Potash for Cereals

Potash uptake

Potash uptake during establishment is low with winter wheat typically containing around 40-50 kg K₂O/ha over winter, more forward winter barley crops may contain up to 80 kg K2O/ha. Whilst these amounts are relatively small, a deficiency of nutrient at this early stage clearly can have critical effects and where soil K levels are low (mid index 1 or below) some potash should be applied to the seedbed - ideally combine drilled if the soil reserves are very low. As the crop reaches tillering and starts to make vigorous vegetative growth, potash and nitrogen uptake increase very rapidly as shown in the diagram. Potash uptake may be as high as 10 kg/ha per day and on light, low K soils this may outpace the soils natural capability to supply. In such circumstances spring NK top dressing makes good sense (see PDA Leaflet 22). This is also good practice on these sandy, low K-retentive soils because split autumn and spring application minimises risk of potash loss under excessive winter rainfall. Peak potash uptake with cereals occurs around late flowering stage when there may be more than 250 kg/ha of potash in a high yielding crop. If this peak requirement is not available, grain number and grain fill will be affected thus prejudicing both yield and quality. This is a key reason for maintaining the adequate soil reserves which can supply these peak amounts more satisfactorily than fresh fertiliser applications. After flowering, potash is redistributed around the plant with a general reduction from leaves to grain and stem bases. Potash is also returned to the soil with the senescence and shedding of older leaves. The rate at which this potash redistribution process occurs varies widely with different seasons and affects potash levels in the straw.

Nutrient uptake of a typical 8t/ha wheat crop

Sources and losses of potash

Virtually all the potash supply to a cereal crop is taken up from the soil. Small quantities of potash (less than 5 kg K₂O/ha) are deposited in rainfall, but foliar uptake is not a practical route for the large requirements of this nutrient. Foliar fertiliser applications to cereals are rarely cost-effective. Potash is held in the soil as the positively charged cation, K⁺, which occurs in small amounts (less than 20 kg/ha) in the soil solution and in much larger amounts in readily or slowly available form attached to the clay minerals. An even larger quantity is also present within the clay minerals but this can be discounted in terms of crop supply as it is only released over long time periods by weathering. The rate and quantity of potash that a soil can supply for crop uptake depends upon soil type, texture, soil condition and previous history of manuring.

Potash in fertilisers and manures is 90-100% available, but if not taken up by crop roots, becomes held by the clay minerals and is therefore not at risk to leaching in the same way as nitrogen. Research studies have shown that for most soils only 1 kg K is lost for every 100 mm of through drainage. Except on light, shallow soils any movement of potash below plough depth is likely to be retrieved by deeper rooting crops such as winter cereals. Applications of potash in excess of crop need will therefore remain as an increase in readily and slowly available soil K reserves for the next crop.

The K cycle