Phosphorus release from 12 contrasting European soils and their aggregate size fractions
Two groups of soils were studied, the first being made up of twelve heavily fertilised representative soils from the European Community and the second containing fertilised and unfertilised soils for comparison. The first group demonstrate how the amount of plant available P was generally most closely related to soil P content, rather than soil chemical characteristics. However, the total adsorption capacity tends to decrease in the order moderately acid > calcareous > weakly acid. A very steep increase in solution P concentration above 60 percent saturation was observed and it was calculated that 15 to 30 years of cropping without further fertiliser P additions is required to bring some soils below this level. The second group of soils shows how long term P fertiliser additions cause accumulation of all forms of inorganic P. The comparison of a continuously fertilised soil to one which has had no P additions for three years, suggests that once soils of this type have been fertilised in excess of plant requirements, they can be cropped for several years without decreasing yield due to P deficiency. The aggregate separation indicates how selective erosion of small aggregates leads to increased loss of P in most cases. Mixing of aggregates demonstrated how some have a stronger ability to influence concentration than others and this was linked to their phosphate buffer capacity. Reducing conditions increased P release and this was linked to a combination of soil type, P content and cation release. Fertilisation has increased the available P and the ability to influence solution P most in the fraction at the greatest risk from erosion.