1911 Encyclopædia Britannica/Agriculture/The Rotation of Crops

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 off 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, and the leguminous crops, especially the clover, much more than the root-crops. The greater part of the nitrogen of the cereals is, however, sold off the farm; but perhaps not more than 10 or 15% of that of either the root-crop or the clover (or other forage leguminous crop) is sold off in animal increase or in milk. Most of the nitrogen in the straw of the cereals, and a very large proportion of that of the much more highly nitrogen-yielding crops, returns to the land as manure, for the benefit of future cereals and other crops. As to the source of the nitrogen of the root-crops—the so-called “restorative crops”—these are as dependent as any crop that is grown on available nitrogen within the soil, which is generally supplied by the direct application of nitrogenous manures, natural or artificial. Under such conditions of supply, however, the root-crops, gross feeders as they are, and distributing a very large extent of fibrous feeding root within the soil, avail themselves of a much larger quantity of the nitrogen supplied than the cereal crops would do in similar circumstances. This result is partly due to their period of accumulation and growth extending even months after the period of collection by the ripening cereals has terminated, and at the season when nitrification within the soil is most active, and the accumulation of nitrates in it is the greatest. When a full supply of both mineral constituents and nitrogen is at command, these root-crops assimilate a very large amount of carbon from the atmosphere, and produce, besides nitrogenous food materials, a very large amount of the carbohydrate sugar, as respiratory and fat-forming food for the live stock of the farm.

Table XI.—The Weight and Average Composition of Ordinary Crops, in lb. per Acre.

Crop.

Weight of Crop.

Total  Pure  Ash.

Nitro- gen.

Sul- phur.

Potash.

Soda.

Lime.

Mag- nesia.

Phos- phoric  Acid.

Chlor- ine.

Silica.

At  Harvest.

Dry.

Wheat, grain, 30 bushels

1,800

1530

30

34

2·7

9·3

0·6

1·0

3·6

14·2

0·1

0·6

Wheat,„ straw

3,158

2653

142

16

5·1

19·5

2·0

8·2

3·5

6·9

2·4

96·3

Total crop

4,958

4183

172

50

7·8

28·8

2·6

9·2

7·1

21·1

2·5

96·9

Barley, grain, 40 bushels

2,080

1747

46

35

2·9

9·8

1·1

1·2

4·0

16·0

0·5

11·8

Barley,„ straw.

2,447

2080

111

14

3·2

25·9

3·9

8·0

2·9

4·7

3·6

56·8

Total crop

4,527

3827

157

49

6·1

35·7

5·0

9·2

6·9

20·7

4·1

68·6

Oats, grain, 45 bushels

1,890

1625

51

34

3·2

9·1

0·8

1·8

3·6

13·0

0·5

19·9

Oats,„ straw

2,835

2353

140

18

4·8

37·0

4·6

9·8

5·1

6·4

6·1

65·4

Total crop

4,725

3978

191

52

8·0

46·1

5·4

11·6

8·7

19·4

6·6

85·3

Maize, grain, 30 bushels

1,680

1500

22

28

1·8

6·5

0·2

0·5

3·4

10·0

0·2

0·5

Maize,„ stalks, &c.

2,208

1877

99

15

..

29·8

..

..

..

8·0

..

..

Total crop

3,888

3377

121

43

..

36·3

..

..

..

18·0

..

..

Meadow hay, 11/2 ton

3,360

2822

203

49

5·7

50·9

9·2

32·1

14·4

12·3

14·6

56·9

Red Clover hay, 2 tons

4,480

3763

258

98

9·4

83·4

5·1

90·1

28·2

24·9

9·8

7·0

Beans, grain, 30 bushels

1,920

1613

58

78

4·4

24·3

0·6

2·9

4·2

22·8

1·1

0·4

Beans,„ straw.

2,240

1848

99

29

4·9

42·8

1·7

26·3

5·7

6·3

4·3

6·9

Total crop

4,160

3461

157

107

9·3

67·1

2·3

29·2

9·9

29·1

5·4

7·3

Turnips, root, 17 tons

38,080

3126

218

61

15·2

108·6

17·0

5·5

5·7

22·4

10·9

2·6

Turnips,„ leaf

11,424

1531

146

49

5·7

40·2

7·5

48·5

3·8

10·7

11·2

5·1

Total crop

49,504

4657

364

110

20·9

148·8

24·5

74·0

9·5

33·1

22·1

7·7

Swedes, root, 14 tons

31,360

3349

163

70

14·6

63·3

22·8

19·7

6·8

16·9

6·8

3·1

Swedes,„ leaf

4,704

706

75

28

3·2

16·4

9·2

22·7

2·4

4·8

8·3

3·6

Total crop

36,064

4055

238

98

17·8*

79·7

32·0

42·4

9·2

21·7

15·1

6·7

Mangels, root, 22 tons

49,280

5914

426

98

4·9

222·8

69·4

15·9

18·3

36·4

42·5

8·7

Mangels,„ leaf

18,233

1654

254

51

9·1

77·9

49·3

27·0

24·2

16·5

40·6

9·2

Total crop

67,513

7568

680

149

14·0

300·7

118·7

42·9

42·5

52·9

83·1

17·9

Potatoes, tubers, 6 tons

13,440

3360

127

46

2·7

76·5

3·8

3·4

6·3

21·5

4·4

2·6

 *Calculated from a single analysis only.

The still more highly nitrogenous leguminous crops, although not characteristically benefited by nitrogenous manures, nevertheless contribute much more nitrogen to the total produce of the rotation than any of the other crops comprised in it. It is the leguminous fodder crops—especially clover, which has a much more extended period of growth, and much wider range of collection within the soil and subsoil, than any of the other crops of the rotation—that yield in their produce the largest amount of nitrogen per acre. Much of this is doubtless taken up as nitrate, yet the direct application of nitrate of soda has comparatively little beneficial influence on their growth. The nitric acid is most likely taken up chiefly as nitrate of lime, but probably as nitrate of potash also, and it is significant that the high nitrogen-yielding clover takes up, or at least retains, very little soda. Table XI., from Warington’s Chemistry of the Farm, 19th edition (Vinton and Co.), will serve to illustrate the subjects that have been discussed in this section.

For further information on the routine and details of farming, reference may be made to the articles under the headings of the various crops and implements.