Watch your P's & K's for cereals

Why maintain soil reserves

Adequate reserves of potash must be maintained in the soil because:

	Cereals take up more than 250 kg/ha of potash which is far more than is provided by the fertiliser.			Yields from impoverished soils will often not match those from fertile soils however much fresh potash fertiliser is applied.
	The rate of uptake during tillering and stem extension can be as high as 10 kg/ha/day which cannot be supplied if soil reserves are low.			Potash is not leached from most soils so there is no risk of loss by maintaining reserves.
	Potash reserves in the soil are more efficiently available tothe plant than newly applied fertiliser.			Crops on soils with satisfactory reserves also perform much better under adverse growing conditions and achieve better yields in the more difficult years.

Soil analysis

Knowledge of the fertility level in the soil is essential to determine fertiliser policy. Deciding fertiliser rates without soil analysis is likely to result in poor and costly decisions. The cost of standard analysis for pH, P, K and Mg is normally under £10 per sample. Sampling is needed every 4-5 years, so assuming a field size of 10 ha this represents a cost of only about 20p/ha/year. The value of this knowledge far exceeds these modest costs.

	Soil P Index	mg/l	Yield response to added nutrient	Soil K Index	mg/l
	0	0-9	Large response likely	0	0-60
	1	10-15	Response likely	1	61-120
	2	16-25	No response	2-	121-180
			No response	2+	181-240
	3	26-45	No response, good reserves	3	241-400
	4	46-70	Unnecessarily high reserves	4	401-600

Principles of potash use

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

Soil K reserves should be maintained at a level which will provide an adequate supply to the crop; fertiliser should be used to replace what is removed in the harvested crop in order to maintain these reserves.

	Low reserves Index 0 & 1 Extra fertiliser in addition to that removed should be used to restore fertility
	Target soil K level Index 2- Potash fertiliser rate should equal the amount removed
	Above target reserves Index 2+ & 3 Fertiliser usage should be less than removal
	Over-high reserves Index 4 Fertiliser should be omitted.

Soil analysis is a guide rather than an exact measurement, and there is no precise or fixed point at which yield and quality are reduced. Target fertility levels are set to minimise the risk of reduced returns and as soil reserves fall below these targets, the risk of penalties increases. The losses from such penalties are likely to be much greater than the cost of nutrient saved. The restoration of low fertility is also a long and expensive process. To run such risks is not good management.