ORIGIN AND FORMATION OF SOILS.

Otago Witness, Issue 2046, 11 May 1893, Page 7

ORIGIN AND FORMATION OF SOILS.

(North British Agriculturist, March 8.)

The fourth and last of the series of agricultural lectures at Kirknewton, under the auspices of the Calder District Committee of the Midlothian County Council, was delivered on Wednesday evening last by Mr John Wallace, lecturer on agriculture at the Heriot-Watt College, Edinburgh. Mr Jas. P. Glendinning, of Inveresk, presided, and, notwithstanding the stormy nature of the weather, there was a large attendance of farmers and others. Mr Wallace, who took for his subject the " Origin and Formation of Soils," dealt most fully ' with the whole matter, explaining at the outset that soils might be divided into two classes — local and erratic or transported soils. The former embraced the soils formed by what was known as the weathering of rocks, and the latter, the soils carried to the spot where they were found by one or other of a series of different agencies at work.

LOCAL WEATHERED SOILS.

Local weathered soils were generally the same, chemically speaking, as the rocks upon which they rested. A great many agencies were at work in the weathering down of rocks, but one of the most important was water— not pure water, as weunderstoodit, but natural water which contained carbonic acid in solution. The latter was one of the great agents in the_ disintegration of rocks, the carbonic acid in it having a most marked effect on such substances in the rock as potash, soda, or lime. Even granite, which was one of the hardest and naturally one of the most indestructible rocks which we had, could not resist the solvent action of the carbonic acid. Granite contained a good deal of what was known as felspar, and the felspar a good deal of potash. The carbonic acid, combined with the latter (potash), setting up a process of disintegration, and ultimately forming what they all knew as clay. In the same way natural water had a marked effect on limestone rock, an important geological formation in some parts of England. Carbonate of lime, of which limestone rock was largely composed, was not very readily soluble in ordinary water, but it was extremely solvent in water containing carbonic acid. In the case of other rocks, those, for instance, that contained the lower oxides of iron, they had other .chemical changes taking place, all of which had a tendency to break up the mass and bring about disintegration. Frost had also an important effect; inbreakingup rocks, and so likewise had changes of temperature, but the latter was not so important in this country as in hot, rainless countries where the effect of the sun's rays playing upon rocks was most helpful in the process of disintegration. All rocks did not weather down alike ; even rocks of the same texture would show differences in this respect. It all depended on the composition rather than the texture,

NO VEGETABLE MATTER.

All the ingredients of a soil, with the exception of vegetable matter, could be produced by weathering. Vegetable matter was, however, all important for plant life, and until they could get it the soil was of no practical value. In a newly-weathered soil, only the lowest forms of vegetation grew — those forms which could exist on the nitrogenous compounds got from the atmosphere in times of rain and thunderstorms. These repeated themselves generation after generation, until, with the remains left by the successive generations, there was sufficient vegetable matter present to supply the necessary nitrogenous compounds for the higher plants. The first of the higher plants which would grow would probably be those of the leguminous order, such as wild clovers, vetches, &c., and they were undoubtedly the most profitable in the circumstances. These plants did not depend so much as some of the other plants on the soil for their nitrogen, as they had the power, not very long since discovered, of making use of the free nitrogen of the air by means of a number of wart-like excresences, or nodules, on their roots. It was a feature of these plants, no doubt due to the nitrifying action of the nodules on their roots, that, as a rule, they left the soil richer in nitrogenous compounds than what they found it. The accumulation of vegetable matter in a weathered soil varied with the composition of the rock. Where the rock was rich in mineral substances, then they would also have a greater increase in vegetable matter than in a soil poor in mineral substances. Vegetable matter in the soil was important as food for living organisms as well as for plants. Not the least important of these organisms was the earth worm, which was of great advantage in the pulverising and making of the soil. Darwin estimated that about 50,000 earth worms could live in an acre of land, and that these would be able to bring up 10 to 15 tons of earth per acre per annum, and lay it on the surface. This would only, however, apply to pasture land, the castings being always turned down by the plough. The worms only worked in the firßt sinto 14in of the soil.

TRANSPORTING SOILS.

Erratic or transported soils were formed from materials which did not belong to the rocks upon which they rested. They were brought from a distance. Wind had been known to carry fine sand 200 or 300 miles, and to deposit it in considerable quantities at given points'. At the present moment there were great complaints in some parts of Russia that the fine black earth was in some parts being encroached upon to an alarming extent by sand blown from the deserts in the interior many miles away. This had, no doubt, been caused by the injudicious cutting down of large tracts of forests and the consequent lessening of the rainfall Glaciers were, and had been, another potent force in transporting soils, while they were all familiar with the large quantities "of silt and sediment which had been brought down by the rivers and deposited along their banks and at their mouths, forming belts of rich alluvial soil. These alluvial soils were generally very rich, because they contained considerable quantities of mineral matter, and because they were always in a fine state of division.

CONSTITUENTS OF SOILS.

Soils should contain a variety of mineral substances, such as phosphoric acid, potash, soda, lime, magnesia, iron, &c, but these should always be in such a state that the plant could make use of them. Clay, which was got through the decomposition of the felspars, should also be present. Pure clay, chemically speaking, was of no value to the plants, and it had also some objectionable characteristics, Buch as a tendency to bind too hard in summer and too wet in winter ; but, on the other hand, it had a wonderful power of retaining plant food in an easily soluble state. Clay was not a very good soil if it stood by itself, but when

they had it in about equal parts with sand, it did very well indeed. It was always a good thing to have a fair amount of stones — not what might be called lumps of sand, but small fragments of rock containing the same mineral substances as the rock itself— because these were always getting decomposed and supplying fresh food for the growing plants. If they had not the stones the soil would soon run done, and then there would be nothing left to grow the crops with. Humus, another name for decayed and decaying vegetable matter, was another constituent of the first importance. Soils might contain supto 10 per cent, of this ; but when the percentage fell to about 1 per cent., they might be sure they had a poor soil to deal with. Humus was of no use directly to plants with green leaves, but it was of immense importance indirectly, inasmuch as it provided them in the form of nitrates, with the necessary supplies of nitrogenous compounds, these nitrates being formed in the soil by a process known as nitrification. This nitrification, which was done by bacteria, required certain conditions before it would act properly, and if they neglected these the results would not be satisfactory. One condition was a moderate amount of moisture, not too' much nor too little, and another » favourable temperature— 9Bdeg Fahr. appeared to be the best temperature. One thing to be remembered with regard to nitrates was that the amount the foil was able to produce annually would determine to a certain degree the fertility of the soil. Nitrates were, of course, produced most during the heat of summer, and there could be little doubt that, in districts wherehighfarmingprevailed,and where the land was full of vegetable matter, there must be a considerable loss of these nitrates, formed after the grain crops had been reaped. It was a question whether it would not be profitable to grow another crop, say, of ryegrass, after the other crops had been harvested to catch these nitrates, which were very easily washed away. The ryegrass was sown in spring on the grain crop, and occupied the land after harvest, and might be ploughed down as a green manure in early winter. In many districts of England this was done at the present time, although he would not maintain that catch crops were grown for the sole purpose of catching the later formed nitrates. When land was under cultivation, especially light land, the tendency was for the vegetable matter to decrease. The great aim of the farmer, therefore, should be to keep the proportion of vegetable matter in his soil at as high a standard as possible. This might be done by * liberal application of farmyard manure, or by planting from time to time a rapid-growing crop and ploughing it in. In Ayrshire and the west coast* where high farming prevailed, a crop of mustard was frequently planted and disposed of in this

way. Another method of increasing the vegetable or nitrogenous matter in the soil was by growing leguminous crops in the rotation. Lime, although itself a plant food, was also a constituent of the soil. The presence of lime in a soil hastened the decomposition of felspars and vegetable matter, while it also sweetened soils in which there was an excess of organic acids. It neutralised their evil effects and hastened the action of the nitrifying bacteria. At the close Mr Wallace was awarded a hearty vote of thanks for his address. The Chairman, in proposing the vote, said he should have liked to have heard a little more as to the differences, from a scientific point of view, between fertile and unfertile land. They all knew that adjoining farms differed greatly in quality, and even on one farm they might get two fields alongside of each other which were in marked contrast to each other in this respect. Mr Wallace's suggestion as to growing of catch crops for the purpose of increasing the nitrates in the soil was worthy of consideration, but he did not think it would do in that district. In some of the warmer districts in the south of England that practice was followed to advantage, but in that locality their climate was too uncertain for them to risk a second crop in the course of one season.

Mr Wallace, in reply, said his suggestion as to the planting of ryegrass after the other crops had been harvested had reference more to the catching of the nitrates which were formed in the end of the year than to the increasing of the nitrates. He quite agreed that this could only be done where the climate was suitable, and in districts where high farming prevailed, and where the land was heavily manured. In estimating the quality of a soil a good many things had to be considered — the composition, texture, depth, &c. ■ They might, for instance, have a soil with a good quantity of vegetable matter in it, and yet it might be a comparatively poor soil. It might be too shallow, or the vegetable matter in it might be rendered useless by the presence of an excessive amount of substances inimical to plant life.