Water, Air And Bacteria

Very few plants will grow on wet swampy soils. Most plants need soil conditions that give free movement of water and air. Their roots will not penetrate into soil where the minute spaces between soil particles are filled with stagnant water. Should their roots be planted in such a layer, they will soon perish.

In all fertile soils certain soil bacteria are present and of vital importance. They are of many kinds, the chief being those concerned with the breaking down of organic matter, those concerned with food absorption by plants, and those living on the roots of leguminous plants such as clovers, peas, and beans and enabling those plants to absorb nitrogen from the air. All these helpful bacteria require air, available only in well-drained soil. Faulty drainage fosters bacteria that break down organic matter by putrefaction, with consequent formation of products toxic to plants, instead of plant food.

As water percolates through soil, air follows it into the soil interstices to the full depth of drainage. As roots absorb moisture, air will be drawn into the soil to replace it. The deeper the drainage, the deeper will roots and air penetrate. With deep drainage, plants can draw on deep soil for moisture and food, and consequently be better able to withstand drought and heat.

With inadequate drainage the root systems develop near the surface. During the summer such soil quickly dries by evaporation and plant absorption, and though there may be-ample moisture in the subsoil it is brought up by capillarity too slowly for the plants. With better drainage the roots go deeper and capillarity is more efficient. These two factors provide for more rapid, continuous, and prolonged growth of the plants. Sodden soils are cold; aerated soils are warm. Soil warmth stimulates plant growth, because heat increases plant cells' absorption of moisture together with plant foods in extreme dilution.

Soils That Need Drainage

The need for drainage arises in most intensely cultivated areas, but more especially in the following circumstances: (a) flat lands or basins in which water from adjacent lands collects; (b) lands adjoining higher slopes where the formation allows the water that soaks into the higher ground to seep farther to the lower levels; (c) lands that are subject to periodical overflow of lakes and rivers; {d) flat lands that rest upon an impermeable stratum of rock or clay; (e) lands that are watered artificially, and where it is essential that the surplus water should readily escape. In suburban gardens the necessity for drainage often arises from, or is increased by, the interference with natural flow of subsoil moisture caused by roadways, large underground drains, sewerage excavations, and the like; (/) unduly acid peaty soils.

A garden should not be made until efficient drainage has been established. It would be foolish to spend time, money, and exertion unnecessarily in laying even a simple system of drains, but most gardens will need some such preparation. An easy test can be made by digging holes in scattered parts and leaving them open for further observation. They should be about two feet deep, and will be more informative in the wet season than in the dry, for in the former they will fill after heavy rain, partly by catchment and partly by soakage, and if they do not empty within twelve or, at the most, twenty-four hours after cessation of the rain, they will show that artificial drainage should be established. In dry weather they should be filled with water and refilled after a few hours. The water should disappear within twelve hours.

It is never safe to assume that sloping ground is well drained, for water is sometimes held up by the formation of pockets in the underlying soil.

Surface drainage may be useful in diverting great rushes of water from storms or floods, but open ditches to carry away surface water are not to be recommended as a general rule. Harmful erosion of the best layers of the soil is caused and often a hard crust is left, excluding air. Valuable purposes are served by water percolating through the soil.

Drain Outlets

The success of any system of artificial drainage depends upon there being a suitable outlet for the water, and this is often difficult to find. Sometimes, underlying impermeable rock or clay, there is a permeable drift of gravel or sand. Then the main drain may be discharged into a pit or well. If the water in the pit rises above the level of the drain it must be pumped out. In suburban allotments an outlet is often available into a street drain or a channel.

Surface And Underground Drains

Surface drains are useful in cutting off seepage from higher ground. They should be at the foot of the slope, where the seepage begins to rise, and should skirt the slope as far as necessary to collect the surplus water.

For underground drainage, the tile drain, commonly known as the agricultural drain, is the most efficient form. Pipes, three inches in diameter, are advisable. Cheaper drains are constructed of stones, saplings, and boards. Tile drains will last for generations and are the cheapest in the end. Each tile or pipe, should be straight, hard, smooth inside, and well burnt so as to ring when struck with a trowel. The point of outlet of the main drain should be first fixed by studying the fall of the ground.

For large areas, such as public gardens, the services of a surveyor should be sought to obtain a series of levels. The water-level is a handy instrument for small schemes and is sufficiently accurate. It may be made from a piece of half-inch water piping, about two feet long, with an elbow at each end. Into each elbow, cement a piece of glass tubing about six inches long. A spiked rod attached to the centre completes the implement. When in use, fill the tube with coloured liquid and set in position. The fall of the drainage trench will be indicated by the difference in levels of fluid in the two tubes.

Before digging a drainage trench it should be marked out with a line to keep it straight. Commence digging at the lower end so that any water accumulating in the trench will run away and not mount up, causing inconvenience. The trench should be just wide enough to be easy to work in, about one foot. The depth is governed by the level of the clay or other impervious matter. The average depth is two or three feet, but it should not be less than eighteen inches. The topsoil should be placed on one side and the small amount of subsoil on the other. The latter should be discarded, its place being taken by the pipes and the cinders which are used to cover them. Should clay be put back in the trench it would render the drain less efficient.

In gaining levels the use of all instruments can be dispensed with by simply digging the trench by judgment, testing with water, and adjusting by results. The high spots are carefully levelled until the water does not lie anywhere, but slowly flows away. An occasional short, deep dip may be found in the clay. It should be filled in with pure clay rather than permit it to contain some less dense material that may allow the pipes to sink.

Though seldom necessary, a semicircular trough, two or three inches deep, can be cut in the bottom of the trench. If used, it should be tested with water.

Place the pipes firmly end to end and give each third one a kick with the heel to keep them firmly together. No matter how closely they are placed together they will never be close enough to keep the water out. The ends are not always quite square, but by twisting a pipe it will often be made to fit its neighbour better. If a gap of more than one-eighth of an inch has to be left it is better to arrange for it to be on the bottom than on the top, where it would allow silt to enter the pipe. The pipe at the highest level of the trench should be blocked with a brick. The outlet should be into a mass of cinders or be covered with fine wire-netting to keep out mice and other pests, which might eventually block the pipe.

The drainage scheme will function better if the pipes are covered with cinders, coarse gravel, or brushwood, which will filter silt out of water making its way to the pipes. It is advisable to lay four-inch strips of heavy bagging over the joins.

Although the bagging rots in time, it serves its purpose until the disturbed soil has settled and, after that, silt is unlikely to enter the pipes. Water reaches the pipes through the small spaces between them, and to a small extent through their pores. Pipe-lines should be no more than twelve feet apart. They can be laid fish-tail fashion, emptying into a larger main drain. Usually the smaller pipes are two inches in diameter and the larger three inches; but three-inch pipes may be used throughout and are probably better. Where the ground has a steep slope it is best to run the drains across the slope where they will catch much more soakage. If they run down the slope they are inclined to scour and move.

Keep large trees away from drain-pipes. Poplars, wattles, cypress, and citrus are the worst types of tree.

Efficient drainage is far more important than watering or manuring, and every gardener knows that it is the first prerequisite of success, and that no garden or soil will be harmed by the installation of a drainage system.

Well-drained soil is almost always acid and has a rather coarse crumb-structure, which helps in the process of aeration. Poorly drained soil is almost always alkaline and has a fine crumb-structure, making it inadequately permeable.

In roses, the outstanding symptoms of poor soil drainage are a form of die-back, undersized foliage of pale colour, poor growth, and numerous blind shoots branches that do not produce a terminal flower. Also, the flowers have a lack of brilliance, a marked tendency for the reds to be purplish (they are said to "blue") and the pinks to be mauve, as well as there being a great number of badly formed blooms with petals deficient in number, size, and texture, and with short, weak stems. The die-back affects both old and young wood. In the young growth the new shoot often grows to a length of four to ten inches and then, for no apparent reason, withers at the tip and fairly quickly dies back to its base without having formed even a small flower-bud, in much the same way as a rose behaves when affected by rose wilt. Poor drainage seems to be the commonest cause of blind shoots.

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