Buying Quality Compost

by Brian Rosa, NC DENR Compost Specialist

Boost your yields with a little help from nature’s black gold – just make sure you’re getting the good stuff

Ask questions before you buy to ensure you get the best quality compost. From

Farmers need good, well produced and consistent compost to incorporate into their soil nutrient plan. Organic or non-conventional farmers especially need to use a soil amendment that is high in organic/carbon matter and rich in plant-available nutrients and micronutrients, has good moisture-holding capacity and a diverse microbiological community.

As with everything else, you get what you pay for. Good quality compost is made from good quality inputs or ingredients. Farmers purchasing compost should ask several questions before considering buying any compost.

The first question you should ask is, do you have a waste analysis for your product? A NCDA analysis will tell you a lot about the product: C:N ratio (is it cured), NPK, pH. Some will give you heavy metal contents.

The next question: What are your feedstocks? (manures, municipal yard debris, food residuals, bio-solids, industrial sludges, paper pulp, egg process residuals, ice cream….)? Ask if they have any analysis on these materials, too!

Some of these materials may have components in them that you might not want on or in your fields, such as pharmaceuticals from municipal bio-solids, heavy metals from chicken litter, herbicides in yard debris or farm residuals; contaminants may be present from other sources.

The next question is, “How do you test your end product? What kind of guarantee do you give with your product?” The US Compost Council has a testing program called STA, Seal of Testing Assurance This seal of approval assures the consumer that the product has met certain benchmarks for the way they process their materials. This is a good way to make certain that the individual composter is trying to produce a value-added end product. But, if you are concerned about what goes into your soil, you should always find out as much as you can about what is really in the compost you are about to put on your land to grow food for yourself and others.

The last question you should be asking is the price of the compost.

Look Closely for Quality

Some of the critical components that make up good compost you can see with the naked eye; the microbiology are not so easily seen!

Physically inspect any compost before you purchase. Have a sample sent to you for you to touch, feel, smell and see what you are getting.

Good quality compost should have these physical qualities:

• Consistent appearance, dark brown or black in color

• 40% moisture

• An earthly smell, no offensive odors

• Particle size of < ½” in size

• Stability (capable of being stored without losing its effectiveness)

• No weed seeds

• No visible contaminants

• No phytotoxins (need to have a germination test )

• A pH of 6.0 – 7.8

The invisible quality of compost is a bit harder to determine! Good mature, finished compost will be pathogen (both plant & human) free and low in soluble salt content (preferred < 5dS/m). Compost produced by thermophilic microorganisms in a large, hot composting process can produce a product with these qualities.

But, the thermophilic microbes that are utilized to break down and consume organic materials are not typically found in our soils! Thermophilic microbes are awake and active at >105 F, which is not conducive to plant and soil microbiology. So, these microbes are not of benefit to the farmer, the soil or the plants he wants to grow. The monophilic microorganisms present at ambient temperatures (32 F – 104F) are of more benefit to the soil, plants and farmers. This is not to say that these microbes are not present in compost, but they are not as plentiful as you would like.

The Magic of Microbes and Worms

One way to ensure that your compost, soil and plants are inoculated with these monophilic microbes is to introduce or add worm castings to your compost. Worm castings or vermi-castings are the excrement from worms after organic material has passed through their gut. In the vermi-composting process, those monophilic microorganisms (bacteria, fungi, nematodes) break down organic matter small enough and soft enough for the worms to suck this material into their mouth. As this material passes through the gut of the worm, other microorganisms are introduced to further break this material down. The worm takes what it needs and the material passes through the worm. In this process, a high calcium digestive juice is generated in the gut of the worm to help with digestion, plant and human pathogens are destroyed and many other beneficial microorganisms are developed and excreted. The end result, called worm castings, is humus-like material that is pathogen-free and high in calcium, beneficial plant and soil microbes, plant growth hormones and nutrients. Studies have shown that a 10-20% addition of worm castings to your soil medium will greatly increase germination, plant growth rates and fruit, veggie, and crop production.


When purchasing worm castings, the same quality concerns apply as they do with compost! Ask questions: What is your feedstock?; Where did it come from, how did you process your materials, what documentation is there to support their claims? After all, you are going to be using that material to improve the soil so you can grow food for your family, neighbors and customers. And, don’t they deserve the best?

Brian Rosa is an Organic Recycling Specialist at the NC Department of Environment and Natural Resources. You can reach him at:

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