with/without biosolids aerial

By Dan M. Sullivan, Oregon State University

applying biosolids
Photos courtesy of City of Portland, OR

cows on rangelandThe impact of repeated biosolids application on grass forage yield has been documented over a 15-year period at a field site near Hermiston, OR (Madison Farms).   Annual precipitation at the site is typically 6 to 8 inches per year. Biosolids were supplied by The City of Portland (OR).  Additional monitoring data from this project will soon be available in a new Pacific Northwest Extension publication prepared by OSU and WSU:  Biosolids in Dryland Cropping Systems. Grass forage data was collected annually in the spring (April or May) from paired sites: a control site receiving no biosolids application and a nearby site receiving biosolids application (see Figure below). Forage at the monitoring sites was not heavily grazed.   Grass forage production varied among years, associated with variable timing and amounts of winter and early spring precipitation. Forage yields increased with biosolids application, especially in wet years. Across all years, forage yields averaged 3310 lb/acre with biosolids vs. 740 lb/acre without biosolids.  Plant-available nitrogen supplied by biosolids is likely the major factor responsible for increased grass production. 

forage yield chart
Above: Grass forage yield (dry matter basis) response to biosolids application. Site near Hermiston, OR. Biosolids were applied every 12 to 24 months at a rate of 3 to 4 dry ton per acre.  

Because biosolids applications promote annual grass growth, heavy grazing in early spring is necessary to make use of the increased forage production. Increased growth of some annual grasses (e.g. cheatgrass, Bromus tectorum) is undesirable on most “native” rangeland sites, so biosolids managers must choose dryland application sites thoughtfully.