Source: AgSource Laboratories

Soil test reports hold a key to farm profitability and environmental stewardship, but interpreting the results can be confusing. Jim Friedericks, laboratory manager at AgSource Laboratories in Ellsworth, Iowa, offers five top measures to focus on when reading a soil test report.

1) Soil pH rules.This figure is a measure of the acidity or alkalinity of the soil. The most suitable soils for field crops will have pH readings between 6.3 and 7 pH. Soils higher than 7 are basic soils, while those below 7 are acidic. 

Unbalanced pH can limit availability of phosphorous (P) and other essential plant nutrients such as manganese, copper, boron and zinc. The amount of lime required to correct low soil pH is calculated from the buffer pH and is a gradual process since it involves reactions between soil and lime particles, and the amount of moisture received. 

“Liming applications give farmers some of the best returns on investment they can make for their operations,” Friedericks says. 

2) Phosphorus levels are the next most important measurement to consider. Phosphorus promotes root growth and winter hardiness. Plants deficient in P may appear stunted and often have an abnormal, dark-green color. The target level for P in Midwest crop production is 20-25 ppm. 

However, soil test P isn’t an indication of total P in the soil, Friedericks warns. Rather, the soil test report indicates how much P is available for plant use. It’s also important to make the distinction between elemental phosphorus (P) and phosphate (P2O5) because, while soil test results are usually reported as elemental P, fertilizer recommendations are reported as P2O5. 

“It requires about 18 pounds P2O5 to raise the soil test level by 1 ppm. Determining the amount of fertilization needed to raise the amount of P requires calculation, and farmers should take this into consideration,” Friedericks explains.

3) Potassium (K) provides a plant with nutrients for protein synthesis, photosynthesis and movement of water within plants, Friedericks says, landing it in his top three most important test results. A corn crop takes up nearly as much K as nitrogen (N) in any given year, but because K is retained in the soil, its management is very different than N management. 

Potassium aids in water absorption and retention, therefore it also encourages strong roots and healthy crops. It’s one of the greatest investments in protecting a crop against disease due to its ability to strengthen stalks and stems, thus protecting the plant from lodging. By helping to thicken plant cells, K also makes it more difficult for certain diseases to invade plants when stressed during the growing season.

4) Organic matter is a key measure of soil health and structure. While there isn’t a great deal anyone can do to immediately change organic matter levels, it’s a factor farmers must consider when planning fertility programs. “At the very least, farmers want land management to maintain or increase the organic matter content, as a way to improve soil health, thus reducing crop vulnerabilities,” says Friedericks.

5) Cation Exchange Capacity (CEC) and Base Saturation levels help characterize soil type, Friedericks explains. Taken together, they indicate how particular soils will hold specific plant nutrients in the profile. 

“Because there is a strong correlation between CEC values and the amount of clay and organic matter present in the soil, the higher the CEC the more soil can retain, and make available, moisture for plant growth,” he says. 

Base Saturation measures how completely soil mineral and organic particles are saturated with basic cations, which also is a key indication of whether lime is needed.

“Soil analysis is the most accurate, cost-effective way to optimize crop production, but when people send soil samples to laboratories for analysis, the reports may be difficult to interpret,” Friedericks says. “Different laboratories can use different methods of testing these important factors, and while tests are similar, they won’t always give the same results, so it’s especially important to pay attention to details.”

Additional information on soil analysis and quality soil test reporting is available online at www.agsource.com/laboratories.