Garden Tips: Compost can be a garden’s downfall

While gardeners know that garden soil may benefit from compost, they may not know about the problems that can arise when adding too much or using poor quality compost.

Let’s take time to chat about the pitfalls of adding compost to garden soil.

While compost is considered the holy grail of garden organic matter, there are no set standards for it. The quality of compost varies with the types of materials and the composting processes used.

Mature compost is one where the organic materials are fully broken down into stable organic matter. Quality compost is mature compost that is not high in salts, contains no contaminants from industrial waste, has few weed seeds and can provide plant nutrients.

Discern quality compost by looks. If buying commercially made compost, ask for a copy of the laboratory analysis provided by the compost producer. Look for the carbon to nitrogen ratio (C:N) that indicates the stability of the organic matter (OM). A ratio between 12:1 to 15:1 is an indication that the OM is stable.

If the ratio is less than 10:1, it is an indication that it contains organic materials that are still in the process of decomposing. A ratio above 25:1 shows that the compost contains high carbon materials that break down slowly and will tie-up available nitrogen as it decomposes, depriving plants of nitrogen and hampering plant growth.

On the analysis, look for the EC, or electrical conductivity, of the compost. This is a measure of the soluble salts in the soil. High soluble salt levels are harmful to plant roots. Compost with an EC above 8mmho/cm are high in salts and should be avoided. It is better to buy compost with an EC that is between 0 and 4 mmhos/cm.

Also, pay attention to the percent (by dry weight) of organic matter in the compost. If the percentage is lower than 30 percent, it means that soil or sand have been added to the mix. If higher than 60 percent, it is unfinished or immature compost containing undecomposed organic materials.

Be aware that each batch of compost a producer makes varies in its analysis. One time, the salt levels may be acceptable, and the next time, they may be too high. So check the analysis each time you buy compost, even if it comes from the same supplier or producer.

While local soils often benefit from the addition of quality compost, it is possible to over do it. Too much compost can cause problems, including excess nutrient levels, especially nitrogen and phosphorous, high soluble salts, and excessive levels of organic matter. (Levels of organic matter above 5 percent to 8 percent by weight are too high.)

The rule of thumb when adding compost to soil in vegetable gardens or annual flower beds is to add no more than 2 to 3 inches of quality, low in salt compost. The compost should be thoroughly incorporated into the top 6 to 8 inches of soil before planting.

Done annually, the level of organic matter in the soil will increase. To avoid excessive levels of organic matter, reduce the amount of compost to only 1 inch after three years, or get a soil test to determine the level of organic matter in your soil. As with so many things in life, too much of a good thing can be bad. The same goes for compost.

This story was originally published July 31, 2016 at 7:01 AM with the headline "Garden Tips: Compost can be a garden’s downfall."