Soil phosphorus budgets in organic farming differ according to plot managements and farm types

Abstract

Soil fertility in organic farming systems relies on the use of organic fertilizers and on the introduction of N-fixing crops in rotations. In such systems, the management of phosphorus fertilization can be challenging. Field phosphorus nutrient budgets and soil phosphorus status are useful indicators for assessing the sustainability of fertilization practices. However, the factors driving soil phosphorus budgets of organic fields for different management practices and farm types are still not fully understood. Here we propose to fill this knowledge gap by investigating, through semi-directive interviews, the variability of 5-year soil phosphorus budgets in 177 French organic farms to identify relationships between the P budgets and farm characteristics, covering farm territorial specialization, and soil fertilization practices. Changes in Olsen-P measurements were also computed to test their coherence with the phosphorus budgets. Based on our unprecedented large and diverse dataset, we found contrasting soil P budgets ranging from -32.3 to 50.3 kg·P ha⁻¹ y⁻¹. Our work also revealed that P management was not at the core of farmers’ preoccupations, as 72% of them did not pay any specific attention to P management practices. The two most important factors driving soil phosphorus budgets were the cumulated phosphorus inputs and the frequency of N-fixing crops. We also identified 5 plot types — through a cluster analysis — with contrasting phosphorus management practices and soil phosphorus budgets. Our results highlight the need for a strategic management of phosphorus resources to prevent the occurrence of soil phosphorus imbalances that may be challenging to rectify in the medium to long term. This is particularly important in the case of organic farms that rely on the use of N-fixing crops for their nitrogen fertilization strategy, leading to low or negative soil phosphorus budgets.