The Best Amendments for Gardens with Clay Soil

Often maligned, clay soil does have benefits. The late George Stritikus, horticulturist and former county agent with the Alabama Cooperative Extension System, described clay soil particles as the "savings account for minerals in the soils." In other words, clay retains minerals until roots uptake them to nourish plants.

Although clay is rich in minerals and water-holding capacity, the individual clay particles are so small that they pack down tightly, restricting air and water movement around plant roots. The bottom line is that clay soil holds too much water and not enough air.

If you have clay soil in your garden, you can add materials called "soil amendments" to loosen the clay so you can grow healthier plants. Amendments are not fertilizers, although both are mixed into the soil. Amendments may not add nutritive value to the soil while fertilizers provide plant nutrients.

The best soil amendments for clay soil fulfill a two-fold goal: improving drainage and increasing aeration.

Amendments may be organic or inorganic:

Compost

Pine bark mulch

Grass clippings 

Shredded leaves

Pea gravel

Expanded shale

Gypsum

Don't use sand as a soil amendment for clay soil, or you'll end up with garden soil that resembles concrete.

Unlike mulches, which are spread on the surface of the soil then kept there to suppress weeds and conserve soil moisture, amendments are incorporated into the soil.

Spread a 2-inch layer of a soil amendment on your garden soil, and dig or till it into the soil to a depth of 8 to 10 inches.

Spread a second 2-inch layer of the soil amendment on the soil -- particularly if your garden has heavy clay soil, and mix it into the soil to a depth of 8 to 10 inches.