Required by all plant life, potassium (K) plays a major role in photosynthesis, breakdown of carbohydrates, protein synthesis and disease resistance. Most importantly, it can activate at least 80 enzymes that regulate the rates of major plant growth reactions. The aforementioned activities should all occur in-season, but what can be done to ensure a crop has the proper levels of K to carry out these functions? And should the applications be made in the fall or spring?
Similar to phosphorus (P), considerations of a spring-versus-fall K application should weigh equipment availability, time and soil moisture/compaction concerns. Once those factors are evaluated, what’s the agronomic advantage of applying K in the fall?
Responses to K applications vary among crop types, but alfalfa has historically shown one of the highest K requirements. Commonly exceeding 60 pounds of potassium oxide (KO₂) needed per ton of hay, alfalfa makes efficient use of K through large root systems. One advantage to a fall K application on alfalfa is the decreased likelihood of winterkill, as extra nutrients are made available prior to dormancy.
Beyond nutrient needs by crop, attention should be given to variables in soil types before fall K applications. Some soils, such as coarser-textured soil types, have cation exchange capacities (CECs) of fewer than 6 milleqivalents/100 grams. CECs are a measurement of the ability of a soil to store and release cations per 100 grams of soil, or centimoles of positive charge per kg of soil. Greatly affected by clay and organic matter content, soils with low CEC levels usually require greater amounts of K fertilizer applied in the spring to avoid leaching.