Optimum growth and yield of high quality fruit cannot be obtained without adequate nutrition. The most successful fertilizer program should be based on tissue analysis, knowledge of soil nutrient status through soil analysis combined with university recommendations. The deficiency or excess of an element will cause disturbance in plant metabolism and lead to poor performance.
Used in conjunction with other data and observations, tissue analysis aids in evaluating the nutrient elements of the soil-plant system. It has proven useful in confirming nutritional deficiencies, toxicities or imbalances, identifying “hidden” toxicities and deficiencies where symptoms are not visible, and evaluating the effectiveness of fertilizer programs.
For reliable results and useful interpretation of lab analysis reports, you need to follow the proper procedures for leaf sampling and sample handling. If you don’t, improperly collected leaf samples will provide misleading information about the nutritional status of the trees and fertilizer programs.
Considerable care is needed in taking samples. Chemical analysis values can only be useful if the samples obtained are representative of the blocks they were taken from. The proper sampling, preparation and handling affect the reliability of the chemical analysis, data interpretation, nutritional recommendations and adjustment of fertilizer programs.
Leaf samples also must be taken at the proper time because nutrient levels within leaves continually change. However, leaf mineral concentrations of most nutrients are relatively stable within four to six months after emergence of the spring flush.
In mature blocks, the best time to collect 4- to 6-month-old spring flush leaves is July and August. If you collect them later in the season, the summer flush could be confused with the spring flush.
Each sample should consist of about 100 leaves taken from non-fruiting twigs of 15 to 20 uniform trees of the same variety and rootstock that are under the same fertilizer program. Use a clean brown paper bag. Complete information sheets from the testing lab for each sample, which helps when interpreting the results. Make sure the sample bag and the information sheet have the same identity so they can be matched by the lab.
Sampling techniques for leaves
Immature leaves should be avoided because of their rapidly changing composition.
Abnormal-appearing trees, trees at the edge of the block and trees at the end of rows should not be sampled because they may be coated with soil particles and dust or have other problems.
Do not include diseased, insect-damaged or dead leaves in a sample. Use good judgment.
Select only one leaf from a shoot and remove it with its petiole (leaf stem).
Diagnosing growth disorders
Collect samples from both affected trees as well as normal trees.
Trees selected for sampling should be at similar stages of development and age.
Whenever possible, confine the sampling area to trees close to each other.
Handling of leaf samples
Samples should be collected in clean paper bags and clearly identified.
They should be protected from heat and kept dry and cool (stored in portable ice chests). Store them in a refrigerator overnight if they cannot be washed and oven-dried the same day of collection.
For macronutrient analysis, leaves typically do not need to be washed.
Leaves should be dried in a ventilated oven at 60 to 70 degrees Celsius.
Preparation for analysis
Leaves that have been recently sprayed with micronutrients for fungicidal (copper) or nutritional (manganese or zinc) purposes should not be analyzed for those micronutrients, since it’s difficult to remove all surface contamination from the leaves.
For accurate iron and boron or other micronutrient determination, samples require hand washing, which is best done when leaves are still fresh.
Soil analysis is an important method for gaining basic information about the chemical status of the soil. It is particularly useful when conducted over several years so you can see trends.
Unlike leaf analysis, laboratories use various methods and analytical procedures to analyze soil. In Florida, soil tests for the relatively mobile and readily leached elements, such as nitrogen and potassium (N and K), are of no value.
Soil tests are mainly important for pH, P, Mg, Ca, and Cu. For Florida’s sandy soils, 40 to 60 pounds per acre (20-30 ppm) of P, 70 to 120 pounds per acre (35-60 ppm) of Mg, 500 to 800 pounds per acre (250-400 ppm) of Ca and 5 to 10 pounds per acre (2.5-5 ppm) of Cu are considered adequate for citrus. These are based on the Mehlich-1 or double acid (hydrochloric acid + sulfuric acid) extraction procedure adopted by the University of Florida analytical lab.
A Ca:Mg ratio of 7:1 is desirable, and ratios higher than 10 may induce magnesium-deficiency problems. Copper levels higher than 50 pounds per acre may be toxic to citrus trees if the soil pH is less than 6.
The accuracy of a fertilizer recommendation depends on how well the soil sample represents the grove. In Florida, if soil samples are collected once a year, the best time is at the end of the summer rainy season and before fall fertiization—usually during September and October. But soil sampling may be conducted at the same time as leaf sampling to save time and reduce cost.
Standard procedures for proper sampling, preparation and analysis have to be followed for meaningful interpretations of the test results and accurate recommendations. Each soil sample should consist of 15 to 20 soil cores taken at the dripline of 15 to 20 trees within the area wetted by the irrigation system to a depth of 6 inches.
The area sampled should be uniform in terms of soil and tree characteristics and correspond to the area from which the leaf sample was taken. Individual cores should be mixed thoroughly in a plastic bucket to form a composite sample.
A subsample of an appropriate size should be taken from the composite mixture and put into labeled paper bags supplied by the lab.
Soil samples should be air-dried but not oven-dried before shipping to the laboratory for analysis.
Dr. Mongi Zekri is a multi-county citrus agent with the University of Florida’s Institute of Food and Agricultural Sciences based in LaBelle. He can be reached at firstname.lastname@example.org or at http://flcitrus.ifas.ufl.edu/Mongi’s%20Webpage/Zekri.htm.