When biosolids and other organic residuals are applied to soil, the amount used is determined by the agronomic rate. This means that the number of tons per hectare or acre is determined based on the amount of nutrients in the biosolids/residuals and the amount of nutrients needed by the crop being grown.
Usually, and by law, Class B biosolids are applied based on the amount of nitrogen (N) needed. However, more and more states are requiring calculation of the agronomic rate based on phosphorus (P). Either way, the agronomic rate limits the amount of biosolids or other residual applied, which further reduces risks from excess nutrients that could impact groundwater (e.g. nitrate N) or surface water (P) or from traces of chemical contaminants.
In recent years, animal manures, which have traditionally been applied without any regulation, are being applied in accordance with agronomic rates. Using agronomic rates ensures that the nutrients in biosolids, animal manures, and other residuals are used most efficiently and safely.
Washington State University Application Rate Worksheet (example calculation PDF)