Introduction and Target Audience
Soil testing is a general term, but it specifically includes laboratory analyses of field or landscape samples, interpretations for crop production and landscape quality, and nutrient management recommendations where appropriate. This testing can analyze samples from soils, plants, water, manures, and recyclable waste materials. In Florida, we have more than 100 years of soil fertility research work experience covering a variety of soils, ecosystems, crops, turfgrasses, nutrient requirements, plant nutrient uptake, and impacts on water quality.
This publication aims to guide readers on the contents and sections of a standard soil test report generated by the UF/IFAS Extension Soil Testing Laboratory (ESTL) in Gainesville, Florida. The target audiences for this publication are county Extension agents, Extension specialists, producers, landscape professionals, crop consultants, researchers, students, homeowners, state and local agencies, and all other Florida residents interested in sustainable natural systems.
Soil Sampling
Soil sampling depth should be the top 6 inches for most agronomic and residential applications or 3 to 4 inches for maintenance of frequently mowed lawn or turf. For deep-rooted trees or shrubs, collecting soil from multiple depths may be necessary. Soils vary from spot to spot, so each sampled area is best represented by a composite sample collected from 8 to 15 locations. Soil collected from all these locations should be mixed well in a clean plastic container. The composite sample should be derived from a dried subsample (1 to 2 cups or approximately 1 pint) of this mixture and sent to the ESTL for analysis. Soil samples need to be free of any aboveground vegetation and coarse roots. One sample represents an area that is uniform in management and topography, typically up to 40 acres of a field section in commercial agriculture. The composite sample should not include anything within that area that is different in feel and look (such as areas with poor plant growth, depressions, wet spots, hard layers, etc.). For areas with different features and characteristics, separate samples should be collected and then submitted to the lab with appropriate identification. Analytical procedures and interpretations employed at the ESTL assume that the submitted samples best represent the soil and conditions of the testing site. Note that the analytical procedures, interpretation of the test results, and fertilizer/lime recommendations apply to the entire area that the small soil sample represents. Therefore, the need to collect and send representative soil to the lab cannot be overemphasized.
Structure of a Standard Soil Fertility Test Report from the ESTL
A standard soil test report from the ESTL comprises five distinct sections:
Section 1. Contacts and Sample
The first section shows contact information, sample IDs provided by whoever collected the sample, and the unique Lab ID assigned by the ESTL to each submitted sample. The local Extension agent should be contacted regarding any questions about the report. Unique lab IDs allow the lab to retrieve sample reports up to three years following the first sample submission date, should any need arise. The lab strongly recommends that whoever collected the sample keep a record of the sample location, ID, and other details they provided on the sample submission form so they can follow the lab report(s) they receive after analyses. Unfortunately, neither the ESTL nor the local Extension agent will be able to help someone who cannot retrieve the details of their own IDs.
Section 2. Soil and Buffer pH and Lime Requirement Test Results
The second section comprises test results starting with soil pH. The pH is determined in water at a 1:2 ratio of soil to water by volume, which is the standard method for Florida soils. This measurement is called water pH or simply soil pH. The soil pH provides an estimate of the relative reaction (acidic or alkaline) of the soil sample and the entire soil area that the sample represents. If the soil pH is different from what is required for the plant species identified on the submission form, then the soil pH should be raised or lowered to the desired value, which is referred to as “target pH” on the report. In that case, the ESTL measures a different type of pH called “Buffer pH” in the same soil sample using the Adams-Evans method (AE Buffer). This method involves mixing a solution (buffered to pH 8.0) with the soil sample. The AE solution neutralizes the reserve acidity in the soil sample, thus making the AE Buffer pH different from the standard water pH. In the field or landscape, the soil’s reserve acidity must be neutralized before raising the soil pH to the target value. The AE Buffer pH method provides an accurate measurement of the reserve acidity and, consequently, an accurate estimate of the lime rate needed to raise the soil pH from the current value to the desired value. The required lime rate is determined by models that were calibrated for Florida soils. Therefore, a soil sample must be submitted to the ESTL to determine the lime requirement. This task should not be attempted at a county Extension office where only water pH is determined. Lime requirement cannot be determined without the water pH, the Buffer pH, and the model estimation.
Section 3. Extractable Nutrient Test Values
The third section provides test values for all extractable nutrients in the sample. All mineral (e.g., sandy) soils with water pH up to 7.4 and all samples from Miami-Dade County with a soil pH of up to 8.2 are extracted using the Mehlich-3 extraction procedure. (See Ask IFAS publication CIR 1248, “UF/IFAS Analytical Services Laboratories [ANSERV Labs]: Analytical Procedures and Training Manual.”) Please note that while the extraction procedure for acidic soils and alkaline calcareous soils in Miami-Dade County is the same, the interpretations for those soils are different; therefore, the recommended fertilizer rates are different. (See Ask IFAS publication SL504, “Nutrient Management Recommendations Based on Mehlich-3 Extractant for Calcareous Soil in Miami-Dade County.”) Alkaline soils (soil pH >7.4) from all other counties are currently being extracted by a procedure called Ammonium-bicarbonate-DTPA (AB-DTPA method); therefore, they are interpreted differently from the above two types of soils.
In this section, both macro- and micronutrient test values are provided in milligrams/kilograms (mg/kg) of soil (equivalent to ppm in this context). Nutrients included in the report are phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), zinc (Zn), and manganese (Mn). The nutrients are interpreted based on the soil pH and/or the geographic location of the sample. An interpretation of a soil test means classifying the results for P, K, and Mg into Low (L), Medium (M), or High (H) categories specifically for agronomic production and plant species performance. Interpretation tables for each of the three categories of soils can be found in Ask IFAS publication CIR 1248. Soil test interpretation is the most important section of the soil test report because it serves as the basis for fertilizer recommendations.
Section 4. Recommendations for Application Rates of Lime, Nitrogen, Phosphorus, Potassium, and Magnesium
The fourth section provides recommended rates of lime (calcitic or dolomitic), nitrogen (N), P, K, and Mg. All recommendations are made in pounds per acre (lb/ac) for commercial and large fields or in pounds per 1000 sq ft for landscapes and residential yards.
Lime application for optimum plant growth is required in areas of Florida, where soil pH is not controlled by underlying limestone (CaCO3) rock. The reaction of lime after incorporation within 6 inches of surface soil is slow and can take up to six months to complete and raise the pH to the desired level. Therefore, it is strongly advised to conduct a soil pH or a standard soil fertility test 8 to 12 weeks ahead of planting. If the recommended amount of lime is too small to logistically justify application to the site, it is advisable to wait for a period of six months or until the beginning of the next season and test again to see if the pH readings warrant a higher lime rate that would be economically feasible. Similarly, if the recommended lime rate is high, the total amount can be split into two to three doses and applied at six-week to three-month intervals.
It is recommended that soil be tested for pH at least once a year, if not every growing season, to ensure that the soil pH remains within the range for the crop/plant/grass species being grown. Soil pH is the most important of all soil chemical properties. It provides insight into growing conditions, nutrient availabilities and deficiencies, and soil fertility and productivity in general.
If the soil pH tests higher than the target pH for the intended crop/plant/grass species, then the buffer pH will not be determined. Careful soil pH management is critical for every plant species because it helps avoid poor nutrient management, including irrigation with high pH water and/or over-liming. When soil pH is higher than the target pH and needs to be lowered, application of elemental sulfur is recommended. Similar to lime applications, elemental sulfur can have slow reaction times in soils. Applications can take up to six months to complete and effectively lower the soil pH to the target value.
While stoichiometric calculations show that >300 lb/ac of elemental sulfur can be applied in certain cases, it is not easy to determine the true amount required. Therefore, soil test reports do not provide recommendations for lowering soil pH, and long-term soil management is important. See details regarding the options for lowering soil pH and reasons for increased soil pH values in Ask IFAS publication SL437, “Lowering Soil pH to Optimize Nutrient Management and Crop Production.”
The soil report also provides a recommendation for N fertilizer application. However, note that ESTL does not conduct analytical tests for mineral N as there is no reliable soil test method for soil N in the humid subtropical and tropical climate of Florida. Nitrogen fertilizer recommendations are therefore based on research that calibrated and validated crop N response, leading to optimum crop growth while also minimizing the effects of nutrients on the environment. Work in this area has been conducted for different crops at different locations without considering the typically low inherent content of soil mineral N.
Recommendations for P, K, Mg, and Ca fertilizer are made based on the soil test calibration relative to soil pH (e.g., acidic vs. alkaline) or the field/landscape location where the fertilizer will be applied. The soil report provides recommendations for P in pentoxide form (P2O5) and K in dioxide (K2O) form. While the soil test for Mg is interpreted between Low and High, any recommended rate of Mg can be applied using dolomite (CaMg[CO3]2), a type of liming rock, or through supplemental application of magnesium sulfate (MgSO4 [Epsom salt]), as appropriate. Florida soils rarely need a Ca recommendation because they often contain enough Ca from limestone rock found in subsurface and, at times, surface soils or from lime added to elevate soil pH. However, some soils test low in bioavailable Ca, yet their pH is high enough for the plant species. Therefore, no lime needs to be applied. In such soils, an application of gypsum (CaSO4·2H2O) provides Ca without increasing soil pH.
This section of the soil report provides only guidance for S, B, Cu, Zn, and Mn applications and management because calibration studies worldwide have not been very successful in developing reliable interpretations. The required amounts are relatively very small. Note that the range between deficiency and toxicity of B, Cu, Zn, and Mn is very narrow; therefore, excess applications of these nutrients should always be avoided.
Section 5. Guidelines for Field Management
The fifth and final section of the report comprises management guidance for specific crops, soils, and nutrients. Beyond right rate, fertilizer management guidance is based on the other three of the 4Rs approach: right source, right timing, and right placement. The 4Rs approach considers plant requirements, plant uptake aspects, water quality, and overall sustainability.
Nutrient recommendations in the report are made for straight (single-nutrient) fertilizers, not blends. Therefore, if blended fertilizers are used, nutrient ratios in the blends should match closely with the recommendations in the report.
Reports and Contacts
Soil test reports are sent out electronically within 2–3 business days after the receipt of the samples at the laboratory. The local Extension agent should be contacted for any additional information on the report and its contents. Depending on the question, the agent may seek input from the state specialists in UF/IFAS academic departments or at regional Research and Extension Centers. The ESTL will be able to assist with sending a duplicate copy of the report only to the email address on file, if necessary.