Soil nutrient fertility

	Illustration of the calculated average annual phosphate and potash input/output balance for the main arable crop area in England and Wales, excluding manure inputs.

Much is written in the farming press and advisory literature under the heading of 'fertilisers', but it usually focuses on the rate and timing of nitrogen (N). As we have said before, crops, including grass, are only able to make the most effective and efficient use of N (and often of fungicides as well) if the soil nutrient fertility is satisfactory. It generally seems to be assumed that soil P and K status should be OK, to the extent that these nutrients may not even be mentioned.

Unfortunately there is growing evidence that often soil P and K levels are not OK, and the charts on the right give some clear suggestions as to why. If nutrients which are removed are not replaced then soil fertility will decline. The charts clearly show that for both phosphate and potash the negative balance is now about 22 kg nutrient/ha/year - i.e. 22 kg more nutrient is removed than is replaced, excluding any inputs from manures. This could result in a potentially serious degradation of one of our main national assets: fertile soils. Analysis of the results of a large-scale series of arable soil samples showed almost 30% with a soil P Index of 0 or 1, and almost 50% at Index 0 or 1 for K. A soil at Index 0 or 1 is deficient in that nutrient, and crops will usually not achieve their potential. If yields this harvest were not as high as expected, it may be unfair and misleading just to blame the weather! Simply to assume that soil nutrient fertility (reserves) are adequate is to take an unnecessary risk; it is not expensive to take soil samples for analysis, to be sure. Soil nutrient fertility can be measured and managed; unlike with the weather, it does not require risk-taking!

Updated PDA Leaflet #20: Biosolids and the need for Potash

A copy of the latest revised version of the PDA leaflet on sewage sludge (Biosolids) is now available on line. While sewage sludge can be a useful manure, its potash content is very low, and its inclusion in a manuring programme must recognise that fact.

A copy of the PDA leaflet giving the simple nutrient content and availability tables for most other manures is also included; these figures are taken from the current version of RB209.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: Aug 2007