Phosphorus loss from organic versus inorganic fertilizers used in alleycropping on a Florida Ultisol

The status of residual phosphorus (P) and nitrogen (N) in agricultural soils is an environmental concern due to the possibility of the nutrients reaching adjacent water bodies through leaching or runoff. This concern is of particular importance in sandy and other coarse-textured, highly permeable soils. A 3-year study examined a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alleycropping system situated on an Ultisol in northwest Florida, USA, to determine the effects of tree–root competition and fertilizer application (poultry litter versus inorganic) on soil nutrient status with time. Overall, competition for P between tree and crop roots appeared to be minimal in these soils. However, build-up of P occurred in soil over time. After 2 years of fertilizer application in the alleycropping system, the average concentration of Mehlich 1-P in the surface soil (0–5 cm) increased from 74 to 106 mg kg−1, and water soluble P increased from 5.9 to 7.2 mg kg−1. In addition, accumulation of P in near-surface soils was slightly higher in manure-treated soils after the first year of treatment. A clay layer in the Ultisol at approximately 25 cm largely prevented further downward movement of P. Overall, in terms of potential environmental risks from P accumulation and loss via runoff or leaching in these soils, the organic fertilizer appears to pose a greater long-term risk than the inorganic fertilizer if application is based on plant N requirement (a common practice) rather than P requirement.