This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
While the aim of the open-air cultivator should be to prepare his soil for the reception of plenty of moisture and its ascent afterwards to the roots of his crops, he must also take care that the supply of moisture does not become exhausted during hot and rainless seasons. The experiment of putting a piece of glass on the surface of the soil to collect the moisture arising from it indicates a way in which moisture may be prevented from escaping into the air from the soil. A piece of board or slate would act in the same way as the glass, although the moisture would not be so apparent. Indeed, layers of almost anything put on the surface of the ground will check the moisture escaping from it freely. The cultivator naturally wishes to keep the moisture in the soil, because it is the only means by which the foods from the soil can be transmitted by root action to all parts of the plant, and because it saves him the labour of watering. Sheets of glass, slates, boards, etc, however, are not the most suitable materials for keeping the moisture in the soil. The grower has found that by placing a layer or mulching of more or less decaying manure on the surface of the soil he not only prevents moisture escaping freely, but he also prevents weeds from growing and robbing him of food and moisture. In addition to this, as the manure gradually decays it yields up to the soil certain valuable foods that are sooner or later washed down to the roots by the rains. The manurial layer also prevents the sun from scorching and baking the soil, and, being a bad conductor of heat, this is a consideration in hot seasons. Layers of short grass, leaves, leaf mould, moss, etc, would act in the same way as the manure.
When these materials are not available, it is still possible to conserve the moisture by stirring up the surface of the soil to a depth of two or three inches by means of the hoe or scarifier. At first sight it would seem as if breaking up the surface soil would facilitate and accelerate the escape of moisture from the soil beneath. Such, however, is not the case. When the surface is broken up with the hoe a layer of loose soil is then placed over the more consolidated soil beneath. In the latter the moisture is rising freely; but when it comes in contact with the loose layer the particles of soil in it are no longer so closely bound together that the moisture can pass readily to them. Consequently a check to evaporation from the surface takes place, and the moisture is kept in the soil for a longer period. The loosening of the surface soil indeed produces a kind of soil blanket which checks the rapid absorption of heat from the air, and the rapid evaporation of moisture from the soil at the same time. The roots of the crops are therefore kept in a cool, moist, and highly active condition during the hottest seasons.
In some experiments carried out in America by Professor King on a sandy loam, it was proved that in a hundred days the unmulched soil lost water equal to 6.55 in. of rain, or 655 tons to the acre. When mulched 1 in. deep, the soil only lost water equal to 3.30 in., or 330 tons per acre; at 2 in. deep about 299 tons per acre; at 3 in. 254 tons per acre; and at 4 in. 278 tons per acre. It would thus appear that a mulching or loosening of a soil to a depth of 3 in. gives better results.
It should not be forgotten that, apart from the benefits of retaining moisture in the soil during a hot summer by loosening the ground with the hoe, other advantages are also secured. The loose and more finely powdered soil absorbs the dews more readily, inorganic foods are liberated, and the grubs or pupae of various insect pests are brought to the surface where they are readily pounced upon by the birds.