Soil Analysis: Key to Nutrient Management Planning

Knowledge about the reserves of available nutrients in the soil of a field is fundamental to making appropriate decisions about fertiliser applications. But before getting into the detail of NPKS, it is essential that soil acidity is corrected as necessary by liming, and the first soil variable to measure must be pH. Even on chalk soils patches of acidity can occur in pockets of deeper soil where no chalk is brought up by the plough.

Standard soil analysis, in addition to measuring pH, quantifies the reserves of phosphate, potash and magnesium which are available to the following crops. These reserves can be seen as one of the main measures of soil fertility and they must be maintained if fertility is not to decline. Nitrogen applications, for example, will be less effective if the soil P and K reserves are below critical values, which are discussed in PDA leaflet 24: Soil Analysis Key to Nutrient Management Planning.

If soil P and K reserves are not maintained at least at Index 2 (or at the Moderate level in Scotland) then the full crop potential is unlikely to be achieved, and other inputs will be less efficiently used. Furthermore, this inefficiency may not always be fully overcome in the season by applying extra P and K to the crop; there is no substitute for maintaining a fertile soil.

Another point to consider is that the reason soil reserves decline is of course because of crop offtake. As a general rule P and K do not leave the field in significant quantities except in the harvested crop (and straw). Thus areas in the field which consistently give higher than average yields will also remove more nutrients than the average. It is worthwhile to sample specifically the top-yielding areas in fields to check that they do not have critically low reserves, in addition to carrying out standard whole-field/area analyses.

Soil sampling and analysis is an intrinsic part of good practice and essential to nutrient management planning. It should not simply be used when trying to explain problems, but must form a fundamental part of planning. At this time when margins are so slim, it provides sound information which allows precise decisions to be made, and risks to be minimised.

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

update: June 2005