The soil is permeated with pores or cavities which are filled with air, or water, or both. Too much water or too little water are the chief problems facing the gardener, so that the control of water is essential except in places wherethat can put up with wet, dry or alternately wet and dry conditions have been naturalized.
THE WATER TABLE
When water falls, either as rain or from a sprinkler, some of the water penetrates into the ground and is retained by the soil. It is held by strong physical forces, the proportion of the soil pores filled depending upon the amount of rainwater that is falling. The soil surface is saturated immediately after rain, but in a well-drained soil this state is only tem-porary. The surplus water runs away down the work tracks, cracks and large pores until it reaches rock or some other hard impermeable layer upon which it collects, forming a permanently water-logged layer. The upper surface of this waterlogged layer is known as the ‘water table’, and resembles an underground lake.
The water table may be only a few feet or even a few inches from the surface, or it may be several hundred feet down, according to the area. In soils by rivers the water table may be at the surface during the winter, and, as a result, these soils will be thoroughly marshy. Water-logged soils hold no air and therefore roots cannot grow in them because they become suffocated. On sloping ground where the subsoil is very compact, some of thewater often appears at the bottom of the slope, forming a marshy place. If the water table is tooc near the surface, only a small space will be left for plants to grow in and, as a result, they will be short, shallow-rooted and stunted.
The water table rises and falls but is nearest the surface in the winter. Ideally it should be about 4 ft. below the surface. Some of the deeper roots can then obtain moisture which rises a foot or so from the water table.
Sometimes rain merely runs off the surface without penetrating the soil. This ‘runoff’, as it is called, occurs in crusted soils and can be very serious on slopes. The soil is rather like a tank, the size of which is determined by the depth of rooting. Awill hold about 2 gal. per sq. yd., to a depth of 1 ft., whereas a will hold much more, possibly up to 5 gal. per sq. yd. to a depth 1 ft.
Plants keep drawing on the water held by the soil. A mature tomato plant, which even on a dull day uses about a quarter of a pint of water, on a sunny day will take up to about 4 pints. The 2 gallons in a sandy soil will not, therefore, last long, particularly as some of the water which is held by the soil is not available at all. Much of the water held by the soil is retained very firmly against the action of roots, the force holding the water being known as the moisture tension. As roots gradually exhaust the available water, they have to struggle harder to overcome the moisture tension.
In the smallest pores water is held with the greatest tenacity, often so tightly that plants are unable to extract it. They then give up the unequal struggle and wilt, even though the soil in which they are growing may contain quite a lot of moisture.
WHEN TO WATER
It is generally reckoned that the time to water is when about half the available water in the soil has gone. To find out this stage exactly is very tricky and requires instruments.
When watering do not just sprinkle the top of the soil. Always give enough water to wet the soil to a depth of at least a foot, otherwise the water will merely evaporate from the surface and do no real good. Sprinkling merely encourages roots to grow to the surface, so that they will suffer severely during hot spells by becoming scorched. A dry, sandy soil needs at least -1 to 5 gal. or 2 large bucketfuls per sq. yd. in order to moisten the soil properly to a depth of 1 ft. It is impossible to partially wet a soil, whether it is in open ground or in a pot. The soil is either wet or dry, since it is wetted from the top downward in a well-defined layer. If a pint or so is sprinkled over a square yard the ground may look moist on the surface, but it will certainly be dry below. Watering during hot peri-ods is usually best done in the evening when evaporation by the sun will be at its lowest.
The best way to prevent undue loss of moisture from dry soil is toit with some organic material. A mulch is a layer or blanket of rotted material, lawn mowings, straw or even polythene, which is put on the surface of the soil to prevent evaporation and to conserve moisture. Its function is primarily to keep roots cool and moist, but when it consists of organic material it also acts as food for the soil.
Hoeing, contrary to popular belief, does not prevent loss of moisture, except by killing weeds which use up a tremendous amount of moisture.