What Makes a Good Compost?

Compost quality can be hard to judge. Part of the quality is what you see and what you feel and smell.

It should smell good. A good quality compost should smell good. It should not have any off odors. No smell of rotten eggs, no smell of ammonia.

Compost should look and smell good.

It should look good. It should be dark brown when moist. It should be ALL brown and not some of it brown and some of it not. It should be consistently brown throughout. You should not be able to discern or see any of the products used to make the compost. Leaves and stems should not be discernable. It should be screened so that larger materials have been filtered out. Some composts are screened with multiple screens to include 1/2 inch and smaller particles (1/2 inch minus). Some are screened even smaller than that… 3/8 inch minus or even 1/4 inch. Screening depends on its use. If it is used on let’s say a golf course on greens then large paricles of compost will interfere with the roll of a golf ball when grass is cut at 3/8 or 1/4 inch.

It should be cool. Good quality compost has matured to the point where it no longer produces alot of heat. Compost that is still hot has not finished composting and is immature. Part of the compost process is setting it aside to “cure” after the composting process has completed.

Compost also has qualities that you cannot see.
Pathogens. Generally speaking, commercial composts usually have fewer pathogens that can affect human health than homemade composts. Commercial operations can spend time monitoring and managing a compost pile more effectively than a gardener. Good commercial operations monitor the temperature and moisture contents (and even the oxygen and carbon dioxide levels) so they know when to turn the pile for better aeration, better temperature control and more even processing of the compost. Techniques like in-vessel composting and windrows can generally make a product that has fewer human pathogens in it than static piles that are not monitored carefully.

Chemistry. Even though not considered a fertilizer by law, composts contain plant nutrients. They will add “fertilizer” to a garden. They add lots of other things as well. Trace minerals are present along with organic acids that improve soil chemistry. It is always a good idea to ask for compost test results. All commercial compost operations have them. They should provide a copy if you ask them about it.

There are potentially some things in compost that we need to take a close look at. Salts. All composts will have salt in them. Fertilizers are salts. Some salts are good and some are not as good. Major salts that can be problems for us in our soils are salts of sodium, chloride, sulfates and boron. Salt levels should not be excessive and the salts that concern us should be minimized. I will give more information on these in future postings. Composts that come from large urban centers can contain heavy metals. In commercial composts the level of heavy metals allowed in commercial composts is highly regulated and monitored. They are not in non commercial composts.

Biosolids. Some composts contain biosolids. Biosolids is the preferred name to sludge. This is becoming more and more common as our federal and municipal governments are trying to find an alternative to placing them in landfills. This will become even more common in the future. The use of biosolids is highly regulated in the commercial compost industry with federal limits established by the federal government. I will be discussing this important issue in future postings.

In short, you should and must know what is in your compost before applying it to your gardens. Ask for reports on what is in the compost you are purchasing. Make sure that the facility is submitting samples regularly to compost testing facilities.