ABSTRACT
Summary The process of phosphorus (P) fixation by soil was long thought to result in large losses of P, an idea that led to a common belief that plants can recover only about 10% to 25% of applied P. Because such small recoveries were considered unrealistic and contrary to common farming experience, the efficiency of P utilization in agricultural ecosystems has been investigated. This was done by using (1) a balance sheet approach in long-term P maintenance trials that had reached a steady state, and (2) published data fitted to a common nutrient cycle for nonsteady-state conditions. The efficiency of P utilization in steady-state ecosystems was defined as the amount of P taken up by the plants as a percentage of the total amount of P required to maintain the available soil P pool at equilibrium. The efficiency of P utilization in nonsteady-state ecosystems was defined as plant P uptake, plus gains or losses in the available soil pool, as a percentage of the total P inputs to the soil. New Zealand evidence shows that P utilization in grassland ecosystems ranges from 70% to 90% of the added and recycled P, the remaining 10% to 30% being lost in and from the soil. These findings have been incorporated into fertilizer recommendation schemes that are currently undergoing field testing in New Zealand. The maintenance fertilizer requirements are first determined for 90% of the maximum production and then calculated for other levels of production by the following equation:
where Pm is the fertilizer required, kg P/ha/yr, for a given relative yield (RY) and P90 is fertilizer maintenance requirement for 90% of the maximum yield, kg P/ha/yr. Balance-sheet studies based on nutrient cycles indicate that P is utilized efficiently in most ecosystems. This finding is in conflict with traditional and still-maintained theories of large-scale P fixation by soil. The concept of nutrient efficiency defined in this paper is of practical importance since it can be used to calculate nutrient inputs required to maintain steady production in any agro-ecosystem.
