ORGANIC SOIL SPONGE
Soils high in humus and other organic matter always dry out more slowly than do sandy types that allow water to run through with little or no absorption. An interesting fact is that when soil does a good job of blotting up moisture, this also takes care of the problem of air supply. As already pointed out, true Gardener's Loam can carry large amounts of moisture without losing its capacity for holding air. Soils in gardens vary between 35 and 65 per cent pore space. Water is held in these spaces in three forms-as free water (usually in vapor form except when soil is "drowned" out with excess water), as a film on the surface of soil particles, and as absorbed water inside porous minerals and organic particles. Air and water, while partners, are antagonists too. They move together into pore spaces when conditions are favorable for growth, but at times excess water can drive out air.
We should not think of the air in pore spaces as identical to the air of the atmosphere. For one thing, soil air contains about six or seven times as much carbon dioxide (in soils containing any amount of organic matter) and is slightly poorer in oxygen. This is due to constant absorption of oxygen by roots, leaving carbon dioxide behind. Leaves, as we know, absorb carbon dioxide from the atmosphere and use it to manufacture carbohydrates. If pore spaces are so small that soil "ventilation" is poor, the result is a build-up of carbon dioxide because more oxygen cannot move in to replace that removed by roots.
Small pore passages are the mark of clay soils and other fine-textured types. This makes clay soils difficult to aerate and drain. Large pore spaces are non-capillary; that is, they are normally filled with air because they are so large in cross section that gravity can pull excess water out of them. This is what happens in soils high in sands. The ideal soil condition is one where sand, clay, silt and organic matter provide a soil with large crumb particles. These allow moisture to drain off readily, but inside the crumbs are small air-holding pore spaces which cannot be drowned out. The organic matter serves as a soil sponge to absorb reserve water.
PLANTS CAN DROWN
Some plants, notably cattails, sedges and others that live in bogs, can survive with their roots in water. Others can tolerate a soil where pore spaces are close to saturation but contain some air. Most garden plants, however, do best when practically all the water in pore spaces is in vapor form, when film water is close to capacity, and when absorbed water is high. This is the invaluable "moist, well-drained soil" about which many garden experts write so much and so vaguely.