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.
Water, being essential for all plant growth, must be present in sufficient quantity in the soil, and in such a condition that it can be absorbed by the roots. Water may be in such abundance in some soils that its presence would be more harmful than useful to the crop. Thus in a clayey soil that has been only broken up with the spade or the plough from 6 in. to 12 in. deep there may be so much water present that the soil becomes chilled and waterlogged, and plants fail to grow because the soil bacteria remain inactive (see p. 125).
The quantity of water in a soil depends upon the rainfall. This varies in different parts of the United Kingdom from 24 or 25 in. in the neighbourhood of London, and along the eastern counties of England and Scotland, to 40 in. in the south-western districts; while in the western Highlands, the Lake District, and parts of Wales the annual rainfall is about 80 in. An inch of rain to the acre represents something over 100 tons of water to the acre; so that in the British Islands the amount of water which falls upon an acre of soil varies from 2400 tons to 8000 tons annually. It penetrates the soil more or less readily according to the nature of the soil itself, and the way in which it has been cultivated, Thus on a clayey, uncultivated soil very little rain will pass straight downwards. It will flow away from the surface to the ditches, or remain in pools in the shallow places - -just as it does on roadways and pavements. On a sandy soil, the rain will pass down and between the particles readily until it comes to the water-table beneath; and in loamy or peaty soils a good deal of water will be absorbed.
Different soils will absorb and retain water in 'different proportions, as shown by the following experiment of Schubler: -
Water Absorbed by 100 Parts of Soil.
Evaporation in 4 hr. from 100 lb. of Water at 66° F.
Rich garden soil ...
This is a laboratory experiment, and cannot therefore be regarded as giving the same results as one would find in the open field, and in actual practice. So much depends upon the way the soil has been treated. Where the soil has been deeply dug or trenched far more water will be absorbed than where it has been allowed to become hard and baked on the surface. It therefore pays to cultivate the soil deeply and well if full advantage is to be taken of the rainfall, and if the soil is to store up sufficient moisture for the roots of the crops during hot and rainless summers.
The above table teaches the market gardener that a soil which has been well dressed with organic material like stable manure, peat-moss litter, etc, will absorb and retain moisture for a very long period - but only in accordance as to whether it has been cultivated to a great or little depth. That is the important point for practical growers to bear in mind. Unless the soil has been well opened up by digging, trenching, or subsoil ploughing, it will lose its moisture very rapidly, and crops will suffer intensely in consequence during a hot dry season.