Soil Parent Material Influence on Douglas-Fir Growth Response to Multi-Nutrient Fertilizers on the Inland Northwest, USA.

Douglas-fir (Pseudotsuga menziesii) is an important tree species on the Inland Northwest, however its growth may be limited by nitrogen (N) or by a combination of nutrients, depending on forest soil parent materials.  We fertilized 816 individual Douglas-fir trees (5-15 feet in height in low density stands), across the Inland Northwest, each with one of three fertilizer levels, control, N-only, or a multi-nutrient blend.  The trees were located on a variety of soil parent materials commonly found in the region, allowing us to test the influence of soil parent material on Douglas-fir nutrient status in a two-way factorial design.  Parent materials were considered as combinations of surface deposits of volcanic ash and loess over underlying lithologies.   On some soil parent materials, only N fertilizer was needed to increase growth, while on others a multi-nutrient blend was required to increase growth.  Still on a few parent materials, growth did not respond to nutrient additions.  The N-only fertilizer increased individual Douglas-fir basal area growth by 24% (ash over siltite), 51% (loess over siltite), 79% (ash over loess over siltite), 33% (ash over glacial deposits), 54% (ash over loess over quartzite) and 20% (ash over loess over basalt).  The multi-nutrient fertilizer increased basal area growth by 51% (ash over quartzite), 29% (ash over quartz diorite), 18% (ash over schist), 40% (ash over schist-gneiss), 41% (ash over basalt) and 29% (loess over Tertiary sediments).  No fertilization response was observed on ash over granodiorite, ash over Tertiary sediments, or basalt without surface deposits.  The trees on the sites that did not respond to fertilizer application grew relatively slowly compared to the control growth of the overall study, indicating another growth limitation on those trees, such as moisture and small relative size.  Douglas-firs on siltite, regardless of the combination of surface deposits, only require N to increase growth.  Loessial deposits appear to alleviate multi-nutrient growth limitations on a variety of underlying lithologies, whereas surface deposits of ash alone do not appear to alter nutrient growth limitation, and may even diminish the nutritional contribution of underlying mafic materials.