Principles of Potash Use

Fine tuning

Where more site-specific information is available it may be possible to 'fine tune' the general offtake guidance in this leaflet. For example in the case of sugar beet, recent studies indicate that the tare-house analysis of root K content (x 1.2 to convert to K₂O), used in conjunction with yield provides an accurate indication of potash offtake, see PDA Leaflet 12.

A number of other specific aspects may be considered.

Soil structure

Compaction, soil pans or other adverse structural problems can inhibit root growth and thus their ability to take up nutrients from a large volume of soil. 'Fertility' embraces other factors as well as nutrients, including soil aeration and water availability and organic matter content. Improvement of these factors leads to better biological activity in the soil and more readily available nutrients. These aspects should be a starting point not an after-thought in fertility management.

Soil depth

Soil analysis measures the available nutrients to sampling depth - normally 15 cm for arable soils and 7.5 cm for permanent grassland - and in soil particles less than 2 mm in diameter. The amount of soil in this size range that can be explored by roots is important. Thus soil depth and stoniness can affect nutrient availability, and should be taken into account when interpreting soil analysis results and assessing the amount of nutrient available. Whilst nutrients are generally most concentrated in the plough layer, it is clear that modern high-yielding, well-rooted crops derive considerable amounts of potash from greater depth. It may be valuable to estimate the likely supply of K by analysing samples taken at depth for soil N. However, interpreting the results may not be easy because there is little information on the amount and distribution of roots at depth.

pH

A larger proportion of applied potash tends to become non-exchangeable on neutral and calcareous soils than on soils with pH around 6. Additions of fertiliser to improve low K status will have less impact on such soils and this needs to be considered when calculating fertiliser requirements for these soils.

High magnesium soils

Potassium availability may be affected on soils with very high Mg levels. It has been suggested that where the concentration (mg/l) of soil Mg is more than twice that of soil K, potash availability to the crop may be less than implied by the actual potassium analysis value. In such circumstances no more magnesium in any form should be applied to the rotation and higher applications of K may be used to overcome the imbalance. The application of calcium to replace the magnesium is not recommended, principally because it will also displace K.

Non-exchangeable K

There is still no practical method to estimate the supply of slowly released K, which will vary greatly between soils and between seasons. For some heavy soils it may be sufficient to provide all the needs of combinable crop rotations and permit applications for root rotations to be halved. However, this does not apply to all clay soils - see PDA Leaflet 19. On lighter soils, allowance for non-exchangeable K should only be made where objective information or experience is available.