Potassium soil testing necessary to avoid deficiency problems

Farm Forum

BROOKINGS — Historically, potassium (K) levels were quite high in most South Dakota fields and therefore in recent years crop producers have paid little attention to K levels or fertilization. However, explained Anthony Bly, SDSU Extension Soils Field Specialist, in the late 1990’s, crop K deficiency symptoms, especially in corn were observed first in coarser textured soils in the northeast and east central part of the state.

“Since then, fields in other areas of the state have exhibited K deficiency symptoms and verified with lower K soil test levels even on soils with finer textures. Lower soil K levels have been hastened by the intensive corn/soybean crop rotation and high yields,” Bly said. “Corn and soybean typically have 0.3 and 1.4 pounds fertilizer equivalent potassium (K2O) per bushel, respectively.”

Therefore, Bly explained that harvesting 180 bushels per acre of corn removes approximately 54 pounds K2O per acre while soybean yielding 60 bushels per acre removes approximately 84 pounds K2O per acre. Removing silage or residue from fields will remove even higher levels of K, further decreasing soil test K levels.

Bly said that lower soil test levels and plants exhibiting K deficiency have brought more attention to K soil test methods and recommendations.

“Soil test K has traditionally been determined by drying and grinding the sample and then extracting with ammonium acetate and analyzing for K which is known as the dry method. Some soil testing laboratories outside of South Dakota have brought back the wet or moist K soil test from decades ago,” he said.

Bly explains that the wet soil sample is not dried but still extracted similarly to the dry test. The wet test requires more labor than the dry test and therefore reduces sample results turnaround time. Recent research in Iowa has shown better yield relationships with the wet method compared to the dry soil test. However, Bly said yield relationships with the dry soil test K method in South Dakota is quite good and does not warrant a shift to the wet method.

“Soil sampling and testing is not an exact science, but is the best method to predict crop yield response to fertilizer applications,” Bly said.

Soil test K variability can range from 30 to 40 parts per million from one sampling to another.

“This is frustrating especially when K values are near the response levels which are 160 parts per million for corn and 120 parts per million for soybean,” he said. “However, if one is using a regular soil sampling management program, variability in soil K or any other soil nutrient can be minimized by comparing results to long-term trends for each field or sample point.”

Bly explained that areas within a field can vary considerably in soil test K levels. Lower K levels tend to be on steeper, eroded slopes and areas with coarse soils. Sampling by grid or management zones can save fertilizer dollars by applying the K only where needed in the field.

Crop stresses that limit early root growth such as soil compaction, cool, wet, very dry or very warm conditions can lead to increased K deficiency.

Bly said that plant tissue testing is a good tool along with soil sampling to determine potassium deficiency.

“Potassium leaf deficiency symptoms are sometimes confused with nitrogen,” he said.

To learn more, visit