Dynamics of soil phosphorus measured by ammonium lactate extraction as a function of the soil phosphorus balance and soil properties

A sustainable supply of phosphorus (P) to agricultural soils should maintain soil fertility without jeopardizing the environment. This study analyzed the change in the soil P as a function of the net soil P mass balance, i.e. the input minus removal by the harvested crop. The P available for crops was estimated by the P extracted by ammonium lactate at pH 3.75 (P-AL). First, laboratory and pot-trial data showed that the net change in soil P-AL was only 68% of the change in total soil P, either when P had been added to soil or when P had been mined by plants for 1–2 years, indicating removal to or release from the non-extractable pool depending on the balance. Secondly, data were collated from 33 field trials in Europe with a wide range of cumulative soil P balances (−1200 to +2500 kg P ha−1) in time spans ranging from 3 to 51 years. The average change in P-AL across the negative and positive balance was only 27% (95% confidence interval 25–30%) of the net balance. The change in P-AL was larger as the net balance increased and as the initial P-AL decreased. The slope of the change versus the balance was smaller as time increased and initial P-AL decreased. The other soil characteristics did not affect these changes, and the model developed was not different for negative or positive P balances. Our results suggest that a steady state P-AL concentration at optimal P supply for crops is obtained at a net P balance of about 1–10 kg P ha−1. The P mass balance of a 51 year-old trial showed that the aqua regia soluble P of the 0–90 cm layer accounted for only 64% of the net P balances at the surface. This still incomplete P balance could be related to upward P transport by plant uptake from > 90 cm soil layers, lateral P movement in the field, and changes in the soil P fraction that could not be extracted with aqua regia. This study quantified the long-term dynamics of P-AL, which is useful for agronomic and environmental purposes and policy, and pointed out the importance of non-extractable P and vertical P movement.