Principles of Potash Use

Low fertility soils

The rate at which reserves in a low K soil should be built up is largely an economic decision. Consideration should be given to the likely loss of crop yield and quality of all the crops grown in the rotation on a low K soil, as well as the cost of fertiliser. Soils vary in their capacity to retain potassium (buffering capacity) and require different quantities of potash to increase plant-available K as measured by exchangeable K.

The quantities required may seem surprisingly large but are a consequence of the fact that some of the added K goes into non-exchangeable reserves that are not measured by soil analysis. In practice, low soil K levels should be improved over a period of years using annual remedial amounts (see for example RB 209). Heavier soils are less likely to be found with low K levels but will take longer to restore to a satisfactory level than lighter soils.

Fertiliser policy

Following the principles of manuring stated earlier, fertiliser recommendations comprise two components:-

These two components vary with soil K Index as shown in the table below. Fine-tuning of the soil fertility adjustment could be done where soil is in the upper or lower parts of Indices 1 or 3. For example, at low Index 1, the fertility adjustment could be +125 kg K₂O/ha for vegetables or +35 kg K₂O/ha for arable and grassland.