Soil potassium (K) levels have been routinely tested for many years. Historically, K levels were quite high in most South Dakota fields and therefore crop producers paid little attention to K levels or fertilization. However, 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. Corn and soybean typically have 0.3 and 1.4 lbs K2O/bu, respectively. Therefore, harvesting 180 bu/a corn removes 54 lbs K2O/a while soybean yielding 60 bu/a removes 84 lbs K2O/a. Removing silage or residue from fields will remove even higher levels of K, further decreasing soil test K levels.
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. 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, 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. Soil test K variability can range from 30 to 40 ppm from one sampling to another. This is frustrating especially when K values are near the response levels which are 160 ppm for corn and 120 ppm for soybean. 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. 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. Plant tissue testing is a good tool along with soil sampling to determine potassium deficiency. Potassium leaf deficiency symptoms are sometimes confused with nitrogen.
