published in


March 2014

Soil and Plant Tissue Testing

by Maria L. Silveira

Ona Report - Dr. Maria Silveira

For questions or comments regarding this publication contact: Dr. Maria Silveira, University of Florida, IFAS

Soil testing remains the best tool for monitoring soil fertility levels and providing baseline information for cost-effective fertilization programs that meet forage nutrient requirements and soil fertility status. Routine soil testing can identify nutrient deficiencies and inadequate soil pH conditions that may negatively affect forage production. It can also indicate which nutrients are present at adequate levels in the soil, providing an opportunity to eliminate unnecessary soil amendment application.

A major limitation associated with soil testing is the fact that it typically accounts for the plant available nutrient pool present in the surface (4 to 6 inches) soil layer. However, the subsoil can be an important source of water and nutrients, particularly in perennial crop systems. In addition, some nutrients are highly mobile in the soil and can easily leach into subsoil resulting in nutrient accumulation in deep soil depths. Unlike soil testing, plant analysis can integrate the nutrient pools present at the various soil depths. Because of extensive root system, plant analysis is believed to better assess the overall nutrient status of a perennial forage system while revealing imbalances among nutrients that may affect crop production.

Purpose of Tissue Testing

Plant analysis involves the determination of nutrient concentrations from a particular part or portion of a crop, at a specific time and/or stage of development. The basic principle involved in plant analysis interpretation is that yield will be limited at a critical nutrient concentration for each specific crop. The critical level, defined as the nutrient concentration range in the plant sample below which crop yield is significantly reduced, varies among forage crops. Since various factors can influence crop tissue concentrations, tissue testing should be used with caution and in conjunction with a routine soil testing program. Recent efforts in Florida have shown that when plant tissue analysis was used in combination with soil test, it was a better predictor of P and K availability to the plants (Silveira et al., 2011). Plant tissue analysis is currently being used in Florida to guide P fertilization of established bahiagrass (Paspalum notatum L. Fluegge) pastures. In Louisiana, Mondart et al. (1974) suggested that 90% of maximum bermudagrass [Cynodon dactylon (L.) Pers.] yields were obtained when average tissue P concentration was 2.0 g kg-1. A critical value of 2.6 g kg-1 P has been estimated as the critical limit for dallisgrass (Paspalum dilatatum Poir) (Kelling and Matocha, 1990). When used in conjunction with soil testing, tissue analysis has the potential to be a useful diagnostic tool for developing nutrient management programs that predict when crops need additional nutrients while avoiding unintended impacts of excess fertilization on the environment.

Best Time to Test Soil and Plant Tissue

Although soil can be tested at any time, testing at the same time each year is recommended. Sampling early to mid Fall (mid-October to November –December) is the ideal time to take samples because it provides ample time for lime to be applied (if needed) and achieve effectiveness.  If soil pH needs to be adjusted, 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. Also, avoid taking soil samples when the soil is saturated with water.

Plant tissue samples should be collected at the same time and vicinity as soil samples are taken. The plant part, maturity stage and time of sampling are important factors that can affect plant nutrient composition. Tissue samples should be collected when the plant is actively growing so careful planning is the key.  

Soil and Plant Tissue Sample Collection

Soil and plant tissue 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 and plant tissue 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. For soil testing, 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.

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 aspect is to be consistent and collect every sample as close as possible to the same depth. For each area or field sampled, place all the subsamples (15-20) 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. It is recommended that a routine soil test (pH, lime requirement, and available plant nutrients) be conducted at least every three years. The frequency of soil sampling will depend on several factors including soil type, nitrogen application rate, nitrogen fertilizer source, and forage utilization (grazing versus haying).

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. If it is not possible to mail the tissue samples immediately to the lab, then place them in the refrigerator until ready for shipping. 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.

Sample Submission and Results Interpretation

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.

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. 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/IFAS 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.

The soil and tissue 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:  low, medium, or 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.

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

  Part per million (ppm)
Phosphorus (P) < or equel to 25 26 - 40 > 41
Potassium (K) < or equel to 25 26 - 40 > 41
Magnesium (Mg) < or equel to 10 11 - 23 > 24

Current tissue testing interpretations are only valid for established bahiagrass (Table 2); thus, if the area is managed for other purposes such as hay, sod, or seed production, a different interpretation approach should be used. Tissue analysis has been recently incorporated into the revised IFAS fertilizer recommendations as a management tool to guide proper P fertilization in established bahiagrass pastures. According to the revised IFAS recommendations, tissue analysis should be performed when soil tests low in P (less than 25 ppm of Mehlich-3 extractable P). Assuming the soil pH is within the optimal range for bahiagrass (around 5.5) and the tissue P concentration is below the critical concentration of 0.15%, P fertilization is expected to improve bahiagrass production. Recommended P application rates vary from 25 lb P2O5/A for the low- and medium-N input options (50 and 100 lb N/A, respectively), up to 40 lb P2O5/A for the high-N option (160 lb N/A).

Table 2.Critical concentrations of N, P, and K in bahiagrass tissue (Mackowiak et al., 2013).

ElementCritical Concentration (%)
N < 1.5
P < 0.15
K < 1.2

Once soil tests and/or plant tissue analyses have been conducted, management decisions must then be implemented to insure efficient and effective fertilization strategies for the required forage production goals.  The target or goals of production vary according to numerous factors such as exclusive hay production, hay plus stocking, exclusive stocking by ruminants, desired stocking rate, cow-calf and/or stocker production, etc.  The choice and selection of fertilizer sources and rates and timing of application are governed by availability and cost of product.  The fertilization strategies are therefore driven by production for a targeted dry matter response and by the need to sustain the pasture system. If you need further assistance with interpretation of soil test results or fertilization recommendations, consult with your local county agent or other university personnel.


Kelling, K. A. and Matocha, J. E. 1990. Plant analysis as an aid in fertilizing forage crops. Pages 603-643 in: Soil testing and plant analysis. R. L. Westerman, ed. Soil Science of America, Madison, WI.

Mackowiak, C.L. Blount, A.R., Hanlon, E.A., Silveira, M.L. and Myer, R.O. 2013. Getting the most out of bahiagrass fertilization. Online. Florida Cooperative Extension Service, IFAS, University of Florida. SL 249. http://edis.ifas.ufl.edu/ss469.

Mondart, C. L. Jr., Harris, H. E., Brupbacher, R. H. Jr., and Sedberry, J. E. 1974. Influence of annual applications of phosphorus on the yield and chemical composition of common bermudagrass and on the chemical composition of a Bowie soil. Louisiana Agric. Exp. Sta. Bull. No. 684.

Mylavarapu, R. S., Wright, D., Kidder, G. and Chambliss, C. G. 2013. UF/IFAS Standardized fertilization recommendations for agronomic crops. Online. Florida Cooperative Extension Service, IFAS, University of Florida. SL 129. http://edis.ifas.ufl.edu/ss163.

Silveira, M. L., Obour, A. K., Vendramini, J. M. B. and Sollenberger, L. E. 2011. Using tissue analysis as a tool to predict bahiagrass phosphorus fertilization requirement. J. Plant Nutr. 34:2193-2205.