Predicting Phosphorus Retention in Volcanic-Derived Forest Soils of Northern California.

Plant available phosphorus (P) occurs in anionic forms which are retained when iron and aluminum oxides form insoluble complexes. P-retention is likely to occur under acid conditions in soils containing amorphous (poorly crystalline) materials, namely allophane and imogolite. This condition is common in volcanic-derived forest soils in Pacific Northwest timber regions.  We investigated P-retention in Powellton (Fine-loamy, parasesquic, mesic Andic Haplohumults) and Aiken (Fine, parasesquic, mesic Xeric Haplohumults) soil series. Soil materials were used from the Whitmore and Feather Falls “Garden of Eden” experiments. Some soils received applications of N, P and K beginning in 1985; unfertilized soils were compared as a control. Results show that NaF pH and New Zealand P-retention (NZP) decreased with soil depth. The Aiken series showed higher NaF pH and NZP with depth compared to the Powellton series. This result is consistent with other studies showing that Powellton exhibits higher crystallinity, though parent materials are similar. Simple regressions of NaF pH versus NZP show r² values greater than 0.8. Further work will be presented which probes the relationship of NZP and NaF pH with soil properties (C:N and mineralogy) and treatments (fertilized, unfertilized). We will also present results comparing the NaF field test versus NaF lab test, the former commonly used to identify andic soil properties.