Phosphorus (P) availability during the depletion of soil P

As an essential element for crop growth, phosphorus (P) has been added excessively to many agricultural soils during the last decades. This created an imbalance between P input (P fertilisation) and crop P offtake, which caused an accumulation of P in soils in some regions of western and southern Europe. Currently, a more sustainable and stricter P fertiliser use is promoted, which led to negative soil P balances in some European regions (i.e. depletion). It is expected that the availability of P in a soil P depleting scenario is smaller than in a P accumulation scenario, due to the hysteresis of the sorption-desorption processes.

This webinar will focus on determining crop and soil factors affecting the availability of soil P in a soil P depleting scenario, and the identification of a soil test which adequately predicts this P availability in a P depleting scenario. The following will be discussed during the webinar:

• An evaluation of soil P tests in their capacity to quantify the plant-available P. For this, soils from eleven European long-term field trials, with contrasting soil properties and different P application rates, were used. The following established soil P tests were evaluated: extraction with ammonium oxalate, extraction with ammonium lactate, extraction with 0.5 M NaHCO₃ (Olsen), extraction with 0.01 M CaCl₂, and the diffusive gradient in thin film technique.
• The effect of the crop growth rate on the critical available P before P deficiency occurs in a soil P depleting scenario. This was based on eight Flemish soils which were subjected to a P mining experiment in a greenhouse to obtain depleted soils. Different amounts of nitrogen fertilisers were applied, thereby creating different crop growth rates.

The role of the P desorption rate on soil P availability in a soil P depleting scenario. This was determined by modelling, via a mechanistic nutrient uptake model based on classic nutrient uptake models, but extended with P desorption kinetics and including the plant’s P demand rate as an important factor in determining the P uptake by roots.