Ideal and saturated soil fertility as bench marks in nutrient management: 1. Outline of the framework

This paper presents a framework for nutrient management that takes sustainable soil fertility, environmental protection and balanced plant nutrition as starting points, and integrates concepts from plant physiology, soil chemistry and agronomy. The framework is meant as a tool that can be applied even if no local experimental results are known. Saturated soil fertility and ideal soil fertility constitute the bench marks of the framework. Saturated soil fertility is the fertility at which the soil by itself does exactly satisfy the nutrient demand of a crop producing the target yield, provided no nutrients get lost. Ideal soil fertility is the fertility at which the soil in combination with ‘replacement input’ exactly satisfies that nutrient demand. Replacement input is an input equal to the quantity of nutrients removed in harvested crop.Saturated soil fertility is calculated as a function of target nutrient uptake, and ideal soil fertility as a function of target nutrient uptake and recovery fractions of input nutrients. Because the recovery fractions are high for N, low for P and medium for K, the soil is relatively low in N, high in P, and medium in K at ideal soil fertility. The supplies of N, P and K to the crop by soil and replacement input together are well-balanced. The ratio of ideal soil fertility to saturated soil fertility proves to be 0.2, 0.6 and 0.4 for N, P and K, respectively.In case of nutrient losses, the steady-state soil fertility where nutrient input equals nutrient output by crop and losses is exactly the same as the ideal soil fertility for nutrients that do not accumulate in the soil, and somewhat higher than ideal soil fertility for nutrients that do accumulate. Equations are presented to calculate that steady-state soil fertility and the nutrient input and output.