When I first encountered this concept—that a gardener could use too much compost—I immediately thought, “Is that even possible?” As an organic veggie gardener dealing with soil comprised of decomposed granite punctuated by lumps of sticky clay, too much compost seemed an impossibility. Isn’t compost the answer to all our gardening problems?
It’s true that “add compost” (or other organic matter, such as peat moss or leaf mold) is the best advice for gardeners dealing with either clay or sand. Organic matter opens up the solidly packed clay particles, allowing air and water—and therefore roots—to penetrate what would otherwise be an impervious substrate. In sand, organic matter acts as a sponge, holding on to both water and nutrients that would otherwise quickly drain away.
Compost doesn’t just add humus—its organic matter contains a whole array of chemicals that feed our plants. Plus, compost introduces micro-organisms that break down those chemicals, converting their nutrients into forms accessible to plant roots. Trying to grow plants in soil devoid of organic matter really amounts to in-ground hydroponics.
It’s easy to assume that if some compost is good, more is better. However, there can be too much of a good thing. The ideal gardening loam contains 45% minerals (sand, silt, clay), 25% water, 25% air—and only 5% (by weight; 10% by volume) organic matter. While most Colorado soils start with far less than 5%, it still is only a minor constituent.
The problem is that too much compost throws the soil’s nutrient balance out of whack. Healthy soil contains just the right nutrients in just the right amounts. Randomly adding fertilizers and amendments can actually create problems.
For example, adding compost tends to raise potassium levels. While the proper amount of potassium is essential, too much potassium interferes with the availability of boron and manganese, micronutrients equally necessary for plant growth.
According to the Penn State College of Agricultural Sciences-Coooperative Extension:
Applying too much compost or manure can lead to excessive nutrient levels. Excessive levels in the soil can be as detrimental as deficient levels. … Availability of nutrients to your crops will depend on the balance in your soil. For example, if too much compost is applied and very high phosphorus levels are detected (above 140 pounds per acre for vegetable crops) in the soil, iron and zinc may become deficient. … If [there is] too much potassium, [and] magnesium and or/calcium are high, serious nutrient imbalances can occur. When levels are above about 5% saturation; magnesium levels 15%; and calcium levels at 80%, soil nutrition is beginning to get out of the optimum range. When excessive nitrogen is applied, delayed fruit, pest problems and reduced quality may occur.
So, how much compost should we add? It depends.
There is no way to look at the soil in a vegetable garden and know which nutrients are available in abundance and which ones are lacking. Plus, most gardeners tend to underestimate the amount of organic matter in their soil. Soil tests take the guesswork out of adding compost and other amendments.
Soil tests are available for a reasonable price through both private laboratories and your state extension. In Colorado, CSU offers soil testing through their Soil-Water-Plant Laboratory. You can learn more about the Soil Testing Procedure at the CSU website.
For further reading: The Myth of Soil Amendments, Part III – “Healthy soil has high organic content” and Putting Compost In Its Place at “High Brix Gardens” (please note I don’t endorse everything on the High Brix Gardens website).