Potassium is the only nutrient that most crops take up in amounts similar to nitrogen. It plays a critical role in crop production, especially plant water movement and enzyme activation. A potassium deficiency restricts plant growth and grain filling.
While many of the northern Great Plains soils have an abundance of soil potassium to meet crop needs, some of North America’s lowest-testing soils can also be found in this region.
An abundance of plant-available potassium comes from soil with a large supply of potassium. Originating in soil minerals such as feldspar and mica, positively charged potassium is released and binds to negatively charged soil colloids.
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As growing crops take up soil potassium, the bound potassium moves mainly by diffusion into the zone adjacent to plant roots. Where an abundant potassium supply exists, this process of nutrient release into the zone of plant root uptake generally meets crop requirements. Where soil potassium is low, the ability to maintain adequate solution potassium adjacent to plant roots can be reduced.
Potassium uptake by plants follows a pattern similar to nitrogen, with rapid early uptake followed by limited uptake once the crop has flowered. Under irrigated conditions, the uptake of nutrients such as potassium can occur longer into the growing season, meeting the needs of the high-yielding crop.
However, unlike nitrogen, most of the potassium taken up by plants remains in the leaves, stem and head, with only a small proportion, less than 20 percent, in the harvested grain.
High rates of potassium are removed when the entire above-ground biomass is harvested. Alfalfa is an example, where potassium removal is approximately 60 pounds K2O per acre for each ton of dry matter harvested. A wheat crop removes about 0.35 lb. K2O per bu. harvested, or 17.5 lb. K2O in 50 bu.
While most soil that tests greater than 250 to 300 lb. of potassium per acre is considered sufficient to grow most grain crops, there are isolated areas that are deficient in potassium and require soil amendment.
On soils where responses to potassium fertilizer have been observed, large differences have been recorded in crop and fertilizer placement response. Of the crops that were evaluated, barley is the most responsive to potassium, followed by wheat and canola. As the soil-test potassium level decreases, the response of crops to added fertilizer potassium increases.
Seed row placement of fertilizer potassium has been shown to provide the best crop response at low application rates, followed by band and broadcast application.
Where low soil-test potassium occurs, the best crop responses have been obtained with a large broadcast application along with low rates: 15 lb. K2O per acre applied with the seed.
Potassium responses have also been recorded on soils testing high in potassium. This potassium response is attributed to cold soil in the spring limiting soil potassium availability, field variability where portions of a field will have high soil-test potassium while other portions are low, and the chloride ion found in most potash fertilizers.
Chloride is a micronutrient required for plant growth and development. It can also affect seedling root and leaf diseases, grain filling and final seed weight, plus physiological leaf spot on certain cultivars.
Potassium nutrition is critical to crop production. Where deficiencies exist, nutrient addition has a major effect on crop response.
