Spring Potash Applications

February 2025

For those who got caught out in the autumn, or who have chosen to hold back on phosphate and potash applications due to weather, crop prospects, price or a combination of any of these, it is not too late to benefit from their application this spring. Granted conditions may still need to improve somewhat to achieve this…

Risk of lodging is greater if potash levels are low

Potassium significantly affects the strength of straw through its influence on lignification and cell wall thickness. Any deficiency of potash will reduce straw strength and increase the risk of lodging. This is illustrated by PDA measurements in wheat crops at different levels of potash application over two years (below). It is clearly essential that to get the best from the use of plant growth regulators (PGRs) a crop must not be suffering from a lack of potash.

Figure 1: Effect of potash on lodging in winter wheat

Top-dressing winter crops with potash in spring

On soils with low potassium reserves (K Index 0 or 1) it is recommended that potash fertilisers are incorporated into the seedbed. However, if for some reason this has not been done, and winter-sown crops have not yet received a dressing of potash, it is still beneficial to top-dress with potash in the early spring rather than to omit the dressing altogether. There are a mulitude of product options, however, outside of NK or NKS products, polyhalite provides an important source of sulphur, along with potash, magnesium and calcium. Soils with K Index 2 can be dressed at any convenient time, with an application designed to replace the K removed in the crop – i.e. a ‘maintenance dressing’ to maintain the soil K reserve.

Penalties of K deficiency

• Serious yield loss especially in adverse growing conditions e.g. dry summers.
• Weaker growth resulting in higher lodging risk.
• Lower crop vigour giving greater susceptibility to disease, pests and weed competition.
• Poor grain sample – low grain size, poor specific and 1000 grain weights.
• Inefficient N uptake and utilisation leading to poor nitrogen response and less protein synthesis.
• Adverse effect on marketability.
• Lower tolerance to physiological stress – wet, drought, frost, wind.