IP guidelines must specify agronomically desirable and environmentally acceptable target ranges for soil fertility for at least P and K.
A nutrient allocation plan for P and K for each crop on a plot/field level must be established, taking into consideration:
- The actual field status of soil fertility in relation to agronomically desirable and environmentally acceptable levels (P, K evt Mg).
- The balance approach: Off-farm fertilizer input must only compensate the real exportation and unavoidable technical losses resulting:
- for annual crops in an balance of inputs and exports at a rotational level (including the technical unavoidable losses),
- for perennial crops in an annual balance of inputs and exports.
- Additional inputs can be justified to maintain the desired soil fertility level, Inputs exceeding this plan are unacceptable: for instance small quantities of phosphate are sufficient to cause over-enrichment of surface waters. Phosphate from agricultural land is mostly translocated by erosion of small soil particles.
- The distribution of macro nutrients over the years might be different to the export with crops, as long as the rotational balances are maintained.