Soil acidification, a major issue responsible for cropland degradation, can be efficiently addressed by forest-derived alkaline residues. However, indirect effect of these by-products on phosphorus (P) forms and their availability to crops is not well recognized in temperate acidic soils. Using the Hedley fractionation procedure, a study was conducted to characterize the P forms, after a 40-week laboratory incubation, of a gleyed clay and a podzolised sandy loam soils, each receiving six different forest materials and a calcitic lime. Lime mud, two types of wood ash (papermill biosolids and wood bark), two biochars (maple and pine), and a de-inking paper sludge were applied at a CaCO3 equivalence based-rate to achieve a target pH of 6.5 in each soil. Results indicated that all liming materials decreased organic P (Po) in both soils. However, the inorganic P (Pi) fractions reacted differently depending on soil. For the clay, there was a shift of P from organic pools (NaHCO3-Po and NaOH-Po) and NaOH-Pi towards NaHCO3-Pi, an available P form for crops. For the sandy loam, P from organic pools and NaHCO3-Pi moved towards stable pools (HCl-P + residual P), mostly associated with Ca. This redistribution of P was largely associated with the soil pH increases. Apart from pH, wood ash also contributed to increase soil total Pi owing to their P content. By contrast, pine biochar had minimal effect on soil P fractions. We conclude that addition of forest alkaline residues to cropland improved soil pH and also modified the P availability to plants.
GAGNON et al. (Thu,) studied this question.