Soil is a glorious stew of minerals, organic matter, living organisms, water and air. Of the solids in soil (minerals and organic matter) mineral particles make up close to 90 per cent. The size of these particles is what determines how we characterize our soil. Sand is the largest, silt next down in size and clay is the smallest of the three. A balance of sand, silt and clay makes a soil with terrific texture, an ideal that’s seldom reality.
If you have predominately clay soil, you know how difficult it can be to work with. It’s sticky and wet in the spring, and brick-hard in summer. If it’s tough on our trowels, just imagine how tough it is for plant roots. Roots need oxygen to survive and optimum amounts of water. Tiny clay particles cling to each other so tightly, there’s little room for air to circulate and it’s difficult for excess water to drain away. The upside of clay is that it holds nutrients longer than its sandier counterparts. In clay soils, some plants do better than others — such as lilacs in the photo.
Here are some tips to help you manage and improve clay soil:
1. Don’t dig when it’s wet; you’ll only make bigger, impenetrable clods.
2. Work in plenty of organic matter whenever you plant or renovate a bed. Use a variety of materials: homemade compost, aged manure or shredded leaves are good. Peat moss is not so good because it holds water like a sponge near roots and contains no nutrients. Organic matter attracts earthworms and other organisms; their activities as they move through the soil help lighten the structure.
3. Avoid using a tiller. The tines pulverize the clay, destroying whatever soil structure you might be building, and also create a smooth bottom below the churned up soil. When excess water reaches this slick, smooth bottom, it has difficulty percolating down, creating drainage problems. Hand digging is best (and great exercise).
4. Mulch bare soil between plants to prevent a hard crust from forming. It’s a way to introduce more organic material, too.
5. It’s folly to think you can improve clay by adding sand. For sand to have a significant impact, you need to incorporate at least a two- or three-inch layer of coarse, builder’s sand (enough so that the soil in the root zone is more sand than clay.) That’s a lot of sand and a lot of work. ‘Coarse’ means sand with various particle sizes. Adding just a shovelful or two of play sand or horticultural sand (both fine-textured versions) creates the perfect recipe for bricks (which are manufactured using clay and a bit of sand) exactly what you don’t want.
The bottom line is that clay soil needs to be worked with care and amended with generous lashings of organic matter over time to encourage the tiny particles to bond together into small clumps, which makes more air spaces. It won’t happen overnight, but you should see better soil structure in two or three years. In the meantime, choose trees and shrubs that tolerate heavy soils — those vigorous enough to battle the clay. Lilacs (above) fare well in clay soil.