What do your soil test numbers mean?

By Greg LaBarge, CCA, Ohio State University Extension

Harvest is quickly followed by soil sampling. Soon after samples are submitted to the lab, we have a bunch of numbers to make sense of to decide our nutrient plan for the next 1 to 2 crops. The soil test numbers help us understand soil nutrient holding and exchange capacity, the need for lime, and if we should invest in fertilizer.

Some soil test report information helps us understand the soil’s natural ability to retain and supply nutrients such as organic matter (OM) and cation exchange capacity (CEC). 

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.

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. The CEC value will affect the potassium recommendation.

Soil pH tells us about nutrient availability and soil suitability to grow a chosen crop. Buffer pH is used for the lime recommendation when the soil pH needs to be corrected.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). Figure 3-1 from the Ohio Agronomy Guide shows the effect of pH on the availability of different nutrients. The wider the band, the more nutrient available at a given pH. Correcting a pH below 6.0 can increase crop available nutrients, reducing our need for fertilizer.

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.

Phosphorus and potassium soil test levels give us our fertilizer need when used in conjunction with the Tri-State Fertilizer Recommendation for Corn, Soybean, Wheat, and Alfalfa Bulletin 974.

Phosphorus: If the soil test number is less than 20 parts per million (ppm), then an application of phosphorus fertilizer is recommended due to the increased risk of yield loss at this lower soil test value. If between 20 to 40 ppm, there is no immediate fertilizer need, apply anytime in the rotation to maintain your soil test in this range. If your soil test is above 40 ppm, no fertilizer is needed.

Potassium: To determine potassium need, look back at the soil test report for the Cation Exchange Capacity (CEC). The recommendation will change based on a CEC below 5. Most soils in the state will have a CEC above 5.

With a CEC above 5 and a potassium soil test number less than 120 parts per million (ppm), an application of potassium fertilizer is recommended due to the increased risk of yield loss at this lower soil test value. If between 120 to 170 ppm, there is no immediate fertilizer need, apply anytime in the rotation to maintain your soil test in this range. If your soil test is above 170 ppm, no fertilizer is needed.

With a CEC below 5 and a potassium soil test number less than 100 parts per million (ppm), an application of potassium fertilizer is recommended due to the increased risk of yield loss at this lower soil test value. If between 100 to 130 ppm, there is no immediate fertilizer need, apply anytime in the rotation to maintain your soil test in this range. If your soil test is above 130 ppm, no fertilizer is needed.

More details on soil test report interpretation are in our new factsheet Interpreting a Soil Test Report AGF514 at https://go.osu.edu/usesoiltest. In addition, a free pdf copy of the Tri-State Fertilizer Recommendation for Corn, Soybean, Wheat, and Alfalfa Bulletin 974, is at https://go.osu.edu/tristatefert or visit your local Extension Office for a print copy.