Phosphorus availability from organic amendments: field-based determination of the b coefficient in a mollisol

Abstract

The application of livestock-derived by-products as organic amendments (OA) to agricultural soils is a sustainable strategy to recycle nutrients and promote circular nutrient economies. The OA application rates are commonly based on crop nitrogen (N) demand, despite their N:P ratios often being lower than required by crops, leading to phosphorus (P) overapplications. Effective nutrient management, particularly of P, requires accurate information on the increase in soil extractable P per unit of P applied—referred to as the b coefficient. However, this parameter remains poorly studied for OAs. The objective of this study was to estimate and compare the b coefficient of different OAs under field conditions. A field trial was conducted on a clay loam Mollisol in Balcarce, Buenos Aires, using a randomized complete block design with four replicates, testing three OAs: (i) chicken litter (CL), (ii) pig slurry (PS), and (iii) anaerobic digestate (DIG) from a digestor working with CL and PS, applied at four rates equivalent to 50, 100, 150, and 200 kg N ha⁻¹, based on total N concentration. A control treatment without OA was included. Amendments were applied 15 days before wheat (Triticum aestivum L.) sowing. Soil samples (0–10 cm) were collected at the crop sowing and harvest, and Bray-1 extractable P was determined. The total N to total P ratio (Nt:Pt) of the amendments was 3.6 for CL, 4.5 for PS, and 5.6 for DIG, with DIG showing the most favorable ratio relative to cereal crop needs (~6:1). Soil extractable P increased linearly with the amount of P applied, though the magnitude of increase varied among OAs. The PS led to the greatest increases in extractable P per unit of P applied, with b coefficients at sowing and harvest of 0.70 and 0.56 mg kg⁻¹ per kg ha⁻¹ P applied, respectively. In contrast, CL and DIG exhibited lower b values of 0.39 and 0.22 at sowing and harvest, respectively. Based on harvest values, when soil P pools had stabilized, the P requirement (kg P ha⁻¹ needed to increase 1 mg kg⁻¹ of extractable P) was 1.8 for PS, and 4.8 for both CL and DIG. These results suggest that OAs have contrasting impacts on soil P availability, likely due to differences in P chemical forms. This was further explored by quantifying the concentration of water-soluble P (WSP) in the OAs, which confirmed that the WSP:Pt ratio was 0.5 for PS and 0.3 for CL and DIG. Furthermore, when calculating the b coefficient based on WSP, no differences across OAs was observed (1.4 and 1.0 mg kg⁻¹ per kg ha⁻¹ WSP applied at sowing and harvest, respectively). The observed variability among OAs should be considered for nutrient management in cropping systems receiving OA applications. Further research is needed to validate these findings across a wider range of OA types and soil conditions.