Amending Clay Soil: How to Improve Soil Quality and pH with Coarse Sand and Lime

Planting plants in heavy clay soil is just like standing them on cement: they’ll struggle for a while and then die. If you want your plants to thrive, you have to amend clay soil regularly.

Adding Perlite, Vermiculite, or Sand to Clay

The more organic material you add to clay soil, the more porous it will become, and the better chance plants will have. You may also want to add coarse fragmented particles like perlite or vermiculite.
Coarse sand is a more economical option, based on the formula one part coarse sand and one part organic matter to one part of the existing soil.

If you choose this alternative, remember to use coarse builder's sand. Adding fine sand can harden your soil. Dig the sand and organic material into dry soil, as digging wet clay soil turns it into a muddy mess. Till the sand and organic material together into the soil to a depth of about 18 inches.

If your ground is severely water-retentive, and holds water for more than 3 hours after a rain, the best bet is to dig out and remove the clay soil, replacing it with a mixture of organic matter, coarse sand, and topsoil.

Improving the pH of Clay Soil

Clay soil is naturally acidic (one reason rhododendrons and azaleas flourish in it). Adding lime will raise the pH level of clay soil. Use dolomitic lime if your ground tends to be deficient in magnesium; if not, use calcitic lime. Both should be added a season in advance since lime is absorbed slowly. For example, add lime in the fall to a bed you plan to plant in spring.

Normally, you only need to lime once every 3 years, and testing the soil's pH level each year will tell you how the lime is doing. Most vegetables and flowers thrive in soil with a pH of 6 to 7, an ideal that can take a lot of amending. A pH level above 7 indicates soil that is alkaline, in which case you'll want to lower the pH by adding wettable, powdered sulfur.

Whether or not you suspect the pH level of your soil is problematic, test it regularly. Easy-to-use soil testing kits from a garden center can provide you with a detailed analysis of your soil and recommend the quantities of lime, fertilizers, and other micronutrients to add. Another alternative is to have your soil tested by an expert. Keep an eye on the pH level and nutrient composition of your garden’s soil: one plant's ambrosia is another plant's poison.