Principles of Potash Use

Potash leaching

Potassium does not leach anything like as readily from soils as does nitrate and sulphate. Potash additions not taken up by the crop will be held in the soil by the clay minerals or organic matter as described above. For the great majority of soils, which have a clay content of 5% or more, where normal rates of potash are applied, potassium not used by the crop will remain in the cultivated layer of soil and will not move further down the profile. However, significant losses of potassium can occur when any source of potash (fertiliser, slurry or manure) is applied under adverse conditions i.e. when soil is water-logged, frozen or very dry and deeply cracked. Most of this loss is by surface run-off and can be avoided by following codes of good agricultural practice. Soils with less than 5% clay (the sands and loamy sands) have a much lower retentive capacity for potassium. Such soils, especially if shallow and subjected to rainfall producing large amounts of through-drainage, have a greater risk of potassium loss. On these soils, potash should be applied 'little and often' and applications timed to suit crop uptake and amounts carefully matched to crop offtake.

Soil analysis

Many decades of detailed research work have resulted in the present soil analysis system as the best practical approach for assessing the adequacy or deficiency of soil potassium supply (see PDA Leaflet 24). Soil analysis measures the K in the soil solution and exchangeable K to give a guide to the amount of potassium that is available to the crop. All soils contain much larger total quantities of K as shown in Figure 4. Slowly available reserves (non-exchangeable K) may contribute to plant supply over the course of a season but this release is variable and impractical to measure. For many soils the slowly available reserve provides only a minor annual supply but it can be more important on some very clayey soils (see PDA Leaflet 19).

Soil K and crop response

Fig 6: Response of winter wheat & spring barley to soil K	Fig 7: Response of potatoes & field beans to soil K

The yield response curve to soil potassium supply follows the same principles as for other inputs. At very low levels of available soil K, the response to applied potash is large and the response decreases to nil as soil supply increases. Target levels for soil K which represent the level at which K supply is adequate for full yield but not excessive have been established from large numbers of response experiments for different crops.

As shown by the scatter of points in Figures 6 and 7 for a number of field experiments, the response to soil K can vary. Often variation is due to factors other than the availability of soil K. For this reason the K Index system, as in RB 209, gives a range of soil K values for each Index and is intended to provide a general guide to interpretation.