5.5 and 6.5 because of the effects soil pH has on the relative availability of nutrients. The pH scale is a logarithmic measure of the soil hydrogen ion (H⁺) concentration, meaning that at a pH of 4, the H⁺ concentration is 10 times greater than at pH 5, and 100 times more acidic than at pH 6. Many fertilizers contain nutrient sources that can lower soil pHs, while irrigation water high in carbonates can cause soil pH levels to rise (see Bicarbonates).

To combat soil pH changes and/or to restore soil pH to desired levels, liming materials such as calcite (calcitic limestone, CaCO₃) or dolomite (CaCO₃ MgCO₃) are used to raise pH and elemental sulfur (S) is used to lower pH. On the sandy soils with low organic matter content (1 to 2%) where most leatherleaf fern is grown in Florida, a ton (907 kilograms) of calcite or dolomite per acre will raise the root zone pH by approximately one pH unit. About one-third as much S (660 pounds [302 kilograms]) is needed to lower soil pH by one unit. Long-term use of S-containing nutrient sources can help lower and/or maintain pHs. The physical size of the particles of these pH-adjusting materials have an effect on the rate and duration of the changes that occur. The finer the material, the faster and the more short-lived the change in pH; the coarser the material, the slower the change, but the longer the duration. Since the finer materials react more rapidly, they are more prone to cause injury if applied in excessive amounts.

Water

Irrigation water may contain plant nutrients and these nutrients should be factored into fertilizer management programs. For example, liming programs should take bicarbonates' and carbonates' contributions from irrigation water into account. As mentioned in the Water Quality section, the liming potential of irrigation water can cause soil pH problems in some cases. The nutrient content of recycled water and reclaimed waste-water is usually higher than that of water from deep wells and can save growers money by reducing fertilizer needs. Research is underway to determine if nutrient recycling (and additional cost savings) can be accomplished using horizontal wells installed under ferneries.

Pesticides

Many factors affect the nutrient application rates needed for commercial production of leatherleaf fern. As mentioned previously, the nutrient content of the soil and irrigation water must be taken into consideration. Soil and water test results apply to both potential nutrient deficiencies and excesses, conditions which may reduce growth and/or crop quality. Pesticides are an often overlooked source of nutrients, especially micronutrients, and should be considered when planning fertilization programs. For example, the fungicide mancozeb is commonly used on leatherleaf fern and contains, by weight, 16% manganese (Mn) and 2% zinc (Zn). If this material was applied ten times per year at the rate of 2 pounds (lbs) per acre (2.2 kilograms per hectare^-1) per application (app), then the amounts of Mn and Zn applied

would be:

Figure 18. 

This fungicide could therefore supply all the Mn and part of the Zn needed by the crop under certain production regimes. Other pesticides also contain micronutrients.

Fertilizer Application Methods and Forms

Regardless of application method, large amounts of soluble fertilizers should never be applied at one time since the application of excessive amounts of fertilizer can lead to ground water contamination. Most fertilizers are applied to commercial leatherleaf fern as liquids using the irrigation system (fertigation), because this can be an efficient and labor-saving method of application. Fertigation makes the application of small amounts of fertilizer at frequent intervals economically feasible. Efficient fertigation is dependent upon good uniformity of application by the irrigation system. Although the application of dry fertilizers is more labor intensive than chemigation, dry fertilizers are still used on leatherleaf fern. The development of controlled-release fertilizers that only need to be applied once every six months or once a year changes the application cost differential between liquid and dry fertilizers. Dry fertilizers have an advantage when fertilizing newly planted fern beds because they can be placed where the fern is located; whereas, much of the fertilizer applied using fertigation ends up outside the root zone. Dry fertilizers also have an advantage when applied to established leatherleaf fern beds since they can be placed directly where the leatherleaf fern plants are located. Therefore, costly nutrients are not applied (as they are using fertigation) to aisles and roadways where the nutrients can only nourish weed growth and increase the potential for nutrient leaching.

Research on other crops in Florida has shown no differences in plant response to similar amounts of nutrients whether from liquid or dry sources. Since nutrient uptake from the soil solution is in the form of ions, liquid or dry fertilizer form would not be expected to matter unless liquid fertilizer was specifically applied as a foliar application, in a form that could be taken directly into the crop, and on a crop whose foliage was permeable to the fertilizer. The decision to use liquid and/or dry fertilizer should be based on availability, economics, environmental concerns and other criteria.

Fertilizer Application Timing

Liquid fertilizers should be applied on a regular basis to actively growing leatherleaf fern. Weekly applications are most common since this allows relatively small amounts of fertilizer to be applied at one time. However, during rainy weather weekly fertilizer applications may not be desirable for several reasons. First, irrigation of soils at or near saturation wastes energy (diesel fuel) since the water is not needed. Second, additional wetting of the crop foliage can wash off protective pesticides and increase the potential for disease development. Third, adding water to already-wet soils can lead to saturated water flow and increased leaching of nutrients and pesticides. Fourth, fern quality (frond vase life) has been shown to decrease with increasing growth rate and adding water and fertilizer during the summer rainy season may increase the problem of postharvest wilt. If wet soil conditions cause the postponement of fertilizer applications, the amount of fertilizer that is applied after the postponement should not be increased as this can lead to excessive nitrogen leaching and increased production costs. Research has shown that commercially acceptable leatherleaf fern can be produced for at least one full year using infrequent (monthly) applications of fertilizer at rates as low as 102

pounds nitrogen (N)/acre/year (114 kilograms N/hectare per year), so missing a fertilizer application every now and then should have little or no effect on overall production.

The frequency of application of dry fertilizers should be determined according to the duration of nutrient release. The application frequency can then be used to determine how much fertilizer to apply at each application. Durations of nutrient release are often temperature sensitive and this should be taken into consideration. For example, some controlled-release fertilizers (CRFs) release nutrients faster under Florida conditions and therefore must be applied in smaller amounts at shorter intervals than would be appropriate in cooler climates. Research has shown that applications of, at least some, CRFs should be timed so as to avoid having large stores of nutrients still in the prills at the onset of hot weather. Application of 3- to 4-month release CRFs in early spring or 8- to 9- month release materials in the fall should minimize the amount of residual nutrients available during hot weather.

Whether using liquid or dry fertilizer, fertilizer application amounts can be reduced in the winter when temperatures, light levels, and growth rates are reduced. In fact, ammonia burn of leatherleaf fern has been observed during the winter in ferneries applying excessive amounts of nitrogen fertilizer. The reddish-brown damage to the foliage caused by the fertilizer can be mistaken for a fungal disease.

Fertilizer Sources and Formulations

The nutrients in the soil are generally not available in adequate quantities for commercial leatherleaf fern production, so additional nutrients are supplied using fertilizer. There are numerous sources of the various nutrients needed by leatherleaf fern. Table 5 and References 1, 10, and 27 list many fertilizer and liming sources and their nutrient content. Fertilizer labels/tags also provide information about the nutrient content of each product. The nitrogen (N), phosphoric acid (P₂O₅) soluble potash (K₂O) contents (% by weight) are listed as the first three numbers on the label. For example, an 8-2-8 analysis fertilizer contains 8 pounds (3.6 kilograms) of N, 2 pounds (0.9 kilograms) of P₂O₅ , and 8 pounds of K₂O per 100 pounds (45 kilograms) of fertilizer. The corresponding elemental P and K contents are 0.88 and 6.64 pounds (0.4 and 3 kilograms) per 100 pounds, since P₂O₅ and K₂O contain 44% and 83% of those primary macronutrients, respectively. The contents of the other nutrients in fertilizers are expressed as percentages, also by weight, of the indicated element.

Nutrient Requirements

Crop nutrient requirements (CNR) are defined as the amounts of particular elements that are needed by a crop to produce economic optimum yield of marketable product. CNRs are based on experiments that test the yield response of the crop to various levels of applied nutrients. Unfortunately, little experimental nutritional data for leatherleaf fern are available. This is because it is a minor crop grown commercially only in a small geographic area in Florida and practically nowhere else in the United States. Furthermore, nutritional research on this crop is nonexistent in other countries where it is grown. In addition, it is much more difficult and takes much longer to determine CNRs for perennial crops like leatherleaf fern that are harvested continuously throughout the year than for crops with short crop cycles and/or few harvests. Therefore, suggested fertilization rates for nutrients other than N and K are based on field observations, research on other plants, and tissue analysis of commercial grade leatherleaf fern. The information on N and K is applicable to current leatherleaf fern production methods but has been developed during 6-month- to 1-year-long studies so the long-term effects are not known. However, commercially harvested leatherleaf fern has been grown for several years at the Central Florida Research and Education Center using the recommended N and K rates listed in Table 6.

As mentioned in the introduction, crop nutrient requirements can be satisfied from many sources, with soil, irrigation water, pesticides, and fertilizer being the main ones for leatherleaf fern. Therefore, development of a fertilization plan starts with the results of a properly calibrated soil test made on a representative soil sampling, irrigation water analysis, and determination of other factors that affect nutrient availability and losses.

Table 5. 

Approximate nutrient content of various materials.

	Percentage Composition (see text and Abbreviations)
Material	N	P2O5	K2O	Ca	Mg	S	Na	Other
ammonium nitrate (nitrate of ammonia)	33.5
(mono)ammonium phosphate (MAP)	11.5	47 -61
(di)ammonium phosphate (DAP)	17	52
ammonium sulfate (sulfate of ammonia)	20.5					23
ammonium thiosulfate	19					43
borax (sodium tetraborate decahydrate)							8.9	11 B
calcium-magnesium carbonates (dolomite)				20	11
calcium nitrate (nitrate of lime)	15.5			20
calcium sulfate (gypsum)				22		17
copper oxide								75 - 89 Cu
copper sulfate						13		24 Cu
iron sulfate (ferrous sulfate, copperas)						11		20 Fe
magnesium nitrate	7				7
magnesium sulfate					18	24
mancozeb fungicides								16 Mn, 2 Zn
manganese oxide								33 - 77 Mn
manganese sulfate						13		23 Mn
potassium chloride (muriate of potash)			60					44 Cl
potassium magnesium sulfate (sulfate of potash-magnesia, SPM)			25		10.8	22	0.7	1 Cl
potassium nitrate (nitrate of potash)	13		44
potassium sulfate (sulfate of potash)			48			18		2 Cl
sodium molybdate							14	39 Mo
sodium nitrate (nitrate of soda)	16						27
sodium and potassium nitrates (nitrate of soda-potash)	15		14				13	0.1 B
superphosphate, triple (concentrated)		46		14		2
Tecmangam®						16		28 Mn
urea	45
urea, sulfur-coated	35					20
urea-formaldehyde (urea-form)	38
zinc oxide								80 Zn
zinc sulfate						12		36 Zn

Determining Fertilizer Application Rates

The nutrient application rates listed in Table 6 are guidelines as to the amounts of nutrients that should be applied using fertilizers when these elements are not available from the soil (soil test value of Very Low, see Reference 5) or other sources (see Nutrient Sources and Availability). The lower values in each range are for reduced-intensity production (new plantings, old ferneries) or normal production where the nutrients can be applied directly to the fern beds. Research has shown that the lowest suggested application rates provide the greatest potential for avoiding violations of water quality standards. The values at the upper end of the range are for very high-yielding, intensively managed leatherleaf fern at peak production. Use of the higher rates can lead to ground water contamination if the crop does not grow vigorously enough to utilize the additional nutrients. If elements are available from sources other than fertilizer, those amounts should be subtracted from the values listed in Table 6.

Table 6. 

Annual nutrient application rates for commercial production of established¹ leatherleaf fern in Florida.

Nutrient	Application rate2 - lbs/acre/year [kg/ha-1/yr-1]
N	100 - 350 [112 - 392]
P as P2O5	120 - 150 [134 - 168]
K as K2O	100 - 350 [112 - 392]
Ca	varies3
Mg	50 - 150 [56 - 168]
S	20 - 60 [22 - 67]
B	0.5 - 1.5 [0.6 - 1.7]
Cu	0.3 - 1.1 [0.3 - 1.2]
Fe	1.2 - 6.0 [1.3 - 6.7]
Mn	1 - 4.5 [1 - 5.0]
Zn	0.7 - 4.5 [0.8 - 5]
1 Lower nutrient application rates should be used for newly planted leatherleaf fern. Initial fertilizer applications (other than for pH adjustment prior to planting should not start until feeder roots start developing on the transplanted rhizomes. 2 Use periodic tests to determine how much of each nutrient is available from the soil and water, and reduce fertilizer application amounts accordingly. Some pesticides contain micronutrients and these contributions should also be subtracted from the annual amounts applied using fertilizers. Nutrients should be applied in small amounts or in controlled/slow release forms to minimize leaching and other losses. 3 Using dolomite to maintain soil pHs between 5.5 - 6.5 should supply adequate Ca for leatherleaf fern. Required Ca application rates depend, in part, on the acidifying effects of the nutrient sources used for fertilization.

Once the nutrient application rate has been decided upon, the fertilizer application rate can be determined. It is necessary to know the concentration of the nutrients in the fertilizer and the nutrient-release characteristics for each source to be able to calculate fertilizer application rates. Included below are some sample calculations for nitrogen, the most limiting element of current environmental concern.

Example 1. Management has determined that an annual nitrogen (N) application rate of 300 pounds N/acre/year (336 kilograms N/hectare per year) is needed to produce twelve 1,000 frond cases of leatherleaf fern per acre per week (624,000 fronds/acre/year [1,541,904 fronds/ha^-1/yr^-1]). Soil and water tests indicate that essentially no N is

available from those sources. Therefore, management plans to apply, on an almost weekly basis, an 8-0-8 liquid fertilizer containing micronutrients that weighs approximately 10 pounds/gallon (1.2 kilograms/liter^-1). Calculation of the weekly application (app) rate follows:

Figure 19. 

Of course, the grower would probably decrease this amount some when the fern was growing slowly or during the summer when fertilization might increase fern growth rate at the cost of lower frond quality.

Table 7 lists weekly liquid fertilizer application amounts required to achieve target annual nitrogen application rates.

Example 2. Management decides to use a 1- year-duration controlled-release fertilizer (CRF) source because they think the labor and fuel savings will offset the higher fertilizer cost compared to using liquid fertilizer. In addition, the use of dry fertilizer will allow greater control over irrigation and foliar wetting, thereby providing more management opportunities to try to control disease and postharvest wilt using cultural methods. The CRF contains 18% N and will be applied to the fern beds that cover 70% of the fernery (the remaining 30% is occupied by aisles and roadways). Previous testing indicates that an application rate (to the beds) of 250 pounds N/acre/year [280 kilograms N/hectare per year] is adequate to meet production goals for quality and yield. Calculation of the yearly CRF application rate per treated (fern bed) acre follows:

Figure 20. 

The amount of CRF needed to treat each acre of fernery per year is:

Figure 21. 

Frond (Leaf) Tissue Analysis

Analysis of the elemental composition of leatherleaf fern fronds can sometimes be used to help determine the effects of nutrient management programs and as an aid in diagnosing problems. Although excessive nutrient applications can eventually lead to obvious symptoms (e.g., frond brittleness associated with excess N), some problems (or potential problems) can be detected before visual symptoms appear using tissue analysis. Nutrient deficiencies and imbalances can sometimes be detected using tissue analysis. Table 8 lists desirable ranges for mature leatherleaf fern frond content of ten essential elements.

Table 7. 

Gallons (liters) of liquid fertilizer to apply approximately weekly (50 applications per year) to achieve target nitrogen (N) application rate.

Target N application rate [lbs N/acre/year (kg N/ha per yr)]	Gallons (liters) of liquid fertilizer per acre per week
	6% Nitrogen	8% Nitrogen
100 (112)	3.3 (12.6)	2.5 (9.5)
150 (168)	5.0 (18.9)	3.8 (14.2)
200 (224)	6.7 (25.2)	5.0 (18.9)
250 (280)	8.3 (31.6)	6.2 (23.7)
300 (336)	10.0 (37.8)	7.5 (28.4)
350 (392)	11.7 (44.2)	8.8 (33.1)

Table 8. 

Desirable leaf tissue content (on a dry weight basis) of ten elements found in mature leatherleaf fern fronds.

Element	Frond Content
Primary Nutrients (%)
N	2.0 - 3.0
P	0.22 - 0.40
K	2.3 - 3.4
Secondary Nutrients (%)
Ca	0.3 - 0.7
Mg	0.2 - 0.4
Micronutrients [ppm (mg/kg-1)]
B	25 - 75
Cu	10 - 30
Fe	110 - 400
Mn	40 - 150
Zn	30 - 150

Summary

• Measure irrigation system water application rate (see Irrigation System Calibration).

• Determine soil available water-holding capacity (see Calculating Soil Water Budgets and Appendix A).

• Schedule irrigations using soil moisture measurements (see Monitoring Soil Water Status) or by keeping a soil water budget. Do not irrigate on a calendar-based schedule.

• Apply only enough water to replenish the (available) water deficit in the root zone (see Determining Water Application Amounts).

• Integrate fertigation and chemigation with irrigation events to conserve water, and to minimize irrigation runtimes and foliar wetting durations.

• Consider soil, water, and pesticide nutrient contents when determining fertilizer programs.

• Apply nutrients according to the guidelines listed in Table 6--keeping concerns about water resource contamination in mind when selecting sources and rates.

• Base nutrient application intervals on nutrient release rates from the various fertilizer sources (see Fertilizer Application Timing).

Abbreviations and Symbols

app(s) .................application(s)

B .........................boron

Ca .......................calcium

CEC ....................cation exchange capacity

cm(s) ...................centimeter(s)

Cl .........................chlorine

CNR .....................crop nutrient requirement

Co ........................cobalt

CRF .....................controlled-release fertilizer

Cu ........................copper

°C ........................degrees Celsius

°F .........................degrees Fahrenheit

EPA ......................Envrionmental Protection Agency

Fe .........................iron

ft ...........................foot, feet

g ...........................gram(s)

gal ........................gallon(s)

gpm ......................gallons per minute

ha .........................hectares

hr ..........................hour

K ...........................potassium

kg .........................kilogram

K₂O ......................potassium oxide

lb(s) ......................pound(s)

MCL ......................maximum contamination level

mg ........................milligram

Mg ........................magnesium

Mn ........................manganese

Mo ........................molybdenum

N ..........................nitrogen

Na ........................sodium

oz .........................ounce

% ..........................percent

P ..........................phosphorus

P₂O₅ ....................phosphoric acid

ppm ......................parts per million

rpm .......................revolutions per minute

S ..........................sulfur

wt .........................weight

yr ..........................year

Zn .........................zinc

Conversion Factors

References and Further Reading

1. Anderson, D. L. 1991. Fertilizer and Liming Sources Used in the U.S. Univ. of Fla., Inst. of Food and Agr. Sci., Agr. Expt. Sta. Cir. S-383.

2. Baldwin, R., C. L. Bush, R. B. Hinton, H. F. Huckle, P. Nichols, F. C. Watts, and J. A. Wolfe. 1980. Soil survey of Volusia County, Florida. United States Dept. of Agr., Soil Cons. Serv.

3. Dickey, R. D. 1977. Nutritional Deficiencies of Woody Ornamental Plants Used in Florida Landscapes.

Univ. of Fla., Inst. of Food and Agr. Sci., Agr. Expt. Sta. Bul. 791.

4. Furman, A. L., H. O. White, O. E. Cruz, W. E. Russell, and B. P. Thomas. 1975. Soil survey of Lake County, Florida. United States Dept. of Agr., Soil Cons. Serv.

5. Hanlon, E. A., G. Kidder, and B. L. McNeal. 1990. Soil, Container Media, and Water Testing --

Interpretations of IFAS Standardized Fertilization

Recommendations. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Cir. 817.

6. Henley, R. W., B. Tjia, and L. L. Loadholtz. 1980. Commercial Leatherleaf Fern Production in Florida. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Bul. 191.

7. Jones, J. W., L. H. Allen, S. F. Shih, J. S. Rogers, L. C. Hammond, A. G. Smajstrla, and J. D. Martsolf. 1984. Estimated and Measured Evapotranspiration for Florida Climate, Crops, and Soils. Univ. of Fla., Inst. of Food and Agr. Sci., Agr. Expt. Sta. Bul. 840.

8. Kidder, G., and E. A. Hanlon, Jr. 1985. Neutralizing excess bicarbonates from irrigation water. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Notes in Soil Science No. 18.

9. Kidder, G., and R. D. Rhue. 1983. Interpretation

of IFAS water tests. Univ. of Fla., Inst. of

Food and Agr. Sci., Fla. Coop. Ext. Serv. Notes

in Soil Science No. 10.

10. Meister Publishing. 1994. Farm Chemical Hand-book. Meister Publishing, Willoughby, OH.

11. Pitts, D. J., and A. G. Smajstrla. 1989. Irriga-tion

Systems for Crop Production in Florida: Descriptions and Costs. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Circ. 821.

12. Readle, E. L., R. Baldwin, J. E. Leppo, A. O. Jones, C. J. Heidt, B. F. Grissi, and D. T. Simonson. 1990. Soil survey of Putnam County area, Florida. United States Dept. of Agr., Soil Cons. Serv.

13. Smajstrla, A. G., B. J. Boman, G. A. Clark, D. Z. Haman, D. J. Pitts, and F. S. Zazueta. 1990. Field Evaluations of Irrigation Systems: Solid Set or Portable Sprinkler Systems. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Bul. 266.

14. Smajstrla, A. G., B. J. Boman, G. A. Clark, D. Z. Haman, F. T. Izuno, and F. S. Zazueta. 1988. Basic Irrigation Scheduling in Florida. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Bul. 249.

15. Smajstrla, A. G., D. S. Harrison, and F. X. Duran. 1984. Tensiometers for Soil Moisture Measurement and Irrigation Scheduling. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Cir. 487.

16. Smajstrla, A. G., and R. H. Stamps. 1993. Simulating irrigation requirements of an ornamental fern. Proc. Fla. State Hort. Soc 106:270-273.

17. Smajstrla, A. G., F. S. Zazueta, G. A. Clark, and D. J. Pitts. 1989. Irrigation Scheduling with Evaporation Pans. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Bul. 254.

18. Stamps, R. H. 1992. Commercial leatherleaf fern culture in the United States of America. pp. 243-249. In: Fern Horticulture: Past, Present and Future Perspectives. The Proceedings of the International Symposium on the Cultivation and Propagation of Pteridophytes, London, England. Intercept Ltd., Andover, UK.

19. Stamps, R. H. 1994. Evapotranspiration and nitrogen leaching during leatherleaf fern production in shadehouses. Spec. Pub. SJ 94-SP10. St. Johns River Water Manage. Distr., Palatka, FL.

20. Stamps, R. H. 1989. Icing shadehouses during radiation freezes. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv. Cut Foliage Grower 4(11/12):1-5.

21. Stamps, R. H., W. G. Boggess, and A. G. Smajstrla. 1991. Irrigation management practices in the leatherleaf fern industry. Proc. Fla. State Hort. Soc. 104:328-330.

22. Stamps, R. H., and C. C. Boone. 1996. Determining Irrigation Water Application Rates. Univ. of Fla., Inst. of Food and Agr. Sci., Central Fla. Res. and Ed. Cntr.-Apopka Cut Fol. Res. Note RH-96-C.

23. Stamps, R. H., and D. Z. Haman. 1991. Cold Protection of Leatherleaf Fern in Lake, Putnam, and Volusia Counties, Florida. Spec. Pub. SJ 91-SP15. St. Johns River Water Manage. Distr., Palatka, FL.

24. Stamps, R. H., and D. D. Mathur. 1982. Reduced water application rates and cold protection of leatherleaf fern. Proc. Fla. State Hort. Soc. 95:153-155.

25. Street, J. J., and G. Kidder. 1990. Soils and Plant Nutrition. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv., Soil Sci. Fact Sheet SL-8.

26. Tisdale, S. L. and W. L. Nelson. 1975. Soil Fertility and Fertilizers. MacMillan Publishing, New York, NY.

27. Volk, G. M. and J. B. Sartain. 1977. Fertilizers and Fertilization. Univ. of Fla., Inst. of Food and Agr. Sci., Fla. Coop. Ext. Serv., Bul. 183-C.

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