Potash manuring for arable crops

Background

It is important that potash (K) manuring is optimised so that yield and crop quality are not jeopardised. Easily recognised K deficiency symptoms in growing crops are usually seen only when there is an acute shortage of K. In cereals, pale yellow leaves can be due to mild K deficiency but are often considered to be nitrogen (N) deficiency. Applying N would be highly inappropriate.

This note summarises present knowledge regarding the relationship between soil and crop K and crop requirements for K and suggests a strategy for potash manuring for arable crops.

Soil - crop relationships

Fig 1. Simplified K cycle for crops & soils indicating content of K in different forms

Plant roots take up K, like other nutrients, from the soil solution which is in the pores between soil particles. Actively growing crops have a large demand for K, cereals as much as 5 kg/ha each day. However, the roots of annual arable crops do not have time to exploit the whole volume of soil. So it is essential that K in the soil solution is rapidly replenished from the supply within the soil near to where it is being taken up by the roots.

It is helpful to visualise the relationship between crop and soil K as shown in Fig 1. which illustrates soil K in four pools. One is the soil solution, the other three are associated with soil mineral particles and soil organic matter.

Some of the K taken up by the roots is removed in the harvested produce, some is returned to the soil in crop residues. An agronomically insignificant amount of K comes in rainfall, usually not more than 5kg/ha each year. The rest of the K required by the crop comes from the soil K supplies.