Phosphate fixing capacity of soils: A survey,using the isotopic exchange technique,or soils from north-eastern France

A large variety of soil samples was collected from farms located in two north-eastern regions of France. Their phosphate fertility was assessed by the isotopic exchange kinetics method which allows for the measurement of a r₁/R ratio, where r₁ is the quantity of radioactivity still present in solution one minute after the injection of a quantity of radioactivity R into a soil-solution system at steady-state. The r₁/R parameter gives an estimate of the soil fixing capacity for phosphate ions (PFC). Results of the analysis of 233 soil samples revealed that the great majority of soils from these areas exhibit a high PFC. Neither the soil type nor mode of utilization appeared sufficient for prediction of the PFC. Attempts to express the PFC as a function of other soil parameters (pH, clay, organic matter, calcium carbonate and exchangeable cations), resulted in a significant regression equation involving soil clay content and soluble phosphate. Due to high PFC of soils in the Lorraine area, phosphate fertilizer could be applied at a time as close as possible to the plant requirements.