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Biosolids are safely used to improve soils and provide essential nutrients for plant growth. In some parts of the country, biosolids have been used as fertilizer for more than 70 years. The beneficial use of biosolids is one of the most extensively studied practices in the U.S. because of its relationship to the federal Clean Water Act and wastewater treatment regulations.

Decades of research by scientists throughout the country form the basis of biosolids regulations and best management practices. Ongoing field research, as well as regular monitoring of biosolids quality, provide assurance that biosolids use is sustainable, safe, and effective.


Nitrogen is one of the most commonly used fertilizers. An essential component of many plant structures, lack of nitrogen is often the primary factor that limits plant growth. By increasing the supply of available nitrogen in the soil, it is possible to stimulate growth and produce larger, healthier plants.

Biosolids contain the nutrients that plants need for healthy, vigorous growth. Loop is rich in nitrogen and phosphorus, and has all the micronutrients not typically found in synthetic fertilizers. Similar to using other fertilizers, proper nutrient management is essential for using biosolids effectively. The key is to supply enough nutrients to meet plant needs without adding excess amounts that could pollute water resources. 

Loop contains 6 to 7 percent total nitrogen. About 1% of this is in the form of ammonia nitrogen, which is immediately available to plants. The remainder is organic nitrogen, which is not available until the organic matter decomposes. As this decomposition takes place, the nitrogen is gradually released, supplying additional nitrogen during the growing season.

Lush Green Wheat FieldLoop supplies carbon and nutrients for lush plant growth


King County conducts monthly monitoring of Loop’s physical, chemical, and microbial properties. This testing allows us to verify that Loop is consistently high in quality and in compliance with state and national regulations. Trace metals in Loop are far below the established thresholds for safety.


Regulated Metals in Loop Biosolids