Core Ideas
Phosphorous fertility was evaluated in weathered volcanic soil in winegrape production.
Seasonal Hedley P fractionations, P sorption, microbial biomass carbon, and DOC were investigated.
Application of compost, alone or with TSP, reduced sorption and increased MBC.
Labile Pi responded significantly to fertilization despite advanced pedogenesis.
Significant labile Pi response in weathered soil may be due to low‐Fe lithology.
Phosphorus dynamics and management strategies were investigated in a strongly weathered rhyolitic soil in the northern Coast Ranges of California. Recent conversion of upland, native oak (Quercus spp.) woodland ecosystems to winegrape production has led to a P deficiency in these soils. We investigated the growing season behavior of Hedley phosphorus fractions, the P sorption index (PSI), microbial biomass C (MBC), and dissolved organic C (DOC) in response to a single P application from three sources: composted steer manure (CSM), triple superphosphate (TSP), and mixed compost and triple superphosphate (MIX). Phosphorous sources, equal in P application rate (39 kg ha−1), were applied in the winter of 2012, and soils were sampled in spring, summer, and fall. Labile inorganic P (Pi) (resin Pi + NaHCO3 Pi), Fe/Al‐Pi (NaOH Pi), and total P were significantly (p < 0.01) increased by P application, regardless of source. Calcium‐Pi (HCl Pi), DOC, MBC, and pH were increased, and PSI was reduced, by compost addition. Net extractable Fe/Al‐Pi increased during the study period. Moderately labile organic P (Po) (NaOH Po) declined, independent of fertilization, reflecting background soil P cycling. Calcium‐Pi totals were low (mean 9 mg kg−1 for untreated plots), and other indicators of chemical weathering were high, suggesting an advanced stage of pedogenesis with respect to Walker–Syers P availability. In spite of advanced pedogenesis, labile Pi increased significantly from fertilization and remained elevated throughout the growing season. Results of this study inform P management in highly weathered, P‐fixing soils experiencing P deficiencies.