Ash Cap Influences on Site Productivity and Fertilizer Response in Forests of the Inland Northwest

Data from 139 research sites throughout the Inland Northwest were analyzed for effects of ash cap on site productivity, nutrient availability and fertilization response. Stand productivity and nitrogen (N) fertilizer response were greater on sites with ash cap than on sites without. Where ash was present, depth of ash had no effect on site productivity or N fertilizer response. Site productivity increased with increasing potentially available soil water. Potentially available soil water, in turn, increased with increasing ash depth. Decreasing availability of magnesium (Mg) and calcium (Ca) in the upper 12 inches of soil may have constrained site productivity with increasing ash depth. Soil mineralizable N and ammonium N were unaffected by ash presence or depth but did vary by underlying parent material. Site productivity and N fertilizer response varied by underlying parent material even after accounting for ash presence. Stand volume response to sulfur (S) fertilization decreased with increasing ash depth, suggesting possible S adsorption by ash cap. No growth response or change in foliar K status was discerned following K fertilization. Management recommendations by parent material are provided. Ash Cap Influences on Site Productivity and Fertilizer Response in Forests of the Inland Northwest Mariann T. Garrison-Johnston, Peter G. Mika, Dan L. Miller, Phil Cannon, and Leonard R. Johnson