Basics of Soil and Plant Tissue Testing

Dr. Maria L. Silveira, Soil Scientist, UF/IFAS Range Cattle Research & Education Center 

Adequate soil fertility is one key to successful forage and livestock production in Florida. However, most soils in Florida are deficient to some degree in more than a single essential plant nutrient. All required nutrients must be supplied in adequate amounts to maximize the benefits of nutrient application. Soil testing remains the best management tool for monitoring soil fertility levels. Routine soil tests can identify nutrient deficiencies and inadequate soil pH conditions that may negatively affect forage production. Soil test results also indicate which nutrients are present at adequate levels in the soil, providing an opportunity to eliminate some soil amendments. In addition to the cost savings associated with applying only the required fertilizers, losses and associated environmental problems can be minimized. Based on soil test results, cost-effective fertilization programs can be developed to meet forage nutrient requirements and minimize production costs. Soil tests also provide important information relative to soil pH and recommendations for lime application.

When soil test should be done?

Soil can be tested at any time of the year, however mid-October to November -December is the ideal time to take samples. Sampling early to mid fall provides ample time for lime to be applied and achieve effectiveness. It is important to apply lime at least 3 to 4 months prior to the spring fertilization to allow time for the material to react in the soil. In recently fertilized hay fields, delay sampling at least four to six weeks. Avoid taking soil samples when the soil is saturated with water. Soil should be tested at least every 3 years.

How to collect the samples?

Soil testing is a vital component of soil fertility programs for forage crops; however, the results and interpretation of a soil test are only applicable if the soil samples are properly collected. The test results are only as good as the sample taken. It is very important to submit a soil sample to the laboratory that truly represents the area of interest so that a reliable test and recommendations can be made for the entire area. This can be accomplished by submitting a composite sample. A minimum of 15 to 20 subsamples (0 to 6 inches in depth) 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. Collecting a good, representative soil sample is well worth the time and effort it requires. Soil samples can be taken using a soil probe or a shovel. The most important point is to be consistent and collect every sample as close as possible to the same depth. Place all the subsamples (15-20) for each area in a clean plastic bucket and mix thoroughly. A hand full (~1 pint) of soil should be sent to a reputable laboratory for analysis. If multiple samples are sent to the lab, pack them in sturdy containers to avoid cross-contamination among the samples.

Submitting the samples to the lab

Make sure you fill out the forms and label boxes before sending the samples to the laboratory. Samples should be submitted to the lab as soon as possible after collection. Never leave the sample in your truck or other warm places. When submitting samples to the UF/IFAS Extension Soil Test Lab, it is important to indicate the forage crop to be grown so the fertilizer recommendations will be given. If the samples are submitted to a commercial lab, contact the lab to make sure that the available tests are compatible with the UF/IFAS recommendations. There are many different soil testing procedures and extracting solutions available. Different soil test procedures generate different results and, consequently, fertilizer recommendations. In Florida the standard soil test procedure is called Mehlich-1 (or double acid). Research has shown that Mehlich-1 is best suited for Florida soils, therefore all UF/IFAS recommendations are based on this procedure. 

Tests offered

Soil test generally includes the determination of pH, phosphorus, potassium, calcium, and magnesium. 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. Lime, phosphorus, and potassium application rates are based on soil test results. The only exception is nitrogen fertilization, which is not based on soil test results. Nitrogen fertilization is based on crop management and expected yields. It is recommended that a routine soil test (pH, lime requirement, and available plant nutrients) be conducted at least every three years. 

Soil Test Interpretation

The soil test report indicates 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 For instance, if the soil test indicates that potassium levels are high, that means that crops will not respond to additional potassium fertilization. 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: very low, low, medium, high, or very high. Table 1 represents the current interpretation of soil test results for agronomic crops in Florida. 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. If you need further assistance with interpretation of soil test results or fertilization recommendations, consult with your local county agent or other university personnel. 

Plant Tissue Testing

Plant tissue analysis has been recently incorporated into the revised UF/IFAS fertilizer recommendations as a management tool to guide proper P fertilization in established bahiagrass pastures. We strongly recommend that both plant tissue and soil samples be collected and submitted to the laboratory at the same time. When used in combination with soil test, tissue analysis can better predict when bahiagrass pastures need additional P. Similar to soil testing, tissue samples must be representative of the field. The number of plants to sample in a specific area will depend on the general conditions of plant vigor, soil heterogeneity, and forage management. A truly representative sample can be obtained by sampling a large number of plants so that the sample represents the entire field. Collect at least 1 ounce (30 g) of fresh material. Sampling is not recommended when plants are injured by insects and diseases. To avoid contamination, plants should not be sampled soon after spraying pesticides or herbicides. Care should be taken to minimize soil contamination on the sampled plant material. In addition, plants should not be sampled under temperature or moisture stress. Ideally, samples should be collected during a time of the day when climatic conditions are mild, generally early to mid-morning or early evening. The plant part, maturity stage and time of sampling are also important factors that can affect plant nutrient composition. Forage grasses and hay fields should be sampled prior to seed head emergence or at the optimum stage for forage utilization. As the plant matures, nutrient concentrations decline, so it is critical that plants are sampled at the proper stage of maturity. Care should be taken to select the plant part that accurately reflects the nutrient status of the plant. The top portion of the plant (similar as the cattle would graze) should be sampled. Do not sample seeds as they are not useful for assessing nutrient status of forage crops and may introduce large errors in the report interpretation. If deficiency symptoms are suspected, plants showing these symptoms should be sampled and analyzed separately from “normal” or healthy appearing plants. After sampling, tissue should be placed in properly labeled paper bags and sent immediately to a reputable laboratory for analysis. Avoid plastic bags because they can hold heat and moisture. The same precautions taken for collecting the plant material should be followed for handling the samples. Because fresh plant material may start decomposing shortly after collection, it is important that plant material be sent to the laboratory as quickly as possible. Prior to transporting the samples to the laboratory, plant material should be stored in a refrigerator at 41oF (or 5oC). For more information on bahiagrass tissue sampling and interpretation, please visit http://edis.ifas.ufl.edu/SS475 or contact your local livestock county agent or other university personnel.