Citrus Grove Leaf Tissue and Soil Testing: Sampling, Analysis, and Interpretation
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Citrus Grove Leaf Tissue and Soil Testing: Sampling, Analysis, and Interpretation

   

Citrus Grove Leaf Tissue and Soil Testing: Sampling, Analysis, and Interpretation1

T.A. Obreza, A.K. Alva, E.A. Hanlon, and R.E. Rouse 2

Introduction

Fertilizer use efficiency in Florida citrus groves can be enhanced by "program fertilization," where annual fertilizer applications are scheduled after considering a number of grove characteristics. The information necessary to formulate an efficient fertilization program for a particular grove includes tree age, past production, fertilization history, and diagnostic information. This fact sheet details the value of grove nutritional diagnostic information in determining fertilizer programs that increase fertilizer efficiency while maintaining maximum yield and desirable fruit quality.

Citrus tree nutritional status can be determined using leaf tissue and soil sampling and analysis. There are two types of sampling: Predictive sampling evaluates the effectiveness of the fertilizer program for the current year, and the analytical data are used to adjust the program for the following year. Diagnostic sampling is used if a nutritional problem is suspected. Diagnostic samples detect symptomless detrimental conditions in the tree or confirms the nature of visible symptoms. This fact sheet describes the procedures used to obtain maximum benefit from predictive sampling.

Usefulness of Leaf-tissue and Soil Testing

Leaf tissue testing is useful to evaluate tree nutritional status with respect to most nutrients, but is particularly effective for 1) macronutrients, primarily nitrogen (N) and potassium (K), that readily move with soil water, and 2) the micronutrients copper (Cu), manganese (Mn), zinc (Zn), and iron (Fe). Leaf tissue analysis is a much better indicator of the effectiveness of soil-applied fertilizer for these elements than soil analysis. In addition, if particular elements have not been applied as fertilizer, leaf tissue analysis indicates the availability of those nutrients in the soil. An annual leaf tissue sampling program can establish trends in tree nutrition resulting from fertilizer practices carried for several years.

Both leaf tissue and soil testing can be valuable, but leaf analysis provides more useful information about citrus nutrition than soil analysis. With the results of a soil test, one tries to predict how much of a particular nutrient will be available to plants in the future. Obviously, the further into the future that the prediction is made, the less accurate it will be. Predictive soil testing works best with 1) short term crops, and 2) nutrients which are not very mobile in the soil. Thus, for long-term crops such as citrus, predictive sampling should be used for only those nutrients which have slight mobility in most soils, including phosphorus (P), calcium (Ca), and magnesium (Mg). Soil testing has limited value for the more mobile nutrients such as N and K.

The single most useful parameter measured in a soil testing program is the soil pH, because pH significantly affects micronutrient availability, P availability, and toxicity of metals such as Cu, Mn, and aluminum (Al). Soil pH can change rapidly in the poorly-buffered sandy soils of Florida. The soil pH will increase due to liming or the application of irrigation water high in bicarbonates. Soil pH will decrease with the application of acid fertilizers or soil acidulents such as elemental sulfur.

Leaf-tissue Sampling Programs

The benefits of leaf tissue sampling are fully realized by establishing an annual sampling program. In this way, trends in tree nutrition over several years may be noted.

The grove should be sampled to minimize soil and tree type variability. The sampling scheme is the one area of the nutritional testing process controlled by the individual taking the sample. Thus, the manager needs to ensure that the leaf sample is representative of a particular area. For sampling purposes, the grove should be partitioned into management units of not more than 20 acres. Each unit should contain similar soil series and scion/rootstock types. For small groves, the entire grove may be partitioned into these units, and a sample taken from each.

For large groves, where sampling the entire grove is unfeasible, indicator blocks may be used. An indicator block is a designated zone within a uniform span of grove from which the sample is taken (e.g. a 20-acre block within a uniform 100-acre span of grove). Aerial photos are useful for the selection of these blocks. The sample results obtained from the indicator block are assumed to represent the entire span, and management decisions made from the sample data are applied to the entire span. The same block should be sampled repeatedly in succeeding years.

A more elaborate approach to citrus leaf tissue sampling involves the use of global positioning satellites (GPS) and a geographic information system (GIS). Groves are sampled in a regular, grid-like pattern, and the geographic position of each sample is recorded using GPS technology. After the samples are analyzed, the results are processed with GIS and contour maps are made. The grower uses the maps to determine the spatial variation of tree nutritional status, and areas of high or low nutrition can be identified. This method is more expensive than the traditional sampling described above, but may provide a higher level of information that can improve management decisions.

The standard leaf sample consists of at least 100 four- to six-month-old spring flush leaves taken from non- fruiting twigs. If the majority of the spring flush occurs in March, the best time to sample leaves would be July through September. About 15 to 20 trees should be sampled within each management unit. The time of year for leaf and soil sampling coincides and can be accomplished during the same trip through the grove. It is convenient to remove leaves from the same trees under which soil samples are taken.

Leaves at the edge of grove blocks may be coated with soil particles or dust. They should be avoided when sampling because surface contamination can lead to measurement errors for several nutrients, particularly Fe. Unless the leaves are washed, the analysis will measure what is on the leaves as well as what is in them. For meaningful Fe analysis, leaves need to be hand-washed with detergent and rinsed with deionized water.

A small amount of spray residue left on leaves can cause a large error in micronutrient analysis because of the very low concentrations of these nutrients that exist within leaves. Foliage sprayed with water-soluble Mn, Zn, or Fe compounds can provide accurate results only if leaves are thoroughly rinsed with deionized water prior to analysis. Carry-over of foliar spray residue depends on the time interval between the spray and the sampling; the shorter the interval, the more important leaf rinsing becomes. Copper compounds are more resistant to rinse-off, so leaves need to be hand washed with detergent and rinsed with deionized water for accurate Cu analysis.

In a normal citrus nutritional evaluation, leaf analysis should be used to determine the need for a micronutrient application, not to evaluate the effectiveness of a foliar spray that was applied earlier in the year. Foliar spray effectiveness can be determined through visual inspection of the sprayed leaves for micronutrient deficiency symptoms.

Analytical Procedures for Leaf Samples

If samples require hand-washing (necessary for accurate Fe determination), it is best done when the leaves are still in a fresh condition. Laboratories do not normally hand-wash leaves, so washing should be done by the person taking the sample at the time he/she takes the sample. When the sample arrives at the laboratory, the following steps are typically taken: 1) the leaves are dried and finely-ground; 2) a known weight of tissue is either digested in acid (for N analysis) or ashed in a furnace (for all other elements); 3) the concentration of elements in the digest or ash are measured; 4) nutrient concentrations are expressed as either percentage or parts per million (ppm) in the tissue. Procedures for plant tissue analysis usually do not vary among laboratories because the entire amount of each nutrient in the leaves is measured. Thus, results from different laboratories can be directly compared.

Leaf-analysis Interpretation

Well-defined categories of classification for citrus leaf tissue analysis values from mature, bearing trees exist from years of experimentation in Florida and California. The categories are "deficient," "low," "optimum," "high," and "excess." Leaf analysis standards are shown in Table 1. Remember that this classification applies only to the standard age leaf sample taken from mature trees as described above. The categories are not valid for young, nonbearing trees. Maintenance of leaf sample elemental concentrations in the "optimum" range is desirable. Those consistently above this range indicate possible over-fertilization. Analysis values can be interpreted by the grower and fertilization rates adjusted in the appropriate direction, such that future leaf values reach the "optimum" range.

Soil-sampling Programs

As with leaf sampling, the benefits of soil sampling are fully realized if samples are taken annually from the same production units (or indicator blocks), because trends in soil pH or extractable nutrients can be established.

The traditional soil sampling technique is as follows:

One 6-inch deep soil core is removed from the dripline (within the herbicide band) of 15 to 20 "average" trees scattered throughout the block. The cores should be composited into the same bag and air-dried before being sent for analysis. Samples should be taken in the latter part of the summer rainy season (July-September), before fall fertilization.

This general sampling technique should be modified under certain circumstances. For micro-irrigated groves, the sample should be taken from within the wetted pattern of the sprinkler or drip emitter. This is the zone of highest root concentration and is most subject to changes in pH due to alkaline irrigation water or acidic liquid fertilizers. To provide a better understanding of the soil chemical changes occurring inside the wetted pattern relative to the unirrigated soil, a separate sample should occasionally be taken from outside the pattern for comparison.

Young flatwoods groves present a situation where deeper sampling than normal can be beneficial. Construction of grove beds usually brings subsurface soil to the top of the bed, burying the original soil profile. The new soil surface layer, which may be 4 to 8 inches thick, will often be chemically different (particularly in pH and organic matter content) from the original soil surface. Deeper sampling, with segregation of soil layers, can quantify these differences and provides a characterization of the new soil profile.

As with leaf sampling, soil sampling can also take advantage of GPS/GIS technology. Samples can be taken in a grid pattern throughout a grove to create a map that describes the spatial variability of the analytical results. Yield maps or grove aerial photographs showing localized "excellent," "average," and "poor" production areas can be overlayed on the soil maps to determine if various soil factors can be correlated with tree growth or fruit production.

Analytical Procedure for Soil Samples

Once the soil sample has been sent to the laboratory, the following steps are usually taken: 1) the soil sample is dried; 2) the dry soil is passed through a screen to remove roots, stones, etc.; 3) a known weight of soil is shaken with a chemical extracting solution for a specified time; 4) the concentrations of extracted P, K, Ca, and Mg in the solution are determined (micronutrients are sometimes also included); 5) nutrient concentrations are usually expressed as ppm in the soil.

An important component of the analytical procedure that the laboratory client should know is the extracting solution used. Extractants vary in strength, and not all laboratories use the same type. The amount of nutrients extracted from a soil sample by one laboratory will not be comparable with those of another if the extracting solutions differ.

Common extractants used by laboratories in the southeastern U. S. include: 1) the Mehlich-1 or double- acid solution (used for P, K, Ca, and Mg); 2) neutral ammonium acetate (used for K, Ca, and Mg); 3) Bray P1 and P2 solutions (used for P); and 4) sodium bicarbonate (used for P). The IFAS Extension Soil Testing Laboratory (ESTL) in Gainesville uses the Mehlich-1 extractant.

Extractants remove a portion of the "plant-available" nutrient forms from the soil. It is not critical that different extracting solutions remove different amounts of nutrient elements, as long as the amounts extracted by each solution are correlated to a crop response. This is termed the soil-test calibration process, discussed below. For most citrus grove soils in Florida, the Mehlich-1 and neutral ammonium acetate solutions extract about the same amount of K. Mehlich-1 extracts about 1 to 2 times as much Ca and Mg as neutral ammonium acetate. For P, the Bray P1 solution extracts significantly more than Mehlich-1.

Soil-test Interpretation

A soil test interpretation verbally explains the relative meaning of soil test values. Interpretation uses the categories "very low," "low," "medium," "high," and "very high" to relate to various levels of an extracted nutrient. However, soil test results have no meaning unless they are calibrated with crop response. Each laboratory must have a calibration-based interpretation of its soil test results for each extractant that it uses.

The category "very low" indicates that the soil can supply little of the Crop Nutrient Requirement (CNR), thus most of the nutrient must come from applied fertilizer. The categories "low" and "medium" mean that proportionally more of the CNR can be supplied from the soil, resulting in reduced need for fertilization. When a soil tests "high" or "very high," all of the CNR can be satisfied from the soil alone and no fertilization with that nutrient is required.

While soil extractants (particularly Mehlich-1) have been calibrated for many agronomic and vegetable crops in Florida, they have not been extensively calibrated for citrus. However, there are sufficient data correlating tree performance with soil-test P values to be useful in formulating fertilizer programs. The minimum adequate P soil test levels for citrus groves on acidic sandy soils are 30 ppm P for the Mehlich-1 extractant, 40 ppm P for the Bray P1 extractant, and 65 ppm P for the Bray P2 extractant. This means that above these levels, no response is expected from added fertilizer. P can accumulate in the soil as a result of fertilization over a number of years, therefore testing soils for P is important because of the likelihood that P fertilization can be reduced or eliminated as a grove matures.

Experimental data are not yet adequate to indicate the optimum extractable Ca and Mg levels for citrus. If liming is needed, Mehlich-1 extractable Ca is used to determine the type of limestone to be recommended. Liming the soil to a pH of approximately 6.5 insures a more-than-adequate concentration of Ca for tree growth.

For Ca and Mg, the interpretations used in the IFAS Standard Fertilization Recommendation System for agronomic and vegetable crops may be used as a guide. The minimum adequate levels of Mehlich-1 extractable Ca and Mg in this system are 250 ppm Ca and 30 ppm Mg. An extractable Ca value of >2000 ppm usually means that free calcium carbonate is present, indicating a high probability of an alkaline soil. Ca and Mg can build up through the application of calcitic or dolomitic limestone, or irrigation with water from a limestone aquifer such as the Floridan.

Soil and Leaf-tissue Sampling Checklist

Soil and leaf analysis helps formulate fertilization programs or diagnose nutritional deficiencies. Use this checklist as a guide for starting a soil and leaf tissue testing program:

  1. Sampling programs are most effective if done annually.

  2. Use leaf tissue testing for all nutrients, especially the mobile soil nutrients (N and K) and micronutrients (Cu, Fe, Mn, and Zn).

  3. Use soil testing for pH and immobile soil nutrients (P, Ca, and Mg).

  4. Use the standard sampling procedures for soil and leaves described in this fact sheet.

  5. Be aware that spray residues or dust on leaf surfaces affect sample results; wash leaves for accurate Fe analysis, and avoid leaves with spray residues.

  6. Be aware that a number of soil extracting solutions exist, and they can differ in their ability to extract plant nutrients, especially P.

  7. Test interpretations should be used to make fertilization or liming decisions. Wise use of the analytical information allows optimal citrus production and minimizes the fertilizer pollution of the environment.

Further Reading

Tables

Table 1. Leaf analysis standards for mature, bearing citrus trees based on 4 to 6-month-old spring-cycle leaves from nonfruiting terminals.

Element


 Deficient


 Low


 Optimum


 High


 Excess
N (%)
<2.2
2.2-2.4
2.5-2.7
2.8-3.0
>3.0
P (%)
<0.09
0.09-0.11
0.12-0.16
0.17-0.30
>.30
K (%)
<0.7
0.7-1.1
1.2-1.7
1.8-2.4
>2.4
Ca (%)
<1.5
1.5-2.9
3.0-4.9
5.0-7.0
>7.0
Mg (%)
<0.20
0.20-0.29
0.30-0.49
0.50-0.70
>0.70
Cl (%)
?
?
0.05-0.10
0.11-0.25
>.25
Na (%)
-
-
-
0.15-0.25
>.25
Mn (ppm)
<17
18-24
25-100
101-300
>300
Zn (ppm)
<17
18-24
25-100
101-300
>300
Cu (ppm)
<3
3-4
5-16
17-20
>20
Fe (ppm)
<35
35-59
60-120
121-200
>200
B (ppm)
<20
20-35
36-100
101--200
>200
Mo (ppm)
<0.05
0.06-0.09
0.10-1.0
2.0-5.0
>5.0

Footnotes

1. This document is SL-115, a fact sheet of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: June 1992; revised: October 1999. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.

2. T.A. Obreza, associate professor, E.A. Hanlon, center director and professor, and R.E. Rouse, associate professor, Southwest Florida Research and Education Center, Immokalee; and A.K. Alva, associate professor, Citrus Research and Education Center, Lake Alfred; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. It is not a guarantee or warranty of the products named, and does not signify that they are approved to the exclusion of others of suitable composition.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

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.


Copyright Information

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