Potash for Cereals

Principles of nutrient management

"Let the soil feed the crop, use fertiliser to feed the soil"

Nutrient reserves in the soil resulting from natural fertility and previous manuring are much more effective in supplying nutrient than newly applied fertiliser. Impoverished soils will often not produce the same yields as fertile soils even if higher fertiliser rates are applied. Thus an adequate supply of potash to the crop should be ensured by maintaining an adequate reserve in the soil. Fertiliser (and manures) should be used simply to maintain this reserve by replacing what is removed in the harvested crop. For impoverished soils extra fertiliser should be used to restore fertility to a satisfactory level. Where nutrient levels are more than adequate, fertiliser usage should be reduced below removal levels, or even omitted.

Unlike nitrogen, soil potash reserves are not subject to loss or major change from season to season and thus maintenance of an adequate reserve is a practical strategy on most soils.

Soil analysis

Response of winter wheat & spring barley to soil K

Knowledge of the fertility level in the soil is clearly essential to determine fertiliser policy. It is impossible and irresponsible to guess correct fertiliser use without soil analysis, which should be undertaken every 4-5 years. Soil analysis provides a measure of the nutrient level available to the crop - the total level of potash (and phosphate) in any soil is very much larger but is not relevant to plant use. The physical condition of a soil is also of vital importance, e.g. structure, depth of soil, level of organic matter and stone content. These must be considered first before interpreting soil analysis.

A very large number of cereal trials on different soil types in different regions has been undertaken to establish how cereal performance is related to the available nutrient supply as measured by standard methods of soil analysis.

Yields increase with increasing soil K up to what is described as a critical level, beyond which further improvement in soil K fertility has no effect. Similar data confirm the same principle for soil P. Because soil fertility varies within fields and according to soil conditions and climate, soil analysis is not a precise measurement as these data indicate. The index system has therefore been developed to provide the general pattern of crop response to added nutrient. Soil K index 2 is divided into a lower and an upper half denoted by minus and plus signs.