Beneficial Effects of Drainage
Good drainage makes increases in production possible in the following ways:
(i) Cultivation and sowing can be carried out much earlier in the spring and sooner after rain. Even one or two wet spots in an otherwise naturally well drained field delay cultivation, and the draining of these makes possible earlier cultivation of the whole field.
(ii) Poaching of pastures is reduced or eliminated.
(iii) Well drained soil requires much less heat to cause a given rise of temperature than waterlogged soil. Thus in the spring, well drained soil is warmer and earlier germination of seeds and growth of pastures are obtained.
(iv) Deeper penetration of roots gives greater drought resistance and improved tilth. In waterlogged soils the roots spread out near the surface in spring and when the water table falls in summer the crops suffer because these surface roots rapidly use up the capillary water in the soil near the surface, while the absence of a system of deep roots means that capillary water at lower levels is not available to the plants.
(v) Beneficial soil bacteria, necessary for preparing soil elements for plant food, thrive and increase in number in well drained soil, as they require air and warmth.
(vi) Waterlogged soils tend to pack and puddle, and become hard and dense on drying, particularly under the action of implements and stock. Drainage prevents this.
(vii) The improved tilth resulting from good drainage, particularly that due to more profuse and deeper growth of roots, increases the ability of the soil to hold capillary moisture. Thus, after being drained, soil often contains more moisture in dry weather than it did before.
Possible Harmful Effects of Drainage
Many farmers are reluctant to carry out drainage work for fear that their land will become too dry in summer. This very rarely happens, and indeed, as explained above, the reverse is often the case. This is because the summer water table is often several feet down, well below the normal depth of tile or mole drains, which cannot, therefore, possibly
affect the ground-water level in summer The only possibility of adverse effects on the summer water table occurring with drainage of normal depth, e.g., 2 to 3 ft., is where there is normally a high summer water table, higher than the drains. This, however, is rather rare, and the lowering is often so slight as to have little adverse effect on the crops.
It is definitely possible, however, for very deep drains, such as deep open cuts made by draglines, to lower the normal summer water table in free-draining soils very considerably, and thus have an adverse effect on summer growth.
Care should also be taken, in draining peat swamps with open drains, not to make the drains too deep; otherwise the peat will dry out instead of consolidating into soil. The best procedure is to make the drains fairly shallow at first, and then, as the peat consolidates, to deepen the drains slightly year by year.
It is also sometimes possible, by a system of adjustable weirs or dams, to keep the summer water level high in deep dragline drains in peat swamp, thus reducing the risk of too much shrinkage and of burning, and also supplying moisture to plant roots in summer.