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Interpreting a Soil Test Report

AGF-0514
Date: 
08/08/2022
Greg LaBarge, Professor, Field Specialist, Agronomic Systems, Ohio State University Extension

Soil test report formats vary from laboratory to laboratory. However, while report formats differ, any standard soil test report has the critical information needed to make a nutrient recommendation. This fact sheet helps you identify common soil test terms you will find regardless of the report format. The soil test terms are defined along with a general description of how to use the information they provide in order to make better nutrient management decisions on your farm.A corn field has a stunted, lightly colored green crop on the left because of potassium deficiency and a dark green, tall crop on the right as a result of adequate potassium.

Some soil test report information helps us understand the soil's natural ability to retain and supply nutrients such as cation exchange capacity (CEC) and organic matter (OM). Soil pH tells us about nutrient availability and soil suitability to grow a chosen crop. Phosphorous and potassium soil test levels help us know if fertilizer is needed. Base saturation further describes the relationship of soil cations that affect plant nutrient uptake. All these values reported on a soil test report are useful to develop productive and cost-effective crop nutrient recommendations.

Soil Factors that Impact Plant Nutrients

A soil test report shows three soil test factors that define the soil environment. These factors are inherent to the soils due to parent materials and other soil formation factors. Since these factors are inherent, our ability to change them may be limited. We cannot alter CEC by management. Organic matter (OM) can be changed but increasing OM requires many years. In comparison, soil pH in the root zone is relatively easy to adjust by adding agricultural limestone.

Standard soil test results from most laboratories report soil OM and CEC values. These two numbers help us understand the soil environment. Plus, they define the expected response to fertilizer additions. For example, CEC is used in potassium (K) fertilizer decisions. Recommended K rates for soils with a CEC less than 5 are lower due to the potential for K to leach below the root zone.

Table 1. OM and CEC ranges typically found on a soil test report for mineral soils common in Ohio (Lindsey 2017).
Soil Test Variable Typical Ranges
Organic Matter (OM) 1 to 6 percent
Cation Exchange Capacity (CEC)
Coarse texture soil (sand) 1 to 5 milliequivalents (meq) per 100 g
Medium texture soil (silt) 6 to 20 meq/100 g
Fine texture soil (clay) >21 meq/100 g

Organic matter (OM): OM plays an essential role in nutrient cycling and retention. OM accumulation in uncultivated soils is impacted by moisture and temperature due to their influence on plant growth and soil microbes. Under cultivation, management practices such as tillage, crop rotation, and drainage influence OM retention. Generally, a lower percentage of OM exists where intense annual tillage is performed. Therefore, building soil OM requires a long-term commitment to reduced tillage, cover crops, and other management.

Cation Exchange Capacity (CEC): CEC measures the capacity of the soil to hold exchangeable cations (positively charged ions). We report CEC as milliequivalents (meq) per 100 grams of soil. CEC depends on the amount and type of clay plus the percentage of OM. Exchangeable cations include hydrogen, calcium, magnesium, and potassium. Aluminum and iron influence the CEC of acidic soils (pH less than 6.0). The higher the CEC value, the more cations the soil can hold, reducing the likelihood of cation leaching. Liming acidic soils can temporarily increase CEC measured in soil testing.

Soil pH: Soil pH measures active soil acidity in a 1:1 mixture of soil to water. The active acidity value is shown on the soil test report as soil pH (or water pH). For example, a pH value of 7.0 is neutral. Where pH values are above 7.0, the soil is alkaline. pH values below 7.0 are acidic. Soil pH influences the nutrient solubility changing the amount of nutrients in soil solution. For example, adding lime to correct an acidic soil pH will increase plant-available phosphorus. Desirable pH ranges for general crop production are shown in Table 2.

Buffer pH: Buffer pH is measured by mixing soil with a buffering solution to estimate the reserve or potential acidity. The buffer pH value determines the amount of lime needed to correct the pH to a target pH. The target pH is based on what crop is being grown. Table 2 shows buffer pH ranges where no lime is needed. The factsheet, Soil Acidity and Liming for Agronomic Production, AGF-505-07 (Mullen 2016), explains pH, buffer pH, target pH, and other considerations when developing a lime recommendation.

Soil Nutrients Reported on a Soil Test Report

 A general range for each nutrient reported on a standard soil test report is shown below.

Table 2. Standard soil test result parameters with optimal soil test values for most Ohio crops.
Test Parameter Desirable Ranges Use of Measure
pH 6.3–7.0 Water pH (neutral pH = 7.0) affects nutrient availability.
Buffer pH 6.8– 7.0 Used to determine lime requirement.
  When reported as part per million (ppm) When reported as pounds per acre  
Phosphorous (P)
Mehlich 3
20–50 40–100 Used to make P2O5 recommendation.

Potassium (K)
Mehlich 3

100–170 200–340 Used to make a K2O recommendation. CEC is used to determine the desired range.
Calcium (Ca) 200–8,000 400–16,000 Levels less than 200 ppm are a concern. Ca deficiencies are rare in Ohio.
Magnesium (Mg) 50–1,000 100–2,000 Levels less than 20 ppm are a concern. Dolomitic limestone is a significant source of Mg.

Phosphorus, Potassium, and Other Secondary Nutrient Numbers

Table 2 has general guidelines on desirable soil test ranges for phosphorus, potassium, calcium, and magnesium soil test values. The soil test value represents the plant-available nutrient, not the total amount of the nutrient in the soil. The total amount of each nutrient in the soil is several times higher than the value measured by the soil test.

Land grant universities develop nutrient recommendations using on-farm and experiment station studies. The studies correlate phosphorus and potassium soil test values to a crop’s yield response. In addition, the studies define the amount of fertilizer needed. As the soil test value increases, the need for supplemental fertilizer to support yield in a given year decreases. Complete fertility recommendation guidelines for row crops and alfalfa are in Tri-state Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, Bulletin 974 (Culman 2021).

When reviewing soil test reports, take care to align reported soil test values with nutrient recommendation sources. The units and the test used for each nutrient's reported soil test value are two areas of concern. A meaningful change in reporting P and K values occurred with the Tri-state Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, Bulletin 974 (Culman, 2021) update. The P and K values used for recommendation criteria now use a Mehlich 3 soil test reported in parts per million. Most soil testing laboratories use Mehlich-3 as the default extractant. Shifting the Tri-state recommendations to Mehlich-3 should reduce confusion about soil test extractants and lead to more unified recommendations.

To use the Tri-State recommendation tables, a conversion of P and K soil test values is needed if they are not reported as ppm using a Mehlich 3 soil test. Soil test values reported in pounds per acre are converted to ppm by dividing pounds per acre by 2. If phosphorus soil test values are shown using the Bray P1 test, multiply the Bray P1 values by 1.35 to convert the values to Mehlich-3 P values. If potassium values are shown as ammonium acetate, convert them to Mehlich-3 K values by multiplying the ammonium acetate value by 1.14. More discussion on these conversions can be found in Converting between Mehlich-3, Bray P, and Ammonium Acetate Soil Test Values, ANR-75 (Culman 2019). If a laboratory reports values from extractants not mentioned here, contact the lab to understand the values shown in their report.

Using Base Saturation for Calcium, Magnesium, and Potassium Percentages

As defined earlier, base saturation is the extent to which soil adsorption complex is saturated by exchangeable cations other than hydrogen or aluminum. Base saturation is shown as a percentage of the total Cation Exchange Capacity (CEC) and equals 100% when the percentages of Ca, Mg, K, and sodium (Na) are added together.

The calcium to magnesium ratio may be important to decide which liming material to use. The calcium to magnesium ratio is calculated by dividing the base saturation percentage of calcium by the percentage of Mg. Where the Ca:Mg ratio is 1:1 or less (less Ca than Mg), use limestone with a lower percentage of magnesium. Agronomic crops grow in soils with a wide range of Ca:Mg ratios, with the ideal ratio between 6:1 to 10:1.

Magnesium and potassium compete for plant uptake. Therefore, the magnesium to potassium ratio should be greater than 2:1. In other words, the percent base saturation of Mg should be at least two times the percent base saturation of K. Where high K levels exist, we frequently see plant Mg uptake reduced. As a result, the application of Mg fertilizer prevents forage and grain nutrient imbalances that affect animal nutrition. For example, grass tetany is a significant concern when Mg:K is out of balance.

Table 3. Typical base saturation ranges shown on a soil test report.
Nutrient Percentage of Base Saturation Range
Ca 40–80
Mg 10–40
K 1–5

References

Culman, Steve, Anthony Fulford, James Camberato, and Kurt Steinke. 2021. Tri-State Fertilizer Recommendations. Columbus: The Ohio State University.
extensionpubs.osu.edu/search.php?search_query=974&section=product.

Culman, Steve, Meredith Mann, Stuti Sharma, Muhammad Tariq Saeed, Anthony Fulford, Laura Lindsey, Aaron Brooker, et al. 2019. “Converting between Mehlich-3, Bray P, and Ammonium Acetate Soil Test Values” (ANR-75). Ohioline. The Ohio State University.
ohioline.osu.edu/factsheet/anr-75.

Lindsey, L, ed. 2017. Ohio Agronomy Guide, 15th Edition, Bulletin 472. Columbus: The Ohio State University.
extensionpubs.osu.edu/search.php?search_query=472&section=product.

Mullen, Robert, Edwin Lentz, and Maurice Watson. 2016. “Soil Acidity and Liming for Agronomic Production” (AGF-505-07). Ohioline, The Ohio State University.
ohioline.osu.edu/factsheet/AGF-505-07.

Originally posted Aug 8, 2022.
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