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.
We have seen that according to the nature of the soil, and the depth to which it has been cultivated, large or small quantities of water are absorbed and sink to certain depths. If, however, the water were to sink so low as to leave no moisture at all in the root region, plant growth would be impossible. We know, however, that in all well-cultivated soils there is generally a good supply of moisture available, even in the hottest and driest of summers. Whence does it come? Obviously from the supplies deep down below the surface.
The soil may be looked upon as a kind of sponge. It will not only soak in water from overhead, but also from beneath. Consequently we find that when no rain falls, and the weather is hot and dry, a good deal of moisture is rising from the ground. This is easily proved by placing a piece of glass on the surface of the soil. After a short time it will be noticed that the under surface is covered with moisture that could not escape through the glass. If we dig down for 2, 3, or several feet we see no actual water, but we notice that the soil becomes more moist the deeper we go, until eventually we should reach water. It is evident, therefore, that the moisture rises upwards, and passes from particle to particle of the soil. A kind of invisible stream of vapour is constantly rising from the lower regions, and is given off from the surface of the soil into the atmosphere. This upward stream of moisture or vapour is caused chiefly by the evaporation that is going on from the surface owing to the heat of the sun. The top layer of soil particles are the first to lose their moisture, then the next layer, and so on downwards, until, if no rain falls, and the reserve of water beneath fails, the soil becomes as "dry as dust", and the crops collapse.
The ascent of water or moisture in the soil is much the same as it would be in a sponge or in a slab of salt or sugar. The lowest layers in direct contact with the liquid are more saturated than those above them, and the particles composing the different layers have the power of drawing moisture from those immediately below them. According to the nature of the soil this power of raising the water from below upwards varies greatly. Thus, in a heavy clay soil, where the particles are closely pressed together, it is very difficult for the moisture to rise freely. The surface layers lose their moisture after a time, and then, because they are unable to obtain a supply from those beneath, the surface begins to shrink and crack and form fissures in all directions.
In a sandy soil, where the particles, although closely packed, are not cemented together as in clay, moisture is given off very freely, and the upper crust very soon becomes dry and hot, and almost incapable of supporting any plant life.
In a loamy soil, however, which is a mixture of sand, clay, and organic material, moisture arises neither too quickly nor too slowly. It is, therefore, retained round the roots for a longer period.
The ascent of moisture depends not only upon the nature of the soil, but also upon the way in which it has been cultivated. We find even in good garden soils that have been dug only 9 in. or 1 ft. deep, that the moisture soon vanishes from this upper layer. The subsoil beneath is probably too firmly compressed to allow the moisture to travel upwards freely. The same thing is seen in soils that have been ploughed year after year. The upper layer of 6 in. or 9 in. rests upon a very hard "pan", through which water can neither penetrate downwards nor rise upwards.
If, however, we take a soil that has been broken up to a good depth, say 2 and 3 ft., it will be noticed, even in the hottest and driest summers, that there is always sufficient moisture available for the roots of the plants growing on it, and they appear to be as fresh and green as if they were supplied overhead with abundance of water each day.
The way in which the water passes upwards from layer to layer and particle to particle of the soil is known as capillary attraction. The direction of the liquid is always from the wet to the dry, and the finer the particles of soil, and consequently the narrower the interstices between them, the greater the height to which the moisture will rise.
This may be demonstrated by taking some glass tubes with bores of various diameters. If placed on the surface of water it will be noticed that the liquid will rise higher and more quickly in the tube with the smallest bore.
It is possible also that the pressure of the atmosphere has something to do with this ascent of liquid in the soil. The air spaces between the particles may be regarded as so many fine-bored tubes, up which the water passes. Owing, however, to the heat at the surface, the air and the moisture become warmer and lighter, and rise upwards. A kind of vacuum is thus caused, or at any rate both air and moisture are less dense than at a lower depth. The equilibrium between top and bottom is thus upset, and the water and air from beneath rush upward, owing to the pressure of the atmosphere, to fill the vacuum caused, and to restore the balance. During a hot day this process is going on vigorously, and moisture rises to the surface in the same way that oil is drawn up the wick of a lamp by the heat of the flame at the top.
Every gardener who sows the spores of Ferns, or such fine seeds as those of Gloxinias, Begonias, Rhododendrons, etc, takes advantage of the capillarity of the soil, by dipping the seed pots in water, and allowing the moisture to rise upwards to the surface instead of watering overhead.