How to Interpret Yout Soil Test Results

October, 2017
 Dr. Maria L. Silveira, UF/IFAS RCREC, Ona

Soil testing is the best tool for monitoring soil fertility levels. However, interpretation of soil test results can be somewhat confusing. This document provides a brief explanation of the general guidelines for interpreting soil test results.

Field Sampling

The first step in ensuring reliable soil test results is proper soil sample collection. Soil results and interpretation are only reliable if the samples are collected properly. In other words, the test results are only as good as the sample taken. It is very important to submit soil samples to the laboratory that truly represent the area of interest so that test results are reliable and fertilizer recommendations can be made for the entire area. This can be accomplished by submitting a composite sample. A minimum of 15 to 20 subsamples (approximately 6 inches deep) should be collected from each field. Samples should be taken at random in a zigzag pattern over the entire area. Areas that are managed or cropped differently should be sampled separately. Similarly, areas that show clear problem signs (i.e., poor forage production, disease) should also be sampled and analyzed separately. Avoid sampling areas not typical of the total field such as near water, feed, or shade.

 

Soil testing laboratories offer a wide range of parameters. In general, most soil test reports include the determination of pH, buffer pH, phosphorus, potassium, calcium, and magnesium concentrations. Micronutrients (e.g., zinc, copper, iron, and manganese), organic matter, and physical properties (e.g., percentage of sand, silt, and clay) can also be determined. Since laboratories use different test methods, it is very important that the soil testing laboratory selected to conduct the soil test uses the procedures recommended by University of Florida. In Florida, Mehlich-3 is currently the standard soil test procedure to determine the concentration of plant available nutrients. Lime recommendations are based on Adams-Evans procedure. Using a testing procedure not applicable to Florida soils can result in misleading soil testing interpretation and recommendations.

 

Although nutrient concentrations in the soil are not expected to vary considerably from year to year, tracking soil test results over time can provide useful information relative to the effect of liming, fertilization, or past management decisions on the fertility status of the soil. Monitoring changes in pH and level of a particular essential nutrient can also help farmers to determine soil fertility approaches that sustain adequate crop production.

 

Soil pH

Soil pH is the most commonly measured soil property. It describes the relative acidity or alkalinity of the soil and represents one of the most useful and informative soil parameter because its relationship to many aspects of soil fertility and plant growth.

 

The pH scale ranges from 1 to 14, however most inorganic soils in Florida have a pH around 4 to 6. The lower the pH values, the greater the acidity in the soil. Soils with pH less than 5 are considered strongly acid, while values of 5 to 5.5 and 5.5 to 6.5 are considered moderately and lightly acid, respectively. Soil pH range of 6.6 to 7.2 is considered neutral, 7.3 to 8.2 is slightly alkaline and above 8.2 is moderately to strongly alkaline. The relatively acidic nature of most Florida soils is due the parent material as well as the environmental conditions (i.e., intensive rainfall) in which these soils were formed. Agricultural practices such as the use of nitrogen fertilizers and crop removal also contributes to increase soil acidity. Since pH is measured using a logarithmic scale, a decrease of 1 unit of pH results in a 10-fold increase in acidity, so small changes in pH values can have important implications.

 

Buffer pH

In Florida, the Adams-Evans test is used to determine the amount of lime required to adjust the soil pH to the target level.The Adams-Evans test measures the soil acidity in water and in a buffer solution (pH of 8) to determine soil lime requirement. The lower the pH of the buffer solution, the greater the amount of lime needed to raise the soil pH to the desirable level. Soils that contain more clay minerals and organic matter typically require greater amounts of lime to raise the pH.

 

Nutrient Concentrations

Routine soil tests only measure only a portion of the total pool of nutrients in the soil. They do not measure total amounts of nutrients in the soil. Rather, most soil testing procedures are designed to mimic the root uptake of nutrients in the soil.

 

One of the major goal of testing the soil is to determine whether or not crops will respond to fertilization. Extensive research has been done to determine the relationship between available nutrients, fertilization application and yield responses. Of greater importance than the actual nutrient concentration, is the classification of the degree of nutrient sufficiency.The degree of nutrient sufficiency is reported as:low, medium, or high. In general, when soil test level is equal or greater than medium, additional fertilization is not expected to result in an anticipated yield response. Table 1 represents the current interpretation of soil test results for agronomic crops in Florida.

 

Table 1. Current Mehlich-3 soil test interpretation for agronomic crops in Florida (Mylavarapu et al., 2013).

Element Low Medium High
  Part per million (ppm)
Phosphorous (P) ≤ 25 26-40 > 41
Potassium (K) ≤ 25 26-40 > 41
Magnesium (Mg) ≤ 10 11-23 > 24

 

Cation Exchange Capaciy

Cation exchange capacity or CEC refers to the capacity of the soil to hold positively charged ions, also known as cations. Some laboratories calculate the CEC based on the summation of cations (potassium, calcium, magnesium, sodium, and buffer pH) or on an actual CEC determination procedure. Soils that contains more clay or organic matter often exhibit higher CEC than coarse-textured soils.

 

Lime and Fertilizer Recommendations

Soil testing results provide basic soil fertility information so management decisions can be implemented to insure efficient and effective fertilization strategies for the required forage production goals. However, caution should be exercised when interpreting fertilizer recommendations generated by commercial laboratories because they typically use different soil fertility approaches. For instance, while University of Florida fertilizer recommendations are based on crop nutrient requirement, fertilizer recommendations generated by commercial labs (particularly out-of-state) may be target at building up nutrients in the soil. However, given the environmental conditions in Florida associated with the coarse-textured soils, most Florida soils have limited ability to retain nutrients; therefore, for economic and environmental reasons, the nutrient “build up” approach is not appropriate for our conditions. In addition to the soil test results, economic issues (e.g., fertilizer cost, hay prices) must also be considered when choosing the most adequate fertilization strategy. Decisions regarding liming and fertilizer sources should be based on cost, availability, soil characteristics, and forage management.

 

From time to time, we come across some issues associated with analytical error or improper soil collection or handling that result in unusual soil test results. It is important to identify these potential limitations. Drastic changes in soil test results or zero values may indicate an unrepresentative soil sample or a laboratory error. When in doubt, the best approach is to collect and submit a new sample or contact the lab to repeat the analysis. If you need further assistance, please consult with your local county agent or other university personnel.

 

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