Clay soils can be heavy and difficult, but if you persevere they can be very fertile and productive. Ideally your soil will contain at least 20% clay, but no more than 35% (or you can have problems). While clay soils hold on to nutrients well, you may have to adjust the pH of the soil to make the nutrients available.
The downside of clay soil is that roots have a hard time penetrating it because the pores are so small and the particles so densely packed. Additionally, clay soils hold a lot of water (up to eight times as much as a sandy one) and are slow to dry out. This is an advantage in times of drought, but in wet climates it means they get waterlogged easily. Wet clay soils tend to be cold and slow to warm up in spring. It’s no wonder that many plants don’t over-winter well in clay soils.
Use the tips below to turn your clay soil into an environment your plants and other beneficial organisms will love.
Tips for working with clay
• Limit cultivation to times when soil is not too dry or wet. If you work with dry clay soil it will be so hard it is almost impossible to dig and may crack into large chunks that crumble to dust. When this dust gets wet it sets rock hard, almost like plaster. Cultivation of wet clay is even worse. Wet a clay soil compacts very easily when you put any pressure on it, creating a sticky mass that is also hard to penetrate.
• Roughly dig the soil in autumn, when it’s relatively dry, and leave it over the winter for frost to break down the large clumps.
• Add well-decomposed organic matter to help particles cluster together and form larger aggregates. This will improve drainage and aeration.
• Use green manures to improve structure.
• Double dig to incorporate organic matter and calcium.
• Use raised beds to improve aeration and prevent future compaction. Build them high to help them drain and warm up.
• Slope the sight slightly so the soil can drain more quickly.
• Never allow the soil to get so dry that is cracks. These cracks increase evaporation and make the soil hard to re-wet because water simply drains away down the cracks. If it does dry out, cultivate the soil surface.
• You can improve clay soil’s structure by adding a mix of 80% gypsum and 20% dolomitic lime. Use an ounce of this mix per square foot of soil in spring and again in fall. This may be repeated for a second year, while also adding as much organic matter as possible.
• Adding calcium may be useful to improve soil structure temporarily, until you can get sufficient organic matter and soil life into the soil. But this only works if the soil is low in calcium.