NAREI in Focus continues to inform readers about soils this week with focus on clay soil. As a reminder to our readers, soil particles come in different sizes, which contribute to soil texture. Sand particles are the largest while clay particles are the smallest.
Clay is the remains of certain types of rocks that have been eroded and weathered to form fine particles. Most soils contain clay particles. A good mixture of sand, silt, clay and organic matter provides the ideal loamy soil. Problems don’t usually occur until the clay content exceeds 30-35 percent.

Being so small, clay particles pack together not allowing the necessary spaces between them for air and water to flow, especially air that most roots need to function. Clay particles are plate-like with little space between them. This traps water for long periods. It is also hard for most roots to penetrate.

However, there are some positives to clay soils. One such is that its particles hold onto nutrients, making them more fertile. The nutrients are released to plants, producing healthy crops. Nutrients are more likely to be washed through sandy or chalky soils by rain.
Clay soils are less prone to becoming too acidic as they hold appreciable quantities of calcium. But these potential advantages are often outweighed by the physical properties brought about by the small particle size.

All soils hold water because it is attracted to particles by surface tension and chemical forces. Clay particles are very small and therefore can hold more water and are less prone to drought. But, as a result, they are likely to become waterlogged in during rainy conditions, making cultivation difficult.

Clay soils are also slower to warm up as the weather changes, (chalk soils certainly don’t suffer from this) and so germination of seeds may be delayed. Conversely, they retain heat better than many soils once they have warmed up.

The strong attraction of the water to the surfaces of the clay particles also leads to cracking during dry weather. When plants remove water from clay soils the volume of the soil decreases and eventually the soil cracks. During rainy conditions the soil absorbs water and it swells back to its original volume.

In extreme conditions, waterlogged garden clay soil can have a solid “Iron Pan” within the sub-soil caused by leaching down of iron salts into an impenetrable barrier. This, of course, increases water-logging and therefore gardening problems!

Improving clay soil
Clay soils are fertile and potentially very good. The most important operation in improving clay soil is to try and break it up to reduce the terrible wetness in winter and dryness in summer.

Simple digging will help make clay soil more workable, but the effect is ten times greater if a bulky organic material is incorporated at the same time. Use peat, garden compost, manure, spent hops, straw or even coarse grit or seaweed. Even very heavy clay soils can be improved as a result.

Even with improved topsoil, it may still become waterlogged unless the rainwater can drain through the subsoil. If the subsoil is solid clay, or the garden is low lying, some form of drainage may be necessary. In large gardens, open ditches could be used to collect water and if possible, carry it to a nearby stream. In smaller gardens tile drains, perforated plastic or French drains (trenches of pebbles or shingle) leading to soak ways will help greatly.