Andic Soils and Phosphorus Nutrition In Forests Of The Inland Pacific Northwest.

Soils formed in volcanic ash are extensive in mid- to high-elevation forests of the Inland Pacific Northwest and support the most productive forests of the region. These andic soils contain varying quantities of allophane and ferrihydrite, and are defined, in part, by their ability to strongly sorb and retain large quantities of phosphorus. P retention occurs via formation of inner-sphere complexes on high and low affinity sites, and precipitation of Al-phosphate minerals. We examined available soil P, foliar P, and andic soil properties for a number of forest soils across the region exhibiting varying andic influence. P retention increases and available soil P decreases with increasing andic influence. As much as 3615 mg kg^-1 of P is contained in the acid-oxalate-extractable phase of these soils, indicating substantial occlusion of P by nano-crystalline minerals. Despite the strong P sorption capability of andic soils, studies show that foliar P concentrations generally exceed established critical values of 1.52 mg g^-1. Although P and nitrogen foliar nutrient ratios in Ponderosa pine show a negative correlation with increasing andic influence, this is primarily a consequence of increasing foliar biomass rather than an indication of P deficiency. Results suggest that andic forest soils of the region do not limit P availability to the extent that tree growth is adversely affected, contrary to what might be expected. Moreover, limitations on P availability imposed by nano-crystalline minerals appear to be offset by other factors, such as P cycling in litter layers and/or mycorrhizal associations.