Potash for Cereals

Timing

Phosphate

Because phosphate helps root development it is important to ensure adequate supplies of P are available for the developing root system. Unfortunately phosphate is a very immobile nutrient and roots have to grow to where the phosphate is rather than the reverse. Water soluble phosphate in fertilisers may also become less available fairly rapidly in some soils. Hence where soil P is less than adequate, i.e. at less than index 2, phosphate should be applied in the seedbed and at very low soil index is best placed near to the seed. This is of greatest importance on heavy soils and soils with poor structure, as root extension to find the P is even more difficult on such soils. However under good soil and climatic conditions, for soils at the target level of Index 2 maintenance dressings can be applied at any time during the growing season. Rotational manuring may also be considered but policies involving large amounts of phosphate at one time are not desirable if there is any risk of loss of run-off or soil erosion.

Potash

Whilst this nutrient is slightly more mobile than phosphate it is still good sense to apply potash in or before the seedbed where soil reserves are low. In these circumstances on heavy soils or where soil structure is poor and nutrient may be accessed by roots less easily, all the requirements are best applied at or before drilling. For light and shallow soils, where fertiliser needs are often high because of a low soil index, it is good practice to split applications for winter cereals, with some in the seedbed and the remainder accompanying nitrogen top dressing in the spring (see PDA Leaflet 22). This ensures a supply at establishment, avoids possible risk of downward movement over winter, provides the beneficial combination of N and K to assist rapid uptake in the spring and permits more flexibility of product choice to provide optimum rates of P and K. For soils with adequate reserves, i.e. at a soil index of 2, potash requirements can be applied at any time as the objective is simply to replenish soil levels for the following crop. Rotational manuring may be attractive but whilst there are no environmental concerns with K, large single dressings are unwise on light or shallow soils because of risk of loss.

Economics of potash use

Many factors influence the adequacy of potash supply for optimum growth, but the risk of yield (and quality) penalties increases the lower the soil index. The cost of normal potash recommendations as shown in the table is modest and is covered by only a very small yield difference. 1 kg of potash normally costs only 50-60% as much as 1 kg of phosphate or nitrogen and there is a danger that its agronomic importance is placed on the same level. Making relatively small cost savings on potash which may run the risk of greater losses of crop value is not sound management.

Cost of normal potash recommendations

Recent potash response experiments endorse the principles of manuring on which current recommendations are based, e.g. large economic responses are certain at K index 0. Smaller but economic responses are likely at index 1. If replacement dressings are not made at index 2-, soil K will fall to index 1 at a rate depending upon soil type.