Changes in plant-available and easily soluble phosphorus within 1year after P amendment

Diffuse losses of phosphorus (P) from arable land are often related to soil P and P amendments. We follow the in situ dynamics of plant-available [ammonium lactate/acetic acid (P-AL)] and easily soluble (0.01m CaCl2) P during 1year after fertilizer application at five sites in long-term fertility field experiments in Sweden with three different soil P levels and amendment rates and two different crop rotation systems. Clear differences between soils and P treatments were found. These can to a large extent be explained by the amount of applied of P and soil sorption capacity. The bell curve' indicating the development of plant-available P can be described successfully using Gaussian modelling. Strong correlation between plant-available and easily soluble P as determined by the above-mentioned methods shows that the existing agronomic soil test P can be a good indicator even for easily soluble P, especially if other soil properties such as soil sorption capacity are taken into account. From the management standpoint, small increases in both plant-available and easily soluble P as in the P replacement treatment indicate that such a management strategy may reduce environmental risks as a result of P amendments. Increases in P amendments above the agronomic optimum increased plant-available P for a considerable time after P amendment (24months) to indicate high environmental risk.