Clay soil is actually relatively common in most soils that haven’t been well maintained will divert to either a thick clay or a loose sand, depending on the area. Many areas have both, often with the sand on top of clay.
Determining Soil Types
To determine how your soil is composed, simply dig up a shovel full and look. Clay soil will be hard to get into, will come out as one big clump, and will be difficult to push your fingers through. At least, in its worst case.
Sandy soil will be exactly the opposite. Most soils aren’t quite this obvious, but if the soil is easily dug, stays together in a clump that is easily broken with the fingers, and is relatively dark in color, then you likely have perfect gardening soil. If not, you’ll need to make it so.
Reasons to Amend High Clay Soil
High-clay soil is thick, hard to grow in, and very problematic for the three musts of gardening: nutrient delivery, drainage, and easy expansion for plant growth. Clay soil is thick, so plants can’t easily push through it to grow proper roots. It’s water resistant since its thickness and the clay content means water doesn’t absorb into it easily, and when it does, it often results in a mushy soil that quickly becomes very hard again. Finally, because of these properties, it doesn’t delivery nutrition to the plants very well either.
For all these reasons, a gardener hoping to grow things in the soil needs to amend it so that the clay is dispersed and the soil becomes healthy for growth.
Avoiding Sand as a Soil Amendment for Clay Soils
Often, the advice for amending clay soil is to “add sand.” Yet if you have a relatively small 100 square foot garden area that is thick with clay soil, you’ll need many yards of sand to amend it. That’s expensive and does nothing to boost the nutrient value of the soil along the way. As a gardener, your goal isn’t just to loosen the soil, you also want to make it a place where plants will thrive. That means nutrition and sand, frankly, has none.
For this reason, it’s best to amend soil with something other than sand. Organic materials are best, as they add both nutrition and aeration qualities to the soil. Where you live will largely determine what type of amendments are easily had, but humus is almost universally available.
Organic Amendments for Clay Soil
Often sold as a mulch, a compost, etc., it is readily available and can be made up of a variety of plant materials. Most often, it is made up of ground bark, leaf mold, and other compost. It can be added anytime there are no plants in the ground and mixed within the soil.
By itself manure will not amend clay soils completely, but can go a long way towards helping. Loose manure with a lot of grasses still in it is best. Best added in the fall and topped with heavy mulch to “seal it in” for the winter.
Other popular amendments such as peat moss, gypsum, and a lot of gravel are not recommended. Peat moss may exacerbate the bad qualities of clay, gypsum can make the soil alkaline, and gravel can make it hard to work by hand and drain too quickly though it can help in small quantities.
Want to learn more about amending clay soils?
Check out these helpful resources:
Amending Clay Soils, a PDF from North Carolina Cooperative Extension
Myths of Soil Amendments, a PDF from Washington State University
Choosing a Soil Amendment from Colorado State University Extension