Efficient use of nutrients in agricultural production systems

In the 1950s and 1960s, the near universal aim was to increase food production at almost any cost. The introduction of improved cultivars and chemicals to control weeds, pests and diseases justified the use of more fertilizers. Worldwide, the largest increase was in the use of nitrogen (N), partly because it had the largest effect on yield and partly because large quantities were available once fixed N was no longer required for use in the weapons of war. Many developed countries, however, were comparatively quick to satisfy their food needs by the intensification of local production and imports. Interest then turned to issues related to the impact of this intensified agriculture on the environment. Among the more important issues were the increasing concentrations of nitrate in potable water. Although these increases were, simplistically, related to the increased use of fertilizers, they led to a greater interest in fertilizer use efficiency. Increasing fertilizer use efficiency must be seen as a component of integrated plant nutrient management. Integrated because farmers should consider all nutrient sources available to them when deciding fertilizer rates; management because all decisions will have to be taken at the field level by each farmer. The latter requires guidelines. For N, annual field-specific recommendations will be required because after harvest there is little or no mineral N residue from the fertilizer in the soil to benefit subsequent crops and any nitrate can be readily lost from the soil by leaching and denitrification. For many crops, N use efficiency, determined by the difference method, has improved greatly but there is concern that a significant proportion, perhaps 20% for rainfed cereals and more for paddy rice, of the applied fertilizer N still cannot be accounted for. The difference method for assessing efficiency may not be appropriate for nutrients like phosphorus (P) and potassium (K). Calculating the P or K balance and determining its effect on readily soluble P and K in soil may be more useful. This is because only very small amounts of P and K are usually lost from soil and on many soils, a part, at least, of the residue from each application accumulates as a plant available reserve. Thus, a critical level of readily soluble P and K can be determined for each crop on each soil. The guideline to farmers would be to maintain their soils just above the appropriate critical value and assess the efficiency of different nutrient sources on their ability to maintain readily soluble soil reserves.