DIGGING THE DIRT

As Ken Thompson explains, the key to a good soil lies both in its structure and in the life it sustains. But is any of it improved by digging?

- Ken Thompson

When reading up on what soil your plants need, you have probably come across the impossible phrase 'moist but well-drained' and puzzled over what that could possibly mean. The soil all gardeners want is loam, which has a mineral fraction of about 10-20 per cent clay, with the rest consisting of roughly equal amounts of sand and silt. The other major component of soil is organic matter, some of which is added to soils directly by plants, both from chemical exudates from roots and from the death of roots themselves. Dead organic matter also arrives at the soil surface, either directly from plants or via the compost heap.

Space, the final frontier

But arguably the most important part of soil is space, usually full of a mixture of air and water. Loam will typically have a pore space of about 50 per cent, but sandy soils are about 35-40 per cent space, while clays are 50-70 per cent space. If it seems paradoxical that poorly drained clays have more space than well-drained sands, the explanation lies in the size of the pores. Most pores in sandy soil are large transmission pores, which fill up with water when it rains, but quickly drain when it stops. In clay soils, however, a high proportion of pores are very narrow residual pores, which hold on tenaciously to the water they contain. A loam has a nice mixture of pore sizes, including plenty of intermediate-sized storage pores, which are good at both storing water and giving it up to plants when needed.