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The Pamphlet Collection of Sir Robert Stout: Volume 73

Drainage and its Effects on Land

Drainage and its Effects on Land.

It is chiefly in its chemical aspects that the lecturer would take up this subject.

A certain quantity of water is present in all soils, and is quite necessary for the growth of plants, being, indeed, the medium through which they get the mineral part of their food. To us here in New Zealand, where water is so abundant and so well distributed, it would be difficult to realise the desolate condition of the parched plains of some parts of Australia and Africa in a succession of rainless seasons, and the amazing transformations wrought on the face of Nature in such regions by the advent of the copious rains, or by the water distributed by irrigation. But when—as in wet land in our temperate regions—the water is present in excessive quantities, and lies stagnant on the surface of the sodden soil, or soaks from below up to within a few inches of the surface, instead of a blessing it becomes an unmitigated evil.

Such land is always cold, the coldness arising from the following causes:—

1. From a wet soil water is always evaporating away into the air as invisible vapour; and in evaporating it draws heat from the soil, perpetually leaving it colder than it was before. All liquids, indeed, when undergoing evaporation, lower the temperature in this way. Every one has experienced the cooling effect of the evaporation or drying-up of a little ether or chloroform, or alcohol or vinegar, on the hot forehead or on the palm of the hand. Ice, even, is produced in hot countries by the rapid evaporation of water from the surface of straw that is kept constantly moist in a dry wind; and mercury may be frozen by the cold produced by the evaporation of liquid carbonic acid. The general coolness and bracing effect of the atmosphere after a day's rain on a warm dry soil is due to the same cause. It is doubtless also by the constant evaporation of water from the surface of the leaves of living plants that their temperature is kept down in hot weather, and partly to the same agency (evaporation from the leaves) must be attributed the coolness of a leafy forest on a hot summer-day. Indeed, all around us, and at all times, we are sensible of the effects of the evaporation of water in producing cold. In the case of wet land, in which the particles of soil are page 54 kept damp by the water that is always oozing up from the water-table below, a great deal of the heat of the sun and air is spent in continuously evaporating off this water, instead of being usefully employed in warming up the soil itself. Of course, when it is only surface-water that troubles the land, the case, though bad enough, is not quite so hopeless; for here, after a great expenditure of solar heat, the stagnant surface-water at last is all evaporated away, and then, but not till then, the cold land begins to get the benefit of the heat of the sun; but in the case of really wet land, when the water-level in the soil is near the surface, and when, therefore, continual accessions of water are always working up from below, and when, therefore, the water driven off by the heat of the sun and air is always being succeeded by fresh arrival of new supplies, one sees how deplorable the case is. There is then a perpetual loss of heat all the year round, and the soil is maintained in a condition of constant coldness. Many experiments have been made to test the temperature of dry drained land and wet undrained land side by side, and have always shown the greater warmth of the former.

2. Another great cause of the coldness of wet land is the fact that it requires six or seven times as much heat to warm up a pound weight of water, say, from 40° to 70° Fahr. as would be required to heat a pound of dry soil from 40° to 70°. Water, indeed, of all solid or liquid substances, absorbs the most heat with the least effect. When, therefore, there is a good deal of cold water in the soil at the end of a cold winter, it takes a long time to warm such wet soil, as the heat of the sun, instead of doing its proper work of heating the solid soil itself, is absorbed, with but little effect, in trying to heat up the water. Such wet soils are from this cause kept cold for many weeks longer than drained soils close by.

3. A third cause of the coldness of wet land is the bad heat-conducting power of water. It is on surface-water in soil chiefly that this cause operates. The heat of the sun and air, striking on the surface of the water standing in the pools and puddles, very slowly warms up the surface layer of such water; but, as water is a bad conductor, this heat does not readily pass down through the water to the cold soil below. From this cause, in conjunction with the cold produced by the constant evaporation from such standing water, it is quite impossible to warm up the soil till this surface-water has been entirely evaporated away.

4. Wet soils also draw frost, and the snow lies longer on them than on dry soils. Much of the heat of the sun, therefore, is spent in first melting this ice and snow, and then warming up the water after it is melted.

5. The glazing of the newly-turned-up furrow also, seen when wet land is being ploughed, keeps out the heat of the sun and air; and such ploughed land consequently takes a longer time to dry and crumble down and get heated for a good seed-bed.

Now, there is no need to enlarge on the advantages of a warm [unclear: s] over a cold one. It is strikingly better from every point of view (a) Fit for sowing earlier in the spring; (b) a warmer seed-bed; (c) more rapid germination of the seeds; (d) less mortality among the see[unclear: ds]; less liable to be attacked by moisture-loving fungoid growths, and a [unclear: m] speedy passage over the dangerous period of the seed-lobe leaves (the smooth leaves in young turnips, &c.), when the young plants are specially exposed to insect pests; (f) more rapid growth in the warm genial [unclear: so] (g) a stronger plant; (h) more speedy maturity, and a finer quality of produce; (i) an earlier harvest, and, therefore, quicker returns, an[unclear: d] earlier deliverance for the farmer from the haunting fears of some [unclear: ca] strophe to the precious crop—the object of all his labours throughou[unclear: t] year.

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The one remedy for wet land is drainage.

Open surface-drains were dug for drying land from the earliest ages, and open drains of different sizes—from 2ft. to 10ft. deep and corresponding width—are now doing excellent work in Otago and Southland in drying and reclaiming, and thereby converting into first-class soil many lovely fertile flats which were formerly flooded swamps, which sustained no vegetation except the lingering maoriheads and the rushes and other water-loving, worthless rank growths that encumbered the useless mud. In the reclamation of these maorihead swamps all that had to be done, in general, was to take up an open drain of suitable width and depth through the middle of the swamp to catch the head-water as it debouched from the gully, and lead it down to its proper outlet at the lower end of the swamp. Of course the proprietor below must look out for himself and take the water on another stage, thus drying his own land, and so on, till the surface-water—the sole cause of all the mischief—has been safely conveyed to the creek or the river or the sea. Among many edifying examples of such swamp reclamation, the lecturer referred to some large paddocks of magnificent land on the farm of Mr. Soutar at Flag Swamp, near the railway, between Waikouaiti and Palmerston, and open to the inspection of any members of the class that could visit that district. The lecturer had had during the week a long and most enjoyable outing with Mr. Soutar, from whom he received much valuable and practical information on this subject. Although it is perhaps to the drainage-away of the surface waters that these large open drains are mainly applied, yet, by lowering the level of the water-table in land (for several feet in many cases) and drawing water for several chains on each side, they are most usefully employed for draining away underground water as well; and, in a country like this, where the price of labour is high, drain-tiles expensive, and the price of produce low, there is no means left for drying wet lands except by open surface-drains, as it would in many cases be cheaper to buy dry land than to dry wet land by an expensive system of tile-draining. In some cases, however, where there is plenty of the right kind of clay for making sound metallic-ringing drain-pipes, cheap coal for burning them, and an enterprising and energetic young farming population to put their shoulders to the wheel, with broad acres of wet lands to improve, the lecturer thought a great deal of pleasure and profit would accrue from the occupation of the odd hours and days and weeks, now and then, in achieving such results.

There is a great variety in the ways of filling up covered drains, the objects aimed at being, of course, in all cases to keep a clear channel for running water at a certain depth under the surface, and to do so in such a way that the channel may draw constantly into itself the excess of water in the soil above, on each side, and from some depth below itself. A waterway is sometimes made by filling in the bottom of the narrow drain to the height of 1ft. or 18in. with angular broken stones or even unbroken stones of all shapes collected from the paddock itself. Over this is then filled in the earth excavated in the digging of the drains, taking care to place on the surface as much of the old surface-soil as possible. With sufficient fall the water finds its way among such stones well enough for a time, but from the necessary irregularities of the channels there is: probably not much suction, and from the fall of loose earth, &c., the channel is liable to obstruction. Sometimes, when stones are used, a big slab is placed edge down on each side in the bottom of the drain, and another slab laid on top resting on these, forming a channel, more stones being thrown in above these, and then the excavated earth is replaced as before. Or slabs are placed in line in the drain, the lower edge resting page 56 on the bottom on one side, while the upper edge (crossing over at [unclear: a] angle of 45deg.) leans on the opposite wall at a height of 5in. or 6in. above the bottom of the drain. In this case the slab forms one wall and cover, the wall of the drain itself being the other wall. The drains are the filled up as before. Occasionally, where stones are not to be had, bundles of straw or of sticks are placed in the bottom of a drain, the soil is them ieplaced, and the water is allowed to find its way as best it may along the rnterstices. This is but a makeshift and very temporary method, as the straw, &c., will soon rot, and probably aggravate the former evil. Sometimes again, where timber is abundant, two logs or saplings are laid lengthways along the bottom, leaving a space between; a large log now placed on the top of these forms the channel, and the excavated earth is filled in as before. In other cases sods (grass side down) are used as covers, the sods resting on each bank of a narrow cut along the bottom of the drain.

The use of drain-tiles marked a great advance in the systems of draining. These, at first, were simply burnt clay house-tiles, laid lengthways, open side down, in the bottom of the drain. When placed close together, end to end, the rounded tops formed, of course, a continuous arch which supported the soil thrown in above; while there was a clear, close, continuous water-channel maintained below. An improvement on these was afterwards made in supplying flat slabs for a bottom on which the edges of these tiles rested, thus maintaining a sound bottom, not liable to be eroded away by the running water.

The next great improvement (in vogue at the present day) was the substitution of pipes, end to end, for the clumsy tiles. On this subject the lecturer had received a great deal of valuable information from Mr. Douglas, of Mount Royal and Waihao Downs, during a three days' visit to the Palmerston district. The pipes, 2in., 2½in., or 3in. diameter, must, to draw properly, be made of well-pugged, sound clay, be well fired, and, when finished, should ring with a prolonged [unclear: metalli] musical note when struck, instead of the dull "thud" or "dump" emitted by the rotten imitation. In placing the pipes in the drain they must be laid on an even uniformly-graded bottom, end on to each other: and care must be taken that the ends are closely applied to each other, so that no particles of soil can get admission into the narrow tunnel. A ring of plastic, well-worked clay should then be firmly applied round the junctions, to help to keep the pipes in proper position, and to prevent any inflow of solids.

The action of such a tile-drain, with a sufficient gradient or fall to secure a constant flow of water, is somewhat as follows:—

1. It draws water mainly from below, but also from the sides to a distance depending on the nature of the soil and the depth of the drain, &c., as well as from above. Such suction and removal of the underground water, if the drains be at proper intervals and of sufficient depth, will at once lower the level of the stagnant water in the soil, or, which is the same thing, lower the level of the water-table through the whole extent of the drained area. This lowering of the water-level, to the depth of 4ft. or more, will make plenty of room for the roots of the growing crops (even the longest-rooted of them) to push and extend themselves in the dry subsoil to pick up nourishment. Whereas, in the undrained, water-logged soil, with the stagnant water-table only a few inches under the surface, such downward penetration of the roots would be impossible, as they only grow in the direction in which they are to find nourishment, and there is none for them in such stagnant soil water.

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2. A proper system of working-drains enables the plants to withstand the effects of drought in a dry season. This at first sight might seem unlikely; but this is how it happens: In undrained, wet clay land, when the drought sets in and the soil becomes dried, the clay hardens into a dense compact impervious mass, which does not admit a passage downward for the roots; they are thus confined to the 6in. or 8in. of dried-up, heated surface-soil, and soon feel the effects of the drought; but in a well-drained clay soil the clay loses this caking character, and becomes crumbly and permeable, thus readily affording an easy passage to the rootlets downwards to the cooler levels, away from the heating, drying action of the sun and hot air. There the roots, at their terminals (for it is by their extremities or terminals and root-hairs that plants take in their mineral food), have a good climate and plenty of dissolved mineral food, without being, on the one hand, confined to the parched, hot surface-soil or, on the other, poisoned by the stagnant water below the water-level.

3. Drains procure atmospheric nourishment for the soil, and fix it there for the use of plants. They do so in this way: There are always, and especially after thunderstorms, nitric acid and nitrous acid in the atmosphere; and there is also some—but not much—ammonia there. Now, all these are the best fertilising substances known, and they are all very soluble in water. The rain, therefore, washes them down on to the surface of the soil; but, if the soil is wet, undrained land, with stagnant water in small pools lying about, these fertilisers will remain in such water, and the rootlets down below will not get any benefit from them. If, on the other hand, the soil is drained, and therefore dry and open and porous, the rain-water, carrying its fertilising properties with it, will be sucked down towards the drain-pipes, and will, in its passage, come in contact with the rootlet terminals and root-hairs, which will speedily relieve it of its burden of nitrate and nitrite of ammonia. Even without the intervention of the rootlets these fertilisers will be fixed in the porous soil itself. Again—and more important—drains furnish atmospheric food by rendering the soil fit for the operations of the nitrogen-microbe. In a soil unduly wet this microbe cannot work, nor is it active in a cold soil. Since, therefore, drains dry and warm the land, as already explained, they fit it for the working of these caterers of nitrogen for the use of the leguminous plants. Not only so, but, since these microbes must have free access to the air, whence to draw this nitrogen, and since the air cannot penetrate or circulate through a water-logged soil (either surface or subsoil water), it is evident that one of the essential conditions of healthy microbe-life is absent; but when the soil is dry and porous the air gets free access away down, and the downward suction of the drain-pipes aids this very materially, so that fresh portions of air are always passing down through the zone (1ft. or 2ft.) occupied by these friends of the farmer, and bringing to them the supplies of nitrogen which it is their function to pass into the plant. In these respects, therefore, and by these processes, nitrogen (partly in the form of ammonia and nitric and nitrous acid, and partly as nitrogenous organic matter) is supplied from the atmosphere by a proper system of drainage and the growth of leguminous plants.

4. Drains also provide a deeper soil, and therefore a larger mass of earthy material whence the plants can take their mineral food; for in a wet soil the roots will not go deep down, as there is too much water there for them; they will not, as a rule, indeed, penetrate the water-table at all, or, if they do so, it will usually be at a great expense of vitality; but in a dry soil, in which by a system of drains the water-table is lowered to a depth of, say, 4ft. from the surface, the roots have all this depth of soil from which to extract their proper food.

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5. Drains hasten the manufacture of soil from the rocky materials of the land itself. All soils contain a large admixture of small particles of such materials; and the stones of all sizes, lying among the loose earth, are of the same nature. Indeed, the whole soil may be looked at as a mixture of clay and sand and stones of various sizes, from a grain to several ounces or even pounds in weight, and particles of organic matter—fragments of the roots and stems and leaves of former generations of plants—mixed with the roots and débris of plants living now in the soil. Now, into a wet stagnant soil the air, with its oxygen and carbonic-acid gas, does not penetrate; but, if the soil be well dried by draining, and therefore loose and porous, the carbonic-acid gas of the air, sucked down by the drains, will disintegrate the silicates of which the stony particles are composed, and from them set free the potash and lime and phosphates they contain, rendering these, for the first time, available for plant-food. At the same time, the oxygen of the air, thus sucked down by the drains, will attack the dead organic matter, converting its carbon into carbonic acid, its nitrogen into some serviceable form, its phosphorus into phosphoric acid, and setting free its lime and potash.

6. Drains, also, by this drying, warming, mellowing, fertilising, soil-making action, qualify the land for growing crops of a more valuable character than the same land could carry in its undrained, wet condition. Examples of this are patent everywhere: such lands may grow good ryegrass, red clover, and superior pasture grasses, instead of the Yorkshire fog of their natural undrained state. They become adapted for growing great crops of barley and wheat instead of the ill-conditioned, thin, stunted oats of their unregenerate days.

7. Drains also help to keep the land clean; for in a wet soil and subsoil the moisture-loving weeds and weedy grasses find a congenial home, and from such a habitat it is impossible to eradicate them, as their rootlets and root-hairs cling so closely to the wet clod and interweave themselves with it, penetrating it by their fine slender white filaments in all directions, and embracing it so tightly and holding it so tenaciously that it is with the utmost difficulty you can shake them asunder; and after yon have succeeded in doing so you will generally find that a few of the broken root-threads have remained in the clod, and will, as soon as you turn your back, start a fresh colony for another generation of weeds. On drain-dried soil, on the other hand, everything is porous and loose, and it is quite an easy matter to kill the obnoxious weeds by bringing them to the surface and shaking the loose soil from them, so as to expose their roots to the drying and killing action of the sun and winds.

8. Another and most important service rendered by drains is to protect the cultivated plants from the ravages of fungus pests—such as the rust and mildew on wheat, oats, and barley, the potato disease, the finger-and-toe disease in turnips, and smut in grain. All these, being kinds of fungus in various states of development, whose growth is favoured by a wet soil, have their congenial nidus in undrained land. Free exposure to a dry air is injurious to them at all stages of their life, and, other things being equal, they will have greater difficulty in maintaining and propagating themselves in such a circulation of dry air as is provided by the suction of the drains acting on a dry, porous, loose soil.

9. Drains sweeten sour lands by, in the first place, drawing off the acid, stagnant water that soaks it, and drawing down in its place fresh, sweet rain-water from the atmosphere; and, secondly, by sucking down the oxygen of the atmosphere, which in its passage will gradually oxidize and destroy the obscure acids that give sourness to a wet soil.