UC Davis

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.