No Such Thing as Too Much Compost?

Hi. In response to my October 23rd video entitled “Am I Using Too Much Compost & Mulch”, there were a number of comments that could be summed up by the phrase “there’s no such thing as too much compost”. This is something I’ve likely said myself in the past. So, today I thought I’d try to give you a better idea of what led me to address this topic in a video. I decided to make the video in response to a viewer who suggested that too much compost could lead to a buildup of excessive nutrients in the soil, resulting in plant and soil health problems as well as pollution of adjacent natural waters. As a reference, the viewer provided a link to Dr. Linda Chalker-Scott’s article “The Myth of Soil Amendments, Part III”. Dr. Chalker-Scott’s article focuses primarily on permanent landscape installations, but touches briefly on annual vegetable gardens. Regarding vegetable gardens, she states that “the annual incorporation of compost makes sense here. However, one needs to have an idea of what the soil already contains before more material is added.” It’s hard to argue with this, especially considering how little most agriculture extension offices charge for a soil test. So, I decided that in the spring I’d have our soil tested to determine if the amount of compost we’ve been using has led to large nutrient surpluses, or if there are other issues that need to be addressed. Some of you might be thinking “wouldn’t large nutrient surpluses be a good thing”? This gets us back to the potential problems I mentioned briefly a moment ago: plant and soil health problems and pollution of adjacent natural waters. As an illustration of these problems, Dr. Chalker-Scott describes an organic demonstration garden in which a large amount of nutrient-rich compost was added to the soil. “Last fall we collected soil samples from a local organic demonstration garden and sent them out for nutrient analysis; this garden had recently experienced some soil and plant health problems. Every single one of the sites that was tested came back with nutrient readings off the scale. In large capital letters the report warned “DO NOT FERTILIZE THIS SOIL.” The excessive addition of nutrient-rich compost to this landscape contributed not only to plant health problems but to nutrient loading of adjacent natural waters.” Much to my surprise, the nutrient surplus and resulting problems occurred despite the fact that no additional fertilizers were used. Now let’s look at some specific plant health problems that can occur. In the case of the demonstration garden, the vegetation was excessively lush but there were few plant resources going into defensive compounds that would help the plants defend themselves from pests. As a result, the garden had many pest problems. In addition, phosphorus surpluses from excessive compost applications can lead to nutrient imbalances. For example, excess phosphorus can interfere with a plant’s ability to uptake iron and manganese, resulting in deficiencies in these minerals and leaf chlorosis. And, as Stephen Legaree and I discussed in our recent mycorrhizae video, excess phosphorus inhibits the formation of mycorrhizae, which assist plants in acquiring water and nutrients. Without mycorrhizal relationships, plants have to put more energy into establishing root systems and acquiring nutrients on their own. Finally, phosphorus surpluses can contribute to non-point source pollution. To quote Dr. Chalker-Scott’s article “The Myth of Phosphate Fertilizer”, “Excess phosphate will eventually find its way into waterways. aquatic plants are most often limited by phosphate and the addition of phosphate will induce algal blooms (eutrophication). Such blooms are always followed by increased bacterial activity, resulting in lowered oxygen levels and the eventual death of fish and other animals.” So, yes, there is such a thing as too much compost. Though we haven’t seen plant health problems related to nutrient surpluses, the best way to determine if we can reduce our use of compost is with a soil test. Plants only uptake nutrients they need, so if the soil test shows nutrient surpluses, we can save time and effort by reducing the amount of compost we make and apply to the garden. We shall see in the spring. Well, that’s all for now. Thank you very much for watching, and until next time remember you can change the world one yard at a time.

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