Soil Test Results

 

Soil testing methods are still about the same as they were 100 years ago and researchers and farmers are still struggling to interpret the numbers. However, a soil test is a good place to start to optimize crop yields. However, follow that up with scouting, tissue tests, and sap tests to verify and refine your analysis.

Most standardized soil test report values for pH, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Soil test values are reported in Parts Per Million (PPM) or pounds per acre. The PPM times 2 equal pounds per acre. Actual soil nutrient levels generally are much higher; however, not all soil nutrients are available for plant uptake.

Soil pH measures the acidity of a soil. A soil that is neutral has a pH of 7, while an acid soil has lower values and a basic soil has higher values. The pH values can range from 1 (highly acid) to 14 (highly basic), although most soil range from pH 5.0 to 8.0. To improve soil productivity and the release of nutrients, a pH between 6.3 and 7.0 is desirable. Buffer pH (6.8 to 7.0) or Lime Test Index (68 to 70) is the indicator used to determine how much lime is needed to correct soil pH. Most soils in Western Ohio have limestone in the subsoil so they tend to be less acidic than Eastern Ohio with more shale (acidic) soils as the parent mater.

Cation Exchange Capacity (CEC) is a measure of how many positive charged nutrients a soil can hold, measured in milliequivalents (meq). CEC ranges from 1 to 30+ with sandy coarse textured soils holding less nutrients (CEC=5), medium textured silty soils having a CEC around 10, and fine textured, heavy clay or high organic matter soils having more nutrients (CEC = 20-30+). Soil organic matter (SOM) levels increases the CEC and holds more nutrients. SOM levels range from 1-6%.

Farmers should strive to optimize calcium (Ca) and magnesium (Mg) soil levels. Ideally, a goal is 70% Ca base saturation and a 12-15% Mg base saturation. Soils in Northwest Ohio (clay soils) tend to be high in Mg which makes soils tight, sticky, hard and dense. Air and water cannot move in tight soils which tend to have poor drainage. Soils that are Ca and Mg balanced are open, have stable soil aggregates (soil crumbles), good drainage, oxygen rich for deeper roots, and have higher microbial activity. It’s not only pH, its good soil structure which allows healthy plants to grow. Good Ca levels attract the clay particles together to form good soil aggregates while high Mg causes soil to be structureless, like a concrete slab, so air and water cannot move freely. On sandy soils and ridges, Mg levels may be too low, and these soils may need more Mg fertilizer. Remember that Mg is the central element needed to make chlorophyll.

Phosphorus (P) is more soluble than it was 20-30 years ago. MAP and DAP fertilizer have highly soluble P plus with less soil inversion, the top 2 inches are more acid, leading to soluble P. Under the new guidelines, adequate soil test P values have been revised downward slightly and now use Mehlich-3 and advise a 20-40 Mehlich-3 P soil test levels as adequate.

On potassium (K), added K tends to help improve yields. Generally, K base saturation should be between 2-5%, but higher end at 4-5% can be desirable for higher yields. Potassium (K) soil test levels have been dropping but Tri-state fertilizer recommendations have eliminated K build up. Why are K soil test levels dropping? Growing more soybeans (a high K remover), higher crop yields, more wheat with straw removal, and even less corn borer means crops are producing better yields and thus removing more K. application of K Fertilizer may also get tied up in the clay soil structure when soils are wet and fall tillage occurs (K Induction). Adequate soil K improves N uptake in plants.

Due to the passage of the 1908’s Clean Air Act, free atmospheric Sulfur (S) has declined from 24# S/acre in 1985 to about 4-7#/acre in 2022. Sulfate forms in the spring when crops need the S. Building soil organic matter by growing cover crops may help. Adding spring applied gypsum (calcium sulfate) is more beneficial than fall applications. Calcium and sulfate are mobile and may be lost or tied up deep in the soil. For more information on fertilizer guidelines, see the Tristate Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, E-2567.