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I'm going to be amending my garden soil soon, after I take care of all the rocks, so I'll be dumping in a lot of compost to fix the horrible clay soil I'm left with.

Should I be worried about adding too much compost, or should I just add as much as I possibly can?

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It is very hard to add too much good compost, I don't think you will have that problem. But, a healthy soil does have a good amount of mineral content. If what you are amending is mostly subsoil, the ratio of compost/soil can be very high.

When organic matter decomposes, it makes humic acid, which is commonly used in labs to break apart minerals. Subsoil is almost all mineral, but soil tests often show a very small part of those minerals are available to plants. This makes most people want to add powdered minerals to the soil, to bring up the number to where it should be, not realizing that all the minerals are there, all they need is to apply organic matter, especially with some that is still actively decomposing. This will drastically increase the amount of available minerals in the existing soil. The humus rich material will also aid in drainage and water retaining capacity, as well as providing the plants with the ideal root zone.

So it's important that there is still some mineral soil mixed in, at least for most plants. And it helps to have decomposing plant matter on the soil at all times, like quickly decomposing mulches (grass clippings come to mind), to keep the available mineral content up where it needs to be for best plant health.

Also, as decomposing plant matter produces acid, compost is often acidic, so I'd test the soil after the compost has been added, to see if it is at a good level for the growth of the plants you intend to cultivate in that area.

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  • I'd like to add that the organic matter will host and support healthy soil biology. It is the microorganisms in the soil that are responsible for nutrient cycling and making nutrients available to your plants. – That Idiot Sep 26 '14 at 19:47
  • @ThatIdiot feel free. – J. Musser Sep 26 '14 at 19:48
  • Got timed out in my last edit session looking for source, but excessive compost can cause levels of some nutrients to rise to the point that they become polluters. And though it is more relevant in turf (which use very little phosphorus) applications of compost, repeated/excessive applications have been shown to cause elevated phosphorous in soils where phosphorous is not rapidly removed/used by plants. newenglandvfc.org/pdf_proceedings/SoilOrganicAmend.pdf – That Idiot Sep 26 '14 at 19:58
  • @ThatIdiot Good point, where your compost contains high levels of phosphorus. Some compost is (very) low in npk, and basically adds organic matter. Some compost, on the other hand, is almost fertilizer quality. – J. Musser Sep 26 '14 at 20:01

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