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The Pamphlet Collection of Sir Robert Stout: Volume 73

Rotation of Crops

Rotation of Crops.

Without going very deeply into the subject the lecturer would put before his audience a few points that are worthy of consideration in connection with the changing of crops from year to year in a regular manner. In doing so he would confine himself as much as possible to the chemical aspects of the subject.

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Some plants require much of one kind of food from the soil, some much of another kind. Some plants, again, can take their food from greater depths in the soil than others. Growing the same kind of crop, grain, or potatoes, year after year without change, exhausts the soil of particular ingredients, while leaving an abundance of other ingredients just suited to the requirements of another kind of crop.

It is for the purpose of utilising in a regular way all the useful kinds of plant-food in the soil that the rotation systems were first established. These did not come into practice suddenly, but were slowly evolved from the experience of farmers, and are as numerous and varied as are the conditions of soil and climate themselves. How the balance of mineral food-constituents is maintained by a change of crops may be seen by looking separately at one or two of these plant-requisites. Magnesia is required by all the cultivated plants, and without it they cannot grow to maturity. But the quantity of magnesia required and removed from the soil by a good crop of one kind is quite different from the quantity that would be taken up by a good crop of something else. A crop of potatoes, for example, yielding 10 tons to the acre, or of clover-hay yielding 3 tons, or of mangels yielding 20 tons to the acre, would remove from the soil about three times as much magnesia as would be taken up by a crop of wheat, or oats, or barley, of, say 50 bushels to the acre. If, therefore, the soil is, from any cause, such as a succession of crops of potatoes, or mangels, or clover-hay, poor in soluble or available magnesia, it would evidently be injudicious to waste time and labour in trying to force any of these or other magnesia-loving crops from the impoverished soil. There might, however, still be in such soil quite enough soluble magnesia for the requirements of a good crop of wheat, or oats, or barley. The proper procedure would then be, of course, by manuring with kainit or other Stassfurt fertiliser, to restore to the soil the magnesia that it lacks; or to grow a cereal crop which does not require much magnesia; or, probably, better still, to lay down in grass for a few years, and thus give time for the disintegrating and decomposing action of the air and frost and rain to reduce to the soluble state a fresh contribution of magnesia from the insoluble soil itself. And as with magnesia so with potash. A crop of 50 bushels of wheat to the acre requires and removes from every acre on which it grows, say, 17lb. of potash; whilst a 10-ton crop of potatoes requires and removes 140lb. of potash, and a 3-ton crop of meadow-hay 100lb. of potash, and a 20-ton crop of turnips or mangels from 150lb. to 200lb. of potash. It is at once seen that if the amount of potash in the soil is low from any cause-as by the withdrawal of too much of that constituent by injudicious potato-growing, or perhaps the soil was originally poor in potash-potatoes or turnips or mangels will be an unprofitable crop and will aggravate, the evil. Wheat, therefore, or another grain crop that does not require much potash, will suggest itself, or a rest for a few years under grass. With either of these the insoluble potash of the raw soil; will have time to become dissolved before the next call will be made upon it. Or if the soil be really destitute of potash in every form, which can only be revealed by an analysis, then the remedy will be in supplying the missing ingredient in the form of one of the potash manures. The same kind of considerations applies of course to the phosphoric acid in the soil, the lime, the sulphates, and all the other mineral plant-requisites. "The minimum rules the crop." If potash be the minimum in the soil—that is, if there is plenty of every kind of plant-food—phosphoric acid, lime, magnesia, nitrogen, &c.—except potash, and there is just enough soluble potash for 3 tons of potatoes to the acre, then, notwithstanding the page 49 abundance of phosphates, lime, magnesia, and nitrogen, the yield will not exceed 3 tons of potatoes to the acre. In such soil, however, there is enough potash and every other requisite for 50 bushels of wheat, or oats, or barley to the acre, and one of these of course, and not potatoes, is the crop to grow on such land.

There is, however, another consideration that arises when we take the case of peas or beans, lucerne, lupines, vetches, the clovers, or other leguminous plants, and the part they play in the rotation of crops. Nitrogen is one of the least abundant, yet most necessary, ingredients of a soil. And if we exclude all the leguminous plants, and consider only the supply of nitrogen to the cereals (wheat, oats, &c.), potatoes, and turnips, the same rule will apply to the treatment of soil deficient in this element as the lecturer indicated above for potash or lime—namely, administer nitrogenous manures in the form of ammonia or nitrates, or grow crops that do not require much nitrogen. But the recent revolutionary discoveries of Hellriegel (mentioned in a previous lecture) have now put nitrogen and the systems of rotation on a different footing.

It did not require a Hellriegel to tell the farmer that a good crop of peas and beans or clover would put the land in better heart for a big crop of wheat to follow.

The old Romans of two or three thousand years ago were aware that a good crop of lupines insured a good crop of grain; and the frequent and favourable mention in the oldest writings extant of pulse (leguminous) as food for man suggests the probability that even in the days of the Bible leguminous crops were grown in rotation with grain. However that may be, certain it is that for hundreds of years the fertilising and regenerative action of leguminous crops on the fields that had been run down by overcropping with the cereals has been recognised in all civilised countries where grain is grown for the food of man. It is indeed quite possible that the great old historical populations of Assyria, Persia, and Palestine, and other once fertile regions, persisted in growing crops of legumes (pulse, &c.) and grain till, stimulated by the never-failing abundance of nitrogen from the atmosphere, they had abstracted from the soil so much of some of its other necessary ingredients (phosphoric acid, for example) that there was not enough left to grow further crops; and hence the impoverished condition of these countries now, and the consequent sparsity and meagreness of their half-starved populations. This impoverishment of these Eastern lands may also have contributed to their relegation to the state of sandy deserts.

There is no feature of the systems of rotation, as practised in modern times, so remarkable as the frequency with which we find a leguminous crop either preceding or succeeding a cereal crop, and in many of the most approved rotations even taking its place between two of them. The following system of rotation practised for many years with the best advantage shows this position of legumes (peas, beans, vetches, clovers):—