Effects of residual struvite granules and soil grinding on soil test phosphorus across contrasting soils in an incubation study

Phosphorus (P) is an essential crop nutrient derived from non-renewable phosphate rock, our continued reliance on it poses severe long-term risks to global food security. Struvite (NH4MgPO4·6H2O) recovered from wastewater is a promising, slow-release alternative. However, its low solubility challenges standard soil testing, as residual granules can artificially elevate or conceal extractable P concentrations depending on sample preparation. This study evaluated the effect of residual struvite granules and soil grinding on Olsen-P concentrations over 140 days in two contrasting soils. An incubation experiment was conducted using 20 g soil samples, four fertilizer treatments (i.e., Mono Ammonium Phosphate (MAP), Struvite with residual granules retained for extraction, Struvite with residual granules removed before extraction, and control; P application rate 215 mg kg-1), and two sample preparation methods (grinding or no grinding). The results showed that when residual struvite granules were retained in the soil sample for extraction, grinding produced very high early Olsen-P concentrations (182 mg kg-1), whereas unground samples showed much lower values (54.5 mg kg-1) because intact granules did not fully interact with the extractant. Under grinding, the retained and removed treatments remained significantly different until Day 84; without grinding, they differed only on Day 1, indicating that physical fragmentation strongly drives early extractable P. In contrast, Olsen-P in MAP treatments decreased steadily over time, with no effect of grinding. These findings highlight how sample preparation and extraction conditions shape the interpretation of P availability from slow-release struvite fertilizers in soil tests.