Soil Olsen P response to different phosphorus fertilisation strategies in long‐term experiments in NW Italy

Soil available P for crops, derived from P inputs and regulated by soil chemical, physical and biological factors, represents a small fraction of the total P content. Soil P balance (P input minus P output) is potentially a good descriptor of fluctuations in the available P pool, making it possible to compare experiments that differ in management and duration. Using a data set compiled from three long-term experiments in NW Italy that tested different P fertilization types (nil, mineral, organic and manures), cropping systems and P balances in different soils, this paper focused on (a) proposing an empirical exponential model to describe the relationship between variables and (b) providing an agronomical interpretation of optimized parameter values, to set the basis for a unified interpretation of long-term dynamics of soil plant-available P. The pooled data fit well in an exponential model that identified three factors supportive of an agronomic interpretation, two linked to soil attributes and one that differentiated fertilizer types. Soil characteristics were found to affect minimum soil Olsen P values in depletion and response rates with additions, while fertilizer type had a multiplier effect on cumulative P balance. Farmyard manure was shown to build the available P pool better than other organic sources (bovine slurry, commercial organic fertilizers, green manure) and inorganic fertilizers, owing to a high C/P. The exponential model provided a good theoretical basis, but further studies are needed to correlate parameter values to soil (e.g., carbonates, Al and Fe oxide content) and fertilizer (e.g., C:P, lignin:P) characteristics and to verify the model applicability to different conditions.