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?
ILLUSTRATION JILL CALDER
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.
The larger pores are crucial to soil function, providing aeration, drainage and water for plant growth. If there aren’t enough, either drainage or water supply (or both) will be a problem. Pores also provide pathways for penetration of soil by roots, and many are created by the death of old roots, while others arise from the activities of earthworms.
Life in soil
But pores are just one part of soil structure, and the key to good structure is plenty of life; plant, animal and microbial. Roots themselves help to provide structure, and also secrete mucilage that sticks soil particles together. Root exudates in the rhizosphere (the zone immediately surrounding each root) lead to vigorous growth of bacteria, which in turn secrete organic glues that help to bind soil particles together. Earthworms incorporate organic matter into soil, and in their guts, clay and organic matter are i
