Loop is the nutrient-rich ingredient in GroCo, a fluffy, rich, stable and odor-free compost. It’s commercially available for use in flower and vegetable gardens and landscaping.

Compost made with Loop, sold as GroCo compost, provides organic matter to soils and improves the physical properties of both sandy and clayey soils. Compost made with Loop is appreciated by gardeners and commercial landscapers for its ability to aerate soil, retain moisture, and produce beautiful blooms. Compost made with Loop also helps to protect Puget Sound because it helps urban soils retain rainwater like a sponge, which reduces the amount of dirty stormwater flowing into Puget Sound. With a full suite of nutrients, compost made with Loop has enriched Pacific Northwest landscapes since 1976.

How GroCo compost is created

To create GroCo compost, one part Loop is mixed with three parts sawdust from local lumber mills. At GroCo Inc., the sawdust-Loop mixture is stacked into large piles that are turned regularly to keep them aerated and hot. In addition to laboratory testing, the temperature of the piles is regularly monitored to ensure that disease causing organisms are killed so that it meets EPA pathogen reduction requirements. After all pathogens have been killed off by high heat, the compost piles are left to mature for about a year. Once GroCo compost is stable, mature, and pathogen-free, it’s ready for sale. All of the nutrients found in Loop are also found in GroCo compost, although at lower concentrations because of the blending with sawdust.

GroCo is made from sawdust and biosolids. Other common composts are made from yard trimmings, food scraps, or manure. All composts provide some nutrients for plants and help build the soil, but GroCo compost is special because it is made with Loop.

How to use GroCo

Add it to your garden soil, containers, or indoor pots to turn your dirt around and get lush plant growth.

  • For annual flower and vegetable beds add 1-3” to the top of your garden soil and till it in 6-12” deep each spring.
  • For turf add 1/2-1” and rake evenly across the grass once a year.
  • For perennial shrubs and trees, add 1-3” to the soil surface. As with any compost product or bark mulch, be sure to leave a 3” border around plant stems and trunks.
  • Freshen up indoor plants by sprinkling a bit 1/2-2” of compost to the surface of the pot.

Total Nitrogen: 1.24%
Plant Available Nitrogen: 0.24%
Organic Nitrogen (slow-release): 1.00%
Phosphorus (as P2O5): 2.04%
Potassium (as K2O): 0.19%
Sulfur: 0.33%
Calcium: 0.90%
Magnesium: 0.23%
pH: 6.8
Organic matter: 85.6%
C to N ratio: 35

All values are reported on a dry weight basis.

Who uses GroCo

Gardener and soil scientist, Kate Kurtz, discusses why she chooses GroCo compost made with Loop® biosolids in her home vegetable garden. For Kate, it’s a choice of environmental ethics and she knows it grows a lush, beautiful garden


Local gardeners and landscapers have also used GroCo compost for decades, and GroCo is used on a ton of awesome community projects. GroCo compost and recycled water are two endlessly renewable resources used at our City Soil Farm, located at the King County South Treatment Plant in Renton.

To learn more about City Soil and to see who else turns their dirt around with GroCo compost made with Loop, visit Our Partners.