Compost Can Create Less Nutrients for Plants

Several years ago, when my husband and I started gardening, we created our own raised garden bed soil. My husband actually was our family gardener at first and after a few years, I took on the hobby. We took advice to make our soil using 1/3 peat moss, 1/3 compost, and 1/3 vermiculite or perlite. According to the book I read, if we used that combination, no fertilizers would be needed.

This soil mix seemed to work pretty well. We had garden beds at our previous house, but when we moved into our current house in 2009 we used the same mix for creating our new raised beds. Before every spring planting, we would add some compost to add nutrients to the soil. While we had many harvests, a lot of success, and some failures, it seemed that some of the vegetables that had higher nutrient needs weren’t growing as well.

Last fall, I found my county conservation district provides 5 free laboratory soil tests for county residents. They send soil samples to A&L Western Laboratories in Portland, Oregon. The test includes analysis and recommendations for Nitrogen, Phosphate, Potassium, Magnesium, Calcium, Sodium, Sulfur, Organic Matter, pH, and Cation Exchange Capacity.

I submitted my soil for testing in November. It was a challenge because I had to wait for a time for the soil to not be saturated (it seemed to pour constantly) and a day when I was driving to work. At work I could place the sample in the refrigerator before dropping it off at the conservation district office on the way home. The ideal time to get the soil test would have been in October so that I could add the fertilizers early to let the soil nutrients stabilize before spring planting.

I anxiously waited for the results, which eventually revealed that my test results were not accurate because I should have requested a “nursery media” analysis. So, after requesting the correct test, which provided very different results, I determined that my soil was deficient in nutrients my plants need.

First, my soil is very high in organic matter (compost), but it was very low on nitrogen, potassium, and phosphorus. I was really surprised; doesn’t organic matter and nutrients come hand in hand? After having a discussion with a person at the county conservation district office, he told me to hold off on adding compost for a couple of years.

The nitrogen and organic matter from my media soil test results

What was going on? After some research, I found that as organic material continues to decompose, it actually uses up more nutrients. For example, the bacteria that breaks down the organic matter require a lot of nitrogen. The bacteria will use up the nitrogen before the plants can and will release the stored nitrogen after the bacteria die.

So adding compost right before planting new crops may actually reduce the nitrogen available for the plant. Depending on what the compost is made of, it can take long periods of time for the soil to restore its available nitrogen. A simple solution to this problem is to add compost in the fall after harvest, instead of right before planting. Additionally, adding additional nitrogen to the soil speeds up the decomposition of the organic matter.

The carbon-nitrogen (C/N) ratio determines which materials take longer for the bacteria to go into action.

A higher C/N takes longer to decompose

Have you had the same problem with compost?



Carbon to Nitrogen Ratios – USDA Natural Resources Conservation Service

Master Composting Program – Oregon State University Extension Service

What Kind of Harm Can Come From Soil Organic Matter? – Plant and Soil Sciences, administered by the University of California-Davis and by the National Science Foundation (NSF)

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