As part of the FAS Agricultural Laboratory’s ongoing commitment to provide our customers with accurate, evidence-based recommendations, we are constantly refining our methods and interpretations. One of our most recently introduced analyses is the Reserve-K estimates which are part of your routine soil fertility analysis report, along with adjustments to K fertiliser recommendations.
What is Reserve-K?
Reserve-K is an estimate of slowly exchangeable K held between layers of clay in minerals such as illite (think of it as the cheese between two slices of bread). These types of soils are more common in the irrigated regions where there are soils that have not been extensively weathered, or in floodplains and valley-bottoms, where these K-rich minerals sometimes accumulate. In soils with high amounts Reserve-K, K for crop uptake can be resupplied from these reserves (the slices of bread can be opened and the cheese removed). The figure below outlines the process of K being slowly released from these layers to become ‘readily available’ to the plant.
The Reserve-K test
The FAS Agricultural Laboratory developed a system that applied reductions to the recommended K-fertiliser rate based on the amount of Reserve-K and Exchangeable K in a sample. The approach is category-based, applying either no reduction (when there is low exchangeable K or Reserve-K amounts) or reducing K fertiliser recommendations by either 30, 60 or 100% as Reserve-K amounts increased from one category to the next. While a convenient approach, some concern over large K-fertiliser reductions being applied for small changes in Reserve-K values (particular at the category boundaries) were noted.
Improvements to the adjustment scale
A recent review of this approach thus led to conversion of the category-based adjustment to a sliding scale of K-fertiliser rate reductions. This approach better aligns the amount by which the K-fertiliser recommendation is reduced to the amount of Reserve-K in a sample. This means that for smaller amounts of Reserve-K, reductions to the K recommendation are lesser, and as Reserve K increases, the amount to reduce will proportionally increase. Where Reserve K is low (or if exchangeable K is very low), no reductions to K-fertiliser are recommended. In instances where Reserve-K is very high, a maximum reduction of 90% has been introduced to ensure that your crop still has a small supply of immediately available K for early crop growth. The difference in approaches is graphically presented in the figure below.
For more information on K management and Reserve-K see Information Sheet 7.5, available from our Publications page.