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Irrigation Scheduling for Tropical Fruit Groves in South Florida 1
Yuncong Li 2The annual average precipitation in south Florida is about 55 inches, and two-thirds occurs during the hot, humid, summer growing season between May and October. The dry period encompasses November to April when irrigation is essential for assuring the productivity of tropical fruit trees. Most tropical fruit trees are grown on gravelly or sandy soils, which have very low water holding capacities. Optimal scheduling of irrigation is essential to allow growers to apply the amount of water actually needed by tree crops.
When should trees be irrigated?
When trees suffer water stress their condition is reflected in their visual appearance. However, often by the time that visual symptoms of water stress are strongly developed, it is already too late to achieve complete recovery. Examination of the soil 3-4 inches below the soil surface under tree canopy by the "kick and feel" method can be helpful in deciding whether to irrigate, but the reliability of this method depends heavily on the growers experience. Irrigation can be determined by calendar methods and time of year (for example, every 4th day in May), by tree phenology (for example, every 3rd day during the flowering stage and every 4th day during the vegetative growth stage). However, these methods fail to make adjustments for climate variations and their effects on water use by tree. Therefore, direct measurements of soil moisture should be used for determining when irrigation is needed. For this purpose two devices have been evaluated in south Florida; one being the tensiometer and the other the recently developed multi-sensor capacitance probes.Tensiometer: This device directly measures soil matric (water) potential or tension in the root zone. Proper calibration, installation and maintenance of a tensiometer are essential for its use in scheduling irrigation in tropical fruit groves in south Florida. In general, the following guidelines can be used to interpret tensiometer readings for irrigation scheduling in gravelly or sandy soils: 1) At tensiometer readings of 0-8 cbars, the soils are saturated or nearly saturated as a result of recent rain or irrigation. Therefore irrigation should be discontinued to prevent the wasting of water and the leaching of nutrients; 2) At readings of 10-20 cbars, the crops should be irrigated. Irrigation should be initiated at 10-15 cbars during flowering and fruit set and at 15-20 cbars at other times; and 3) At readings of 30 cbars and higher the plants probably show symptoms of water stress, and should be irrigated without delay.
Multi-sensor capacitance probe (EnviroSCAN): Despite its high cost and sophistication of operation, the multi-sensor capacitance probe is very useful for irrigation scheduling in tropical fruit groves. The instrument takes advantage of the fact that the dielectric constant of water is 100, that of air is 1 and that of dry soil is in the range of 4 to 6. Thus the probe measures the electrical capacitance of the surrounding soil-air-water mixture, and the instrument converts this reading into the percentage of water in soil. A computer program provides a graph showing how the moisture level is changing with time. Irrigation should be initiated when the soil water content has been depleted to approximately the mid-point between field capacity and the "onset of plant stress." The onset of plant stress is indicated by the leveling off of the graph.
How much water do trees need?
The optimal amount of irrigation is that which is just enough to meet the needs of the trees, but not so much that water is wasted by deep percolation or runoff. Therefore, the grower must determine how much water the trees actually need. Water requirements of trees can be calculated from evapotranspiration (ET) data (Table 1 ) and tree spacing using the following equation:A = ET × L × W × 0.622
Where:
A - Amount of water required (gal/tree/day)
ET - Evapotranspiration from Table 1 (in/day)
L - Spacing between trees within the row (ft)
W - Spacing between rows (ft)
For example, one can calculate the daily water requirement in April for mature lychee trees planted at spacing of 20 ft by 25 ft as follows. ET is 0.19 in/day in April. Therefore,
A = 0.19 in/day × 20 ft × 25 ft × 0.622 = 59 gal/tree/day.
One also can calculate water requirement of young trees with 50% canopy coverage as follows:
59 gal/tree/day × 0.50 = 30 gal/tree/day.
How much available water does your soil hold?
Available water capacity (AW) is the amount of soil water, which can be extracted by the trees. In general, sandy or gravelly soils have very low available water capacities (Table 2 ). Irrigation should bring the soil moisture only up to the available water capacity.In general, the amount of water, which your soil holds depends on the available water capacity of your soil and the depth of tree's root zone. For example, mature lychee trees are planted at spacings of 20 ft by 25 ft in an Oldsmar fine sand soil in Palm Beach County, and the tree root zone is 2 ft. The irrigation wetting area has a diameter of 16 feet: The amount of water which can be held by soil is 2 ft × 0.75 in/ft = 1.5 in or this amount times the irrigation wetting area = 188 gallons per tree.
For tropical fruit trees planted in trenches, soil volume is an important factor for calculating the amount of water held by soil. Here is an example involving mature lychee trees planted at spacing of 20 ft by 25 ft at the intersections of trenches (6 feet wide and 16 inches deep) in a Krome very gravelly loam soil in Miami-Dade County.
Total soil volume (SV) = (D × W × H) + [(D-W) × W × H] = 208 ft3
Where:
D - Diameter of wetting area (ft)
W - width of trenches (ft)
H - depth of trenches (ft)
The amount of water hold by soil (AWS) can be calculated as:
AWS = AW × SV × 0.622 = 1.2 in/ft × 208 ft3 × 0.622 = 155 gal/tree.
Where:
AWS - Amount of water held by the soil (gal/tree)
AW - Available water capacity (in/ft)
SV - Total soil volume (ft3)
If you turn on the irrigation at 50% soil moisture depletion (approximately 15 cbar), the amount of water your soil can hold is 188 gal/tree × 50% = 94 gal/tree in the first example and 155 gal/tree × 50% = 78 gal/tree in the second.
Irrigation Scheduling
The optimum interval between irrigation events can be calculated based on the amount of water needed by the tree and amount of water your soil can hold. For example, calculate the irrigation interval the young lychee grove in Krome very gravelly loam soil in Miami-Dade County (30gal/tree/day water requirement):Total number of irrigations needed per month (TT) = A /AWS = 900 gal/tree/month /78 gal/tree = 11 times/month or every 3rd day.
Where:
TT - total irrigations needed per month
A - Amount of water required ( 30 gal/tree/day = 900 gal/tree/month),
AWS - Amount of water held by the soil (gal/tree)
If the delivery rate (DR) of a sprinkler is 30 gal/hr, then the run time (RT) for each irrigation event will be:
RT = A/ DR /TT = 900 gal/tree/month /30 gal/hr / 11 times/month = 2.7 hr
Where:
RT - Run time of the pump (hr)
A - Amount of water required (gal/tree/month)
DR - Delivery rate of the sprinkler (gal/hr)
TT - total number of irrigations per month.
Therefore, optimum schedule for irrigating a young lychee grove is one irrigation every 3rd day for 2.7 hr each time.
Natural Resources Conservation Service, USDA in Homestead, Fl has developed a computer program to calculate when and how much a tropical fruit grove should be irrigated based on the calculations given in this document. You may contact NRCS for a copy of the program at 305-242-1197.
Tables
Table 1. Estimated average daily evapotranspiration (ET) for south Florida.
Month Inches/day January 0.10 February 0.13 March 0.16 April 0.19 May 0.19 June 0.18 July 0.18 August 0.17 September 0.15 October 0.14 November 0.12 December 0.10 Table 2. Available water capacities (Inches of water per foot of soil depth) for various soil types.
Soil Range (in/ft)
Average (in/ft)
Gravelly loam 1.0-1.4 1.2 Marl 1.2-2.4 1.8 Peats and mucks 2.0-3.0 2.5 Sand or fine sand 0.4-1.0 0.75
Footnotes
1. This document is Fact Sheet TR001, Florida Cooperative Service, Institute of Food and Agricultural Science, University of Florida. Publication date: May 2000. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.2. Y.C. Li, Assistant Professor, Tropical Research and Education Center, Homestead, FL. Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
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