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.

	Soil P (Olsen test)	Timing guidelines
	Under 14 mg P/litre	All in seedbed
	14-18 mg P/litre	Annually, in seedbed or in spring
	Over 18 mg P/litre	At any time (rotational manuring possible)

Potash

Whilst this nutrient is slightly more mobile than phosphate it is still good policy to apply potash into 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 requirement for potash is 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 nitrogen and potash to assist rapid uptake in the spring and permits more flexibility of product choice to provide optimum rates of phosphate and potash. 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 potash, large single dressings are unwise on light or shallow soils because of risk of loss.

Soil K	Timing guidelines
Under 100 mg K/litre	Winter cereals -	Light soils: 50% in seedbed, rest in spring
		Other soils: all in the seedbed
	Spring cereals -	Light soils: between January & sowing
		Other soils: between autumn & sowing
100-150 mg K/litre	All cereals -	annually, at any time
Over 150 mg K/litre	All cereals -	at any time (rotational manuring possible)