DEFRA figures reveal that the national balance for phosphate in England has been declining over the last few decades and is now only about 5% of the 230 kt surplus from 1990.
Whilst a net balance between nutrient inputs and outputs may be seen as ideal, this now marginal surplus is an average for the country and does not give the true picture at an individual level, according to the Potash Development Association, which says farm level balances are a better indication for each business and whilst some are likely to find themselves still in a positive balance, there are plenty of other businesses which are now in a deficit position.
Whilst the published trend is for phosphate (all inputs and outputs), it is likely to be similar for potash. Any negative balance will deplete soil reserves and if not corrected will result in potential yield and financial penalties down the line.
Crops take up nutrients during their growth (uptake) and some of the quantities taken up are removed from the field at harvest (offtake). Crop offtakes are often overlooked but the implications should not be ignored.
Despite UK soils usually containing large total quantities of phosphorus and potash, most of this will be unavailable in the short to medium term. Complete restocking is extremely rare, although certain clay soils can release useful quantities of potassium from pools not measured by soil analysis.
Data from the British Survey of Fertiliser Practice show significant reductions in major nutrient use in Great Britain over the past 40 years. The overall rate of fertiliser phosphate has decreased by around 67% and that of fertiliser potash by around 55%. Rates of phosphate and potash application have remained greater in Scotland than in England and Wales.
The latest data provided by NRM shows that around a quarter of all arable soils are below the target level for P, K and Mg.
Optimal yields are only likely to be achievable where soils contain sufficient phosphorus and potassium in a readily available form so that they are able to release enough of these nutrients into the soil solution on a daily basis to satisfy crop uptake requirements. Nitrogen is also likely to be used less efficiently where potash levels are suboptimal, reducing the returns from this valuable input and increasing the potential for negative environmental effects, according to the PDA.
Potash reserves in the soil are more effective at supplying the plant than fresh fertiliser applications. Impoverished soils often do not produce the same yields as fertile soils even if higher fertiliser rates are applied so an adequate reserve of phosphate and potash to feed a potato crop should be maintained in the soil by using fertilisers to replace what is removed, the PDA stresses.
For soils with low reserves, extra fertiliser should be used to restore fertility to target levels. For soils with high reserves, fertiliser usage should be reduced or omitted, it adds.
Source: PDA
