The frozen ground with variable snow cover across the state has led to some farmers choosing to take the time to spread delayed fall potassium applications. The frozen ground provides ideal conditions for reduced compaction from heavy fertilizer equipment. Winter can be a good time to spread limeas well as potash on fields where the potential for runoff is minimal, such as relatively flat fields with some residue cover. Runoff of lime or potassium (K) is not an environmental hazard, but rather is an economic loss of fertility inputs. Wintertime applications of phosphorus (P) containing fertilizers, however, should be avoided when possible because of the negative environmental effectsof P runoff. Be cautious of wintertime applications in excessive wind, which can disrupt the spread pattern for broadcast spreaders.
Application rates of lime and potash are best determined from a recent soil test. The opportunity for wintertime fertilizer spreading is a great example of why soil testing in the fall can be advantageous. If you don’t have a recent soil test, you can use expected crop removal rates for the upcoming crops in the rotation as a guideline for potash applications. Potassium crop removal rates can be calculated based on a yield prediction and crop-specific removal rates per yield unit, which are available in Table 1.2-8 of the Agronomy Guideand in Potassium for Crop Production. Applying at crop removal rates will approximately maintain your current soil test K level over time. If your starting point soil test K level is low, however, this approach won’t help you build up levels into the optimal range. Conversely, if your soil test K levels are already above optimal (but you didn’t know it from lacking a soil test), then continuing to purchase and apply K fertilizer is unlikely to have a positive economic return and can lead to livestock health issues if forage crops are in the rotation (see below). There are no good ways to calculate a lime application rate without a soil test. So if you don’t have a recent soil test recommendation for a liming rate then you are better off delaying your lime application until you can get your soil tested.
Potassium is a positively charged ion and can be stored on soil cation exchange sites, meaning it does not leach in most of our Pennsylvania soils. High organic, muck soils and sandy soils allow free, or exchangeable potassium to leach through the soil. Therefore, avoid winter or fall potash application on sandy and muck soils. On silt or clay loam soils, potassium is held between clay particles more tightly and can be stored. This is a sticky situation, so to speak. Plants are indiscriminant users of potassium. Therefore, typical Pennsylvania soils are prone to high potassium from over-fertilization. This can cause problems for cattle. Crops uptake high levels of K causing nutrition imbalances in cattle, often referred to as grass tetany. Proper balancing in the ration can compensate for these issues. However, it is cheaper and easier to attempt to avoid over-fertilizing ground in the first place. Using a soil test, we can ensure we need potassium.
To spread in winter, we need to confirm we need potassium, our soils do not leach and that the conditions are appropriate for spreading equipment, taking both environmental conditions and safe operation into consideration with these cold temperatures and snow.
Photo caption: Potassium in Soil Solution, Penn State