PRELIMINARY DRIS NORMS FOR COASTAL DOUGLAS-FIR SOILS IN WASHINGTON AND OREGON

The Diagnosis and Recommendation Integrated System (DRIS) has been use d successfully in agricultural crops and holds promise for use in fore st stands. This study used soil tests to develop DRIS norms and evalua te their effectiveness in coastal Douglas-fir (Pseudotsuga menziesii ( Mirb.) France) forests. DRIS norms for nitrogen, phosphorus, potassium , and calcium were developed using soil test and site index data from 72 soil series that commonly support Douglas-fir in western Washington . The norms were tested using soil test and stand basal area growth re sponse data from 20 thinned and 30 unthinned N fertilizer test sites i n coastal Washington and Oregon. Response to urea fertilizer in thinne d stands averaged 34% and 43% for 224 and 448 kg N . ha-1, respectivel y, when N was identified as the most limiting nutrient. When N was not the most limiting nutrient, N response averaged 8% and 10% for 224 an d 448 kg N . ha-1, respectively. Results were similar in unthinned sta nds and thinned stands, although response to fertilizer appeared to be slightly less in unthinned stands when N was the most limiting nutrie nt. DRIS correctly classified 25 of the 33 sites (76%) where N fertili zer increased growth by more than 15%. More importantly, 13 of the 17 (76%) sites that responded by less than 15% were correctly identified by DRIS. The results clearly indicate that N fertilizer response is de pendent on the interactions (balance) between soil nutrients at a give n site. Future soil diagnostic work needs to focus on techniques, like DRIS, that provide an assessment of these interactions.