Nutrients in Salmonid Ecosystems: Sustaining Production and Biodiversity

Evaluations of Slow-Release Fertilizer for Rehabilitating Oligotrophic Streams

Megan S. Sterling and Kenneth I. Ashley


Abstract.—A solid briquette fertilizer for use in the Pacific Northwest streams and elsewhere was identified from a variety of slow-release formulations (26 were tested with varying N:P2O5:K2O ratios and binders) using indoor trough and controlled field experiments. The use of a slow-release fertilizer is an innovative method for adding inorganic nutrients to nutrientpoor (oligotrophic) streams to increase autotrophic production and aid in the restoration of salmonid populations. A series of indoor trough experiments demonstrated that the majority of samples containing binders of molasses, hydrated lime, vegetable oil, bentonite, starch, acrawax, candle wax, and Daratak® XB-3631 (unpolymerized Saran™) dissolved too slowly. The fastest dissolution rates occurred with fertilizer briquettes having no binder or vegetable oil. Further trough and field studies using fertilizer with no binder and vegetable oil as binder examined the effects of varying N:P2O5:K2O ratios. Dissolution rates were varied by using different percentages of magnesium ammonium phosphate (MagAmP; its formula 7:40:0 N:P2O5:K2O) and urea (46:0:0). Optimal continual nutrient release for a period of four months was achieved with a fertilizer formulation of 17:30:0 (percent by weight N:P2O5:K2O), with a ratio of 75% MagAmP to 25% urea, and containing no binder. The dissolution rate for this product ranged from 4.6% to 6.6% per week (for field and trough experiments, respectively) in water of 0.15 m/s average velocity. These studies indicate that a slow-release fertilizer product can be manufactured to last approximately four months when applied in the spring to stimulate autotrophic production in nutrient deficient streams, thereby increasing forage and salmonid production.