In the spirit of impartial examination, let us examine popular amendments for clay soil and find out which ones work, and which ones don’t.
Adding sand to clay to improve texture is misguided gardening advice! For sand to work, you have to use coarse sand, and add at least 30 percent by volume to lighten clay soil. Most sustainable landscape architects suggest 50 percent by volume of sand added to your soil.
My verdict: Ridiculously time-consuming and expensive advice.
Adding gypsum (calcium sulfate) to loosen clay soil is another age-old garden advice. How does gypsum work? And does it stand up under scrutiny?
The calcium in the gypsum flocculates the tiny clay particles into larger ones, loosening the clay soil: improving structure and drainage. And it only works if your soil is low in calcium, says Doug Green.
If your soil is based on limestone parent material (like most of the mid-west) then gypsum isn’t going to work for you. If your soil is a Southern US soil – based on different parent material like iron, the gypsum will work
My verdict: it depends on what kind of clay soil you have.
Compost and other organic amendments
The most common advice given to gardeners with clay soil is to amend it with organic matter. Lots of organic matter. And you hear it over and over again. But does it work?
Owen Dell says that for clay soil, adding organic matter (OM) provides no long-term benefit. I suspect he means amending (i.e. mixing in organic matter – manure, compost, leaf mold – into the soil).
Dr. Linda Chalker-Scott, Urban Horticulture Extension specialist warns gardeners that if OM is applied in excess, it will cause pollution problems just like commercial fertilizers do. A heavily amended soil (33% OM) releases much greater amounts of nitrogen, phosphorus and other nutrients than an ideal soil (i.e. 5% OM).
My verdict: The jury is still out on the long-term effect of organic matter on clay soil.
However, Dell admits that adding organic mulch to clay soil will modify the clay soil, but with the caveat that it takes years or decades before the clay soil will loosen up.
My verdict: Use mulch. It will eventually loosen up the clay soil and it may not take as long as Owen Dell claims.
In the meantime, while we mulch, still select plants that thrive on clay soils and learn to garden effectively with clay soil.
Clay soil isn’t all bad.
Gardening and growing with clay soil (next week’s blog)
Doug Green. Gypsum and Clay Soil.
Dr. Linda Chalker-Scott. Washington State Extension. Myth of Soil Amendments Part III. Healthy soil has high organic matter.
Owen Dell and Associates. Sustainable Landscaping.