1911 Encyclopædia Britannica/Agriculture/Crops and Cropping
Crops and Cropping.
The greater freedom of cropping and the less close adherence to the formal system of rotation of crops, which characterize the early years of the 10th century, rest upon a scientific basis. Experimental inquiry has done much to enlighten the farmer as to the requirements of plant-life, and to enable him to see how best to meet these requirements in the case of field crops. He cannot afford to ignore the results that have been gradually accumulated—the truths that have been slowly established—at the agricultural experiment stations in various parts of the world. Of these stations the greatest, and the oldest now existing, is that at Rothamsted, Harpenden, Herts, England, which was founded in 1843 by Sir John Bennet Lawes. The results of more than half a century of sustained experimental inquiry were communicated to the world by Lawes and his collaborator, Sir J. H. Gilbert, in about 130 separate papers or reports, many of which were published, from 1847 onwards, in the Journal of the Royal Agricultural Society of England.
In the case of plants the method of procedure was to grow some of the most important crops of rotation, each separately year after year, for many years in succession on the same land, (a) without manure, (b) with farmyard manure and (c) with a great variety of chemical manures; the same description of manure being, as a rule, applied year after year on the same plot. Experiments on an actual course of rotation, without manure, and with different manures, have also been made. Wheat, barley, oats, beans, clover and other leguminous plants, turnips, sugar beet, mange's, potatoes and grass crops have thus been experimented upon. Incidentally there have been extensive sampling and analysing of soils, investigations into rainfall and the composition of drainage waters, inquiries into the amount of water transpired by plants, and experiments on the assimilation of free nitrogen.
Cereals.—Amongst the field experiments there is, perhaps, not one of more universal interest than that in which wheat was grown for fifty-seven years in succession, (a) without manure, (b) with farmyard manure and (c) with various artificial manures. The results show that, unlike leguminous crops such as beans or clover, wheat may be successfully grown for many years in succession on ordinary arable land, provided suitable manures be applied and the land be kept clean. Even without manure the average produce over forty-six years, 1852–1897, was nearly thirteen bushels per acre, or about the average yield per acre of the wheat lands of the whole world. Mineral manures alone give very little increase, nitrogenous manures alone considerably more than mineral manures alone, but the mixture of the two considerably more than either separately. In one case, indeed, the average produce by mixed minerals and nitrogenous manure was more than that by the annual application of farmyard manure; and in seven out of the ten cases in which such mixtures were used the average yield per acre was from over two to over eight bushels more than the average yield of the United Kingdom (assuming this to be about twenty-eight bushels of 60 ℔ per bushel) under ordinary rotation. It is estimated that the reduction in yield of the unmanured plot over the forty years, 1852–1891, after the growth of the crops without manure during the eight preceding years, was, provided it had been uniform throughout, equivalent to a decline of one-sixth of a bushel from year to year due to exhaustion—that is, irrespectively of fluctuations due to season. It is related that a visitor from the United States, talking to Sir John Lawes, said, “Americans have learnt more from this field than from any other agricultural experiment in the world.”
Experiments upon the growth of barley for fifty years in succession on rather heavy ordinary arable soil resulted in showing that the produce by mineral manures alone is larger than that without manure; that nitrogenous manures alone give more produce than mineral manures alone; and that mixtures of mineral and nitrogenous manure give much more than either used alone—generally twice, or more than twice, as much as mineral manures alone. Of mineral constituents, whether used alone or in mixture with nitrogenous manures, phosphates are much more effective than mixtures of salts of potash, soda and magnesia. The average results show that, under all conditions of manuring—excepting with farmyard manure—the produce was less over the later than over the earlier periods of the experiments, an effect partly due to the seasons. But the average produce over forty years of continuous growth of barley was, in all cases where nitrogenous and mineral manures (containing phosphates) were used together, much higher than the average produce of the crop grown in ordinary rotation in the United Kingdom, and very much higher than the average in most other countries when so grown. The requirements of barley within the soil, and its susceptibility to the external influences of season, are very similar to those of its near ally, wheat. Nevertheless there are distinctions of result dependent on differences in the habits of the two plants, and in the conditions of their cultivation accordingly. In the British Isles wheat is, as a rule, sown in the autumn on a heavier soil, and has four or five months in which to distribute its roots, and so it gets possession of a wide range of soil and subsoil before barley is sown in the spring. Barley, on the other hand, is sown in a lighter surface soil, and, with its short period for root-development, relies in a much greater degree on the stores of plant-food within the surface soil. Accordingly it is more susceptible to exhaustion of surface soil as to its nitrogenous, and especially as to its mineral supplies; and in the common practice of agriculture it is found to be more benefited by direct mineral manures, especially phosphatic manures, than is wheat when sown under equal soil conditions. The exhaustion of the soil induced by both barley and wheat is, however, characteristically that of available nitrogen; and when, under the ordinary conditions of manuring and cropping, artificial manure is still required, nitrogenous manures are, as a rule, necessary for both crops, and, for the spring-sown barley, superphosphate also. Although barley is appropriately grown on lighter soils than wheat, good crops, of fair quality, may be grown on the heavier soils after another grain crop by the aid of artificial manures, provided that the land is sufficiently clean. Experiments similar to the foregoing were carried on for many years in succession at Rothamsted upon oats, and gave results which were in general accordance with those on the other cereal crops.
Additional significance to the value of the above experiments on wheat and barley is afforded by the fact that the same series, with but slight modifications, has also been carried out since 1876 at the Woburn (Bedfordshire) experimental farm of the Royal Agricultural Society of England, the soil here being of light sandy character, and thus very different from the heavy soil of Rothamsted. The results for the thirty years, 1877–1906, are in their general features entirely confirmatory of those obtained at Rothamsted.
Root-Crops.—Experiments upon root-crops—chiefly white turnips, Swedish turnips (swedes) and mangels—have resulted in the establishment of the following conclusions. Both the quantity and the quality of the produce, and consequently its feeding value, must depend greatly upon the selection of the best description of roots to be grown, and on the character and the amount of the manures, and especially on the amount of nitrogenous manure employed. At the same time, no hard-and-fast rules can be laid down concerning these points. Independently of the necessary consideration of the general economy of the farm, the choice must be influenced partly by the character of the soil, but very much more by that of the climate. Judgment founded on knowledge and aided by careful observation; both in the field and in the feeding-shed, must be relied upon as the guide of the practical farmer. Over and above the great advantage arising from the opportunity which the growth of root-crops affords for the cleaning of the land, the benefits of growing the root-crop in rotation are due (1) to the large amount of manure applied for its growth, (2) to the large residue of the manure left in the soil for future crops, (3) to the large amount of matter at once returned as manure again in the leaves, (4) to the large amount of food produced, and (5) to the small proportion of the most important manurial constituents of the roots which is retained by store or fattening animals consuming them, the rest returning as manure again; though, when the roots are consumed for the production of milk, a much larger proportion of the constituents is lost to the manure.
Leguminous Crops and the Acquisition of Nitrogen.—The fact that the growth of a leguminous crop, such as red clover, leaves the soil in a higher condition for the subsequent growth of a grain crop—that, indeed, the growth of such a leguminous crop is to a great extent equivalent to the application of a nitrogenous manure for the cereal crop—was in effect known ages ago. Nevertheless it was not till near the approach of the closing decade of the 19th century that the explanation of this long established point of agricultural practice was forthcoming. It was in the year 1886 that Hellriegel and Wilfarth first published in Germany the results of investigations in which they demonstrated that, through the agency of micro-organisms dwelling in nodular outgrowths on the roots of ordinary leguminous plants, the latter are enabled to assimilate the free nitrogen of the air. The existence of the root nodules had long been recognized, but hitherto no adequate explanation had been afforded as to their function.
Since Hellriegel’s striking discovery farm crops have been conveniently classified as nitrogen-accumulating and nitrogen consuming. To the former belong the ordinary leguminous crops—the clovers, beans, peas, vetches or tares, sainfoin, lucerne, for example—which obtain their nitrogen from the air, and are independent of the application of nitrogenous manures, whilst in their roots they accumulate a store of nitrogen which will ultimately become available for future crops of other kinds. It is, in fact, fully established that these leguminous crops acquire a considerable amount of nitrogen by the fixation of the free nitrogen of the atmosphere under the influence of the symbiotic growth of their root-nodule-microbes and the higher plant. The cereal crops (wheat, barley, oats, rye, maize); the cruciferous crops (turnips, cabbage, kale, rape, mustard); the solanaceous crops (potatoes); the chenopodiaceous crops (mangels, sugar-beets), and other non-leguminous crops have, so far as is known, no such power, and are therefore more or less benefited by the direct application of nitrogenous manures. The field experiments on leguminous plants at Rothamsted have shown that land which is, so to speak, exhausted so far as the growth of one leguminous crop is concerned, may still grow very luxuriant crops of another plant of the same natural order, but of different habits of growth, and especially of different character and range of roots. This result, is doubtless largely dependent on the existence, the distribution and the condition of the appropriate microbes for the due infection of the different descriptions of plant, for the micro-organism that dwells symbiotically with one species is not identical with that which similarly dwells with another. It seems certain that success in any system involving a more extended growth of leguminous crops in rotations must be dependent on a considerable variation in the description grown. Other essential conditions of success will commonly include the liberal application of potash and phosphatic manures, and sometimes chalking or liming for the leguminous crop. As to how long the leguminous crop should occupy the land, the extent to which it should be consumed on the land, or the manure from its consumption be returned, and under what conditions the whole or part of it should be ploughed in—these are points which must be decided as they arise in practice. It seems obvious that the lighter and poorer soils would benefit more than the heavier or richer soils by the extended growth of leguminous crops.
Remarkable as Hellriegel’s discovery was, it merely furnished the explanation of a fact which had been empirically established by the husbandman long before, and had received most intelligent application when the old four-course (or Norfolk) rotation was devised. But it gave some impetus to the practice of green manuring with leguminous crops, which are equally capable with such a crop as mustard of enriching the soil in humus, whilst in addition they bring into the soil from the atmosphere a quantity of nitrogen available for the use of subsequent crops of any kind. In Canada and the United States this rational employment of a leguminous crop for ploughing in green is largely resorted to for the amelioration of worn-out wheat lands and other soils, the condition of which has been lowered to an unremunerative level by the repeated growth year after year of a cereal crop. The well-known paper of Lawes, Gilbert and Pugh (1861), “On the Sources of the Nitrogen of Vegetation, with special reference to the Question whether Plants assimilate free or uncombined Nitrogen,” answered the question referred to in the negative. The attitude taken up later on with regard to this problem is set forth in the following words, which are quoted from the Memoranda of the Rothamsted Experiments, 1900 (p. 7):—
“Experiments were commenced in 1857, and conducted for several years in succession, to determine whether plants assimilate free or uncombined nitrogen, and also various collateral points. Plants of the gramineous, the leguminous and of other families were operated upon. The late Dr Pugh took a prominent part in this inquiry. he conclusion arrived at was that our agricultural plants do not themselves directly assimilate the free nitrogen of the air by their leaves.
In recent years, however, the question has assumed quite a new aspect. It now is—whether the free nitrogen of the atmosphere is brought into combination under the influence of micro-organisms, or other low forms, either within the soil or in symbiosis with a. higher plant, thus serving indirectly as a source of nitro en to plants of a higher order. Considering that the results of Hellriegel and Wilfarth on this point were, if confirmed, of great significance and importance, it was decided to make experiments at Rothamsted on somewhat similar lines. Accordingly, a preliminary series was undertaken in 1888; more extended series were conducted in 1889 and in 1890; and the investigation was continued up to the commencement of the year 1895. Further experiments relating to certain aspects of the subject were begun in 1898. The results have shown that, when a soil growing leguminous plants is infected with appropriate organisms, there is a development of the so-called leguminous nodules on the roots of the plants, and, coincidently, increased growth and gain of nitrogen.”
The conclusions of Hellriegel and Wilfarth have thus been confirmed by the later experiences of Rothamsted, and since that time efforts have been directed energetically to the practical application of the discovery. This has taken the form of inoculating the soil with the particular organism required by the particular kind of leguminous crop. To this end the endeavour has been made to produce preparations which shall contain in portable form the organisms required by the several plants, and though, as yet, it can hardly be claimed that they have been generally successful, the work done justifies hopes that the problem will eventually be solved in a practical direction.
Grass.—Another held experiment of singular interest is that relating to the mixed herbage of permanent meadow, for which seven acres of old grass land were set apart in Rothamsted Park in 1856. Of the twenty plots into which this land is divided, two were left without manure from the outset, two received ordinary farmyard manure for a series of years, whilst the remainder each received a different description of artificial or chemical manure, the same being, except in special cases, applied year after year on the same plot. During the growing season the field affords striking evidence of the influence of different manurial dressings. So much, indeed, does the character of the herbage vary from plot to plot that the effect may fairly be described as kaleidoscopic. Repeated analyses have shown how greatly both the botanical constitution and the chemical composition of the mixed herbage vary according to the description of manure applied. They have further shown how dominant is the influence of season. Such, moreover, is the effect of different manures that the gross produce of the mixed herbage is totally different on the respective plots according to the manure employed, both as to the proportion of the various species composing it and as to their condition of development and maturity.
The Rotation of Crops.
The growth, year after year, on the same soil of one kind of plant unfits it for bearing further crops of the kind which has exhausted it, and renders them§ less vigorous and more liable to disease. The farmer therefore arranges his cropping in such a way that roots, or leguminous crops, succeed the cereal crops.
It is not only the conditions of growth, but the uses to which the different crops are put, that have to be considered in the case of rotation. Thus the cereal crops, when grown in rotation, yield more produce for sale in the season of growth than when grown continuously. Moreover, the crops alternated with the cereals accumulate very much more of mineral constituents and of nitrogen in their produce than do the cereals themselves. By far the greater proportion of those constituents remains in circulation in the manure of the farm, whilst the remainder yields highly valuable products for sale in the forms of meat and milk. For this reason these crops are known as “restorative,” cereals the produce of which is sold off the farm being classed as “exhaustive.” With a variety of crops, again, the mechanical operations of the farm, involving horse and hand labour, are better distributed over the year, and are therefore more economically performed. The opportunities which rotation cropping affords for the cleaning of the land from weeds is another distinct element of advantage. Although many different rotations of crops are practised, they may for the most part be considered as little more than local adaptations of the system of alternating root-crops and leguminous crops with cereal crops, as exemplified in the old four-course rotation-roots, barley, clover, wheat.
Under this system the clover is ploughed up in the autumn, the nitrogen stored up in its roots being left in the soil for the nourishment of the cereal crop. The following summer the wheat crop is harvested, and an opportunity is afforded for extirpating weeds which in the three previous years have received little check. Or, where the climate is warm and the soil light, a “catch-crop,” i.e. rye, vetches, winter-oats or some other rapidly-growing crop may be sown in autumn and fed on or otherwise disposed of prior to the root-sowing. On heavy soils, however, the farmer cannot afford to curtail the time necessary for thorough cultivation of the land. The cleaning process is carried on through the next summer by means of successive hoeings of the spring-sown root-crop. As turnips or Swedes may occupy the ground till after Christmas little time is left for the preparation of a seed-bed for barley, but as the latter is a shallow-rooted crop only surface-stirring is required. Clover is sown at the same time or shortly after the cereal and thus occupies the land for two years.
The rotations extending to five, six, seven or more years are, in most cases, only adaptations of the principle to variations of soil, altitude, aspect, climate, markets and other local conditions. They are effected chiefly by some alteration in the description of the root-crop, and perhaps by the introduction of the potato crop; by growing a different cereal, or it may be more than one cereal consecutively; by the growth of some other leguminous crop than clover, since “clover-sickness” may result if that crop is grown at too short intervals, or the intermixture of grass seeds with the clover, and perhaps by the extension by one or more years of the period allotted to this member of the rotation. Whatever the specific rotation, there may in practice be deviations from the plan of retaining on the farm the whole of the root-crops, the straw of the grain crops and the leguminous fodder crops (clover, vetches, sainfoin, &c.) for the production of meat or milk, and, coincidently, for that of manure to be returned to the land. It is equally true that, when under the influence of special local or other demand—proximity to towns, easy railway or other communication, for example—the products which would otherwise be retained on the farm, are exported from it, the import of town or other manures is generally an essential condition of such practice. This system of free sale, indeed, frequently involves full compensation by purchased manures of some kind. Such deviations from the practice of merely selling grain and meat off the farm have much extended in recent years, and will probably continue to do so under the altered conditions of British agriculture, determined by very large imports of grain, increasing imports of meat and of other products of stock-feeding, and very large imports of cattle-food and other agricultural produce. More attention is thus being devoted to dairy produce, not only on grass farms, but on those that are mainly arable.
The benefits that accrue from the practice of rotation are well illustrated in the results obtained from the investigations at Rothamsted into the simple four-course system, which may fairly be regarded as a self-supporting system. Reference may first be made to the important mineral constituents of different crops of the four-course rotation. Of phosphoric acid, the cereal crops take up as much as, or more than, any other crops of the rotation, excepting clover; and the greater portion thus taken up is lost to the farm in the saleable product—the grain. The remainder, that in the straw, as well as that in the roots and the leguminous crops, is supposed to be retained on the farm, excepting the small amount exported in meat and milk. Of potash, each of the rotation crops takes up very much more than of phosphoric acid. But much less potash than phosphoric acid is exported in the cereal grains, much more being retained in the straw, whilst the other products of the rotation-the root and leguminous crops—which are also supposed to be retained on the farm, contain very much more potash than the cereals, and comparatively little of it is exported in meat and milk. Thus the whole of the crops of rotation take up very much more of potash than of phosphoric acid, Whilst probably even less of it is ultimately lost to the land. Of lime, very little is taken up by the cereal crops, and by the root-crops much less than of potash; more by the leguminous than by the other crops, and, by the clover especially, sometimes much more than by all the other crops of the rotation put together. Very little of the lime of the crops, however, goes off in the saleable products of the farm in the case of the self-supporting rotation under consideration. Although, therefore, different, and sometimes very large, amounts of these typical mineral constituents are taken up by the various crops of rotation, there is no material export of any in the saleable products, excepting of phosphoric acid and of potash; and, so far at least as phosphoric acid is concerned, experience has shown that it may be advantageously supplied in purchased manures.
Of nitrogen, the cereal crops take up and retain much less than any of the crops alternated with them, notwithstanding the circumstance that the cereals are very characteristically benefited by nitrogenous manures. The root-crops, indeed, may contain two or more times as much nitrogen as either of the cereals,
- See J. B. Lawes and J. H. Gilbert, Rothamsted Memoirs on Agricultural Chemistry and Physiology, 7 vols. (1893–1899); A. D. Hall, Book of the Rothamsted Experiments (1905).