Caitlin Youngquist

Not all compost is created equal — so how can you tell if what you have will help or harm your soil? There are a few simple ways to assess compost quality based on how it smells and looks.

Beyond sight and smell, laboratory tests and bioassays are helpful to assess for heavy metals or persistent herbicides.

Quality compost is mature and no longer warm to the touch, or steaming when you dig into the pile. If the compost is still actively decomposing it likely contains phytotoxic (toxic to plants) organic acids that will harm plants or seeds. If you are concerned about immature compost, let it sit and cure until it cools (and smells better), or spread it on the soil a few weeks before planting to let the soil microbes start to work and any phytotoxic compounds dissipate. Once the compost is distributed in the soil, decomposition will continue at a slower rate.

Quality compost also smells good. If the compost smells “earthy” and mild it is likely mature and ready to use. If it smells like ammonia, it needs more time to mature. Immature compost is often called “hot” even though it might not be hot in terms of temperature. After the active phase of decomposition is over, the compost still needs to go through a curing phase. During this time, the temperature is no longer elevated, different microbes move in to do the work, and ammonia is converted to nitrate.

The ammonia in immature compost can cause the plants to wilt or look scorched on the edges of leaves, leading to the expression that “hot” compost can “burn” plants.

Quality compost should be uniform and dark in color. The raw materials (food scraps, manure, leaves, etc) should be unrecognizable. The dark color indicates that the raw materials have been thoroughly broken down and humified (converted to humus) by the microbes in the compost pile.

If you make compost at home out of yard and food waste, the finished product may still contain small sticks, corn cobs, fruit pits, etc. While these larger items may be unsightly or inconvenient in the garden they will not cause any harm! They will continue to decompose over time and feed the soil.

Even if your compost looks and smells good, it could contain persistent herbicides or heavy metals. If you make your own compost this is a very low risk, as you know what goes into it.

However, if you are purchasing compost be sure to inquire about laboratory tests for compost quality. All commercial compost facilities should test every batch of compost and provide those test results on request.

It is uncommon to get compost that is contaminated with persistent herbicides. However, on the rare occasion this does happen, the results can be devastating!

Most herbicides break down relatively quickly in compost (within a few weeks). However, there are a few that will persist in soil and compost for several years.

The active ingredients in these persistent herbicides mimic natural plant growth hormones that control plant cell growth and elongation.

If you are concerned that your compost is contaminated with herbicides the easiest way to test it is by growing sensitive plants in a combination of compost and soil to see how they look. This is called a bioassay. Peas and beans are very sensitive to herbicides, germinate relatively quickly, and are easy to handle.

You may have been told that compost and manure are high in salts and will “burn” your plants.

Salts are actually plant nutrients and essential for life in the soil. The problem is that when they are too concentrated in the compost, or the soil, they cause plant stress.

Excess salts cause a condition called physiological drought, and scorching of the leaf margins on sensitive plants (leading to the term “burning”). Compost that is higher in nutrients (ie. higher in salts) is best used as a soil amendment and not for direct planting or as part of a seed starting mix.

If you purchase compost for your garden this year, make sure it looks and smells good, and is well cured. And you can always ask for the results of a nutrient analysis and bioassay.

(Dr. Caitlin Youngquist is a soil scientist and the owner of Dirt Works - a compost and soil company located in Worland, for more information visit dirtworkswyo.com.)