Surface-spring-applied compost replenishes plant available phosphorus, but crop uptake depends on soil moisture

Supplying phosphorus (P) to crops on organic farms with low soil test P (STP) remains a major challenge. This study evaluated surface- spring-applied compost effects on spring wheat (Triticum aestivum L.) grown under low STP (3 ppm) conditions over three years in Manitoba, Canada. Compost application increased grain yield and P uptake in wet but not in drought conditions, where plant-available P remained unattainable to the crop and drought reduced P uptake efficiency (34 vs 79%). Despite poor crop uptake under drought, spring-applied compost increased mid-season STP, demonstrating that surface-applied compost at time of seeding was effective at replenishing STP.