Changes in plant‐available and easily soluble phosphorus within 1 year 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 (0.01 m CaCl2) P during 1 year 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 (2–4 months) to indicate high environmental risk.