CHAPTER XIII._Elements of Agricultural Chemistry

THE ROTATION OF CROPS.

Reference has already been more than once made to the fact that a crop

growing in any soil must necessarily exhaust it to a greater or less

extent by withdrawing from it a certain quantity of the elements to

which its fertility is due. That this is the case has been long admitted

in practice, and it has also been established that the exhausting

effects of different species of plants are very different; that while

some rapidly impoverish the soil, others may be cultivated for a number

of years without material injury, and some even _apparently_ improve it.

Thus, it is a notorious fact that white crops exhaust, while grass

improves the soil; but the improvement in the latter case is really

dependent on the fact, that when the land is laid down in pasture,

nothing is removed from it, the cattle which feed on its produce

restoring all but a minute fraction of the mineral matters contained in

their food; and as the plants derive a part, and in some instances a

very large part, of their organic constituents from the air, the

fertility of the soil must manifestly be increased, or at all events

maintained in its previous state. When, however, the plant, or any

portion of it, is removed from the soil, there must be a reduction of

fertility dependent on the quantity of valuable matters withdrawn by it;

and thus it happens that when a plant has grown on any soil, and has

removed from it a large quantity of nutritive matters, it becomes

incapable of producing an equally large crop of the same species; and if

the attempt be made to grow it in successive years, the land becomes

incapable of producing it at all, and is then said to be thoroughly

exhausted. But if the exhausted land be allowed to lie for some time

without a crop, it regains its fertility more or less rapidly according

to circumstances, and again produces the same plant in remunerative

quantity. The observation of this fact led to the introduction of naked

fallows, which, up to a comparatively recent period, were an essential

feature in agriculture. But after a time it was observed that the land

which had been exhausted by successive crops of one species was not

absolutely barren, but was still capable of producing a luxuriant growth

of other plants. Thus peas, beans, clover, or potatoes, could be

cultivated with success on land which would no longer sustain a crop of

grain, and these plants came into use in place of the naked fallow under

the name of fallow crops. On this was founded the rotation of crops; for

it was clear that a judicious interchange of the plants grown might

enable the soil to regain its fertility for one crop at the time when it

was producing another; and when exhausted for the second, it might be

again ready to bear crops of the first.

The necessity for a rotation of crops has been explained in several

ways. The oldest view is that of Decandolle, who founded his theory on

the fact that the plants excrete certain substances from their roots. He

found that when plants are grown in water, a peculiar matter is thrown

off by the roots; and he believed that this extrementitious substance is

eliminated _because_ it is injurious to the plant, and that, remaining

in the soil, it acts as a poison to those of the same species, and so

prevents the growth of another crop. But this excretion, though

poisonous to the plants from which it is excreted, he believed to be

nutritive to those of another species which is thus enabled to grow

luxuriantly where the others failed. Nothing can be more simple than

this explanation, and it was readily embraced at the time it was

propounded and considered fully satisfactory. But when more minutely

examined, it becomes apparent that the facts on which it is founded are

of a very uncertain character. Decandolle's observations regarding the

radical excretions of plants have not been confirmed by subsequent

observers. On the contrary, it has been shewn that though some plants,

when growing in water, do excrete a particular substance in small

quantity, nothing of the sort appears when they are grown in a siliceous

sand. And hence the inference is, that the peculiar excretion of plants

growing in water is to be viewed as the result of the abnormal method of

their growth rather than as a natural product of vegetation. But even

admitting the existence of these matters, it would be impossible to

accept the explanation founded upon them, because it is a familiar fact

that, on some soils, the repeated growth of particular crops is

perfectly possible, as, for instance, on the virgin soils of America,

from which many successive crops of wheat have been taken; and in these

cases the alleged excretion must have taken place without producing any

deleterious effect on the crop. Besides, it is in the last degree

improbable that these excretions, consisting of soluble organic matters,

should remain in the soil without undergoing decomposition, as all

similar substances do; and even if they did, we cannot, with our present

knowledge of the food of plants, admit the possibility of the direct

absorption of any organic substance whatever. Indeed, the idea of

radical excretions, as an explanation of the rotation of crops, must be

considered as being entirely abandoned.

The necessity for a rotation of crops is now generally attributed to the

different quantities of valuable matters which different plants remove

from the soil, and more especially to their mineral constituents. It has

been already observed that great differences exist in the composition of

the ash of different plants in the section on that subject; and it was

stated that a distinction has been made between lime, potash, and silica

plants, according as one or other of these elements preponderate in

their ashes. The remarkable difference in the proportion of these

elements has been supposed to afford an explanation of rotation. It is

supposed that if a plant requiring a large quantity of any one element,

potash, for example, be grown during a succession of years on the same

soil, it will sooner or later exhaust all, or nearly all, the potash

that soil contains in an _available_ form, and it will consequently

cease to produce a luxuriant crop. But if this plant be replaced by

another which requires only a small quantity of potash and a large

quantity of lime, it will flourish, because it finds what is necessary

to its growth. In the meantime, the changes which are proceeding in the

soil, are liberating new quantities of the inorganic matters from those

forms of combination in which they are not immediately available, and

when after a time the plant which requires potash is again sown on the

soil, it finds a sufficient quantity to serve its purpose. We have

already, in treating of the ashes of plants, pointed out the extent of

the differences which exist; but these will be made more obvious by the

annexed table, giving the quantity of the different mineral matters

contained in the produce of an imperial acre of the different crops.

TABLE shewing the quantities of Mineral Matters and Nitrogen in average

Crops of the principal varieties of Farm Produce.

+---------------+--------------+---------+----------+---------+-------+-------+

+---------------+--------------+---------+----------+---------+-------+-------+

|Wheat--Grain | 28 bushels | 1,680 | 34.12 | 10.11 | 1.20 | 1.04 |

| Straw | 1 ton 3 cwt. | 2,576 | 114.48 | 20.70 | 2.84 | 8.53 |

| Total | ... | ... | 148.60 | 30.81 | 4.04 | 9.57 |

| | | | | | | |

|Barley--Grain | 33 bushels | 1,749 | 44.24 | 9.40 | 0.30 | 0.76 |

| Straw | 18 cwt. | 2,106 | 99.14 | 11.24 | 1.14 | 5.81 |

| Total | ... | ... | 143.38 | 20.64 | 1.44 | 6.57 |

| | | | | | | |

|Oats--Grain | 34 bushels | 1,360 | 48.89 | 11.00 | ... | 5.31 |

| Straw | 1 ton. | 2,240 | 143.53 | 30.71 | 6.10 | 10.29 |

| Total | ... | ... | 192.42 | 41.71 | 6.10 | 15.60 |

| | | | | | | |

|Beans, Peas-- | 25 bushels | 1,650 | 55.97 | 30.00 | 0.31 | 3.01 |

| Straw | 1 ton. | 2,240 | 108.51 | 48.61 | 13.14 | 29.37 |

| Total | ... | ... | 164.48 | 78.61 | 13.45 | 32.38 |

| | | | | | | |

|Turnips--Bulbs | 13-1/2 tons. | 30,240 | 213.75 | 57.35 | 44.71 | 28.60 |

| | | | | | | |

|Potatoes | 3 tons. | 6,720 | 55.58 | 28.92 | 2.85 | 1.20 |

| | | | | | | |

|Hay | 2-1/2 tons. | 5,600 | 391.31 | 129.79 | 4.80 | 35.46 |

+---------------+--------------+---------+----------+---------+-------+-------+

+---------------+-----------+--------+-----------+----------+---------+-------+

| | | | | | | |

| | | -ine | Acid. | -ic Acid.| | -gen. |

+---------------+-----------+--------+-----------+----------+---------+-------+

|Wheat--Grain | 4.80 | ... | 0.32 | 16.22 | 0.43 | 29.20 |

| | | | | | | |

| Straw | 2.23 | ... | 3.55 | 3.16 | 73.47 | 16.13 |

| Total | 7.03 | ... | 3.87 | 19.38 | 73.90 | 45.33 |

| | | | | | | |

|Barley--Grain | 3.10 | 1.12 | 0.85 | 15.52 | 13.19 | 34.98 |

| | | | | | | |

| Straw | 2.75 | 1.30 | 1.10 | 7.22 | 68.58 | 6.03 |

| Total | 5.85 | 2.42 | 1.95 | 22.74 | 81.77 | 41.01 |

| | | | | | | |

|Oats--Grain | 4.04 | 0.20 | ... | 26.07 | 2.27 | 27.54 |

| | | | | | | |

| Straw | 5.50 | 5.55 | 5.18 | 7.35 | 72.85 | 14.10 |

| Total | 9.54 | 5.75 | 5.18 | 33.42 | 75.12 | 41.64 |

| | | | | | | |

|Beans, Peas-- | 4.00 | ... | 1.76 | 16.65 | 0.24 | 46.10 |

| Straw | 3.74 | 7.00 | 2.07 | 0.74 | 3.84 | 26.88 |

| Total | 7.74 | 7.00 | 3.83 | 17.39 | 4.08 | 72.98 |

| | | | | | | |

|Turnips--Bulbs | 4.65 | 10.35 | 39.02 | 22.57 | 6.50 | 60.48 |

| | | | | | | |

|Potatoes | 2.11 | 3.21 | 10.24 | 5.76 | 1.29 | 26.00 |

| | | | | | | |

|Hay | 9.62 | 39.61 | 16.57 | 21.79 | 133.67 | 56.22 |

+---------------+-----------+--------+-----------+----------+---------+-------+

The minor constituents, such as oxide of iron, manganese, etc., have

been omitted as being of little importance; and the quantity of

nitrogen, which is of great moment in estimating the exhaustive effects

of various crops, has been added.

In examining this table, it becomes apparent that while in regard to

some of the elements, the quantities removed by different crops do not

differ to any marked extent, in others the variation is very great. The

cereals and grasses are especially distinguished by the larger quantity

of silica they contain, and the exhaustive effect consequent upon the

removal of both grain and straw from soils which contain but a limited

supply of that substance in an available condition is obvious. It is

clear that under such circumstances the frequent repetition of a cereal

crop may so far diminish the amount of available silica as to render its

cultivation impossible, although the other substances may be present in

sufficient quantity to produce a plentiful crop of any plant which does

not require that element. Beans and peas, turnips and hay, on the other

hand, require a very large quantity of alkalies, and especially of

potash.

Looking more minutely, however, into this matter, certain points attract

attention which appear to be at variance with commonly received

opinions. With the exception of silica, for example, the cereals do not

withdraw from the soil so large a quantity of mineral matters as some of

the so-called fallow crops, and if their straw be returned to the soil

they are by far the least exhaustive of all cultivated plants; and we

thus recognise the justice of that practical rule, which lays it down as

an essential point of good husbandry that the straw ought, as far as

possible, to be consumed on the farm on which it is produced. As regards

the general constituents of the ash, it is also to be remarked that

though differences in their proportions exist, they are by no means so

marked as might be expected; thus there are no plants for which a large

quantity of potash, nitrogen, and phosphoric acid is not required; and

it is not very easy to see how the substitution of the one for the other

should be of much importance in this respect. Indeed, the more minutely

the subject is examined, the more do we become convinced of the

insufficiency of that view which attributes the necessity for a rotation

of crops to differences in chemical composition alone. There can be no

doubt that the nature of the plant and the particular mode in which it

gathers its nutriment, have a most important influence. Certain plants

are almost entirely dependent on the soil for their organic

constituents, while others derive a large proportion of them from the

air, and a plant of the latter class will flourish in a soil in which

one of the former is incapable of growing. In other cases, the structure

and distribution of the roots is the cause of the difference. Some

plants have roots distributed near the surface and exhaust the

superficial layer of the soil, others penetrate into the deeper layers,

and not only derive an abundant supply of food from them, but actually

promote the fertility of the surface soil by the refuse portions of them

which are left upon it. Experience has in this respect arrived at

results which tally with theory, and it is for this reason that the

broad-leafed turnip, which obtains a considerable quantity of its

nutriment from the air, alternates with grain crops which are chiefly

dependent on the soil. It is undoubtedly to some such cause that several

remarkable instances of what may be called natural rotations are to be

attributed. It is well known in Sweden that when a pine forest is

felled, a growth, not of pine but of birch, immediately springs up. Now

the difference in composition of the ash of these trees is not

sufficient to explain this fact, and it must clearly be due to some

difference in the distribution of their roots, or the mode in which they

obtain their food.

Whatever weight may be given to these different explanations of

rotation, there is no doubt about the importance of attending to it, and

there are various practical deductions of much importance to be drawn

from the facts with which we are acquainted. Thus it is to be observed

that the quantities of mineral matters withdrawn by plants of the same

class are generally similar, and hence it may be inferred that crops of

the most opposite class ought as much as possible to alternate with one

another, and each plant should be repeated as seldom as possible, so

that, even when it is necessary to return to the same class, a different

member of it should be employed. Thus, for instance, in place of

immediately repeating wheat, when another grain crop is necessary, it

would theoretically be preferable to employ oats or barley, and to

replace the turnip by mangold-wurzel or some other root. It is obvious,

however, that this system cannot be carried out in practice to its full

extent; for the superior value of individual crops causes the more

frequent repetition of those which make the largest return. But

experience has so far concurred with theory that it has taught the

farmer the advantage of long rotations; and we have seen the successive

introduction of the three, four, five, and six-course shift, and even,

in some instances, of longer periods.

Such is the theory of rotation, and while it will always be most

advantageous to adhere to it, it is by no means necessary that this

should be done in an absolutely rigid manner. In the practice of

agriculture, plants are placed in artificial circumstances, and instead

of allowing them to depend entirely on the soil, they are supplied with

a quantity of manure containing all the elements they require, and if it

be used in sufficiently large quantity, the same crop may be grown year

after year. And accordingly the order of rotation, which is

theoretically the best, may be, and every day is, violated in practice,

although this must necessarily be done at the expense of a certain

quantity of the valuable matters of the manure added, and is so far a

practice which ought theoretically to be avoided. In actual practice,

however, the matter is to be decided on other grounds. The object then

is, not to produce the largest crops, but those which make the largest

money return, and thus it may be practically economical to grow a crop

of high commercial value more frequently than is theoretically

advantageous. In such cases the farmer must seek to do away as far as

possible with the disadvantages which such a course entails, and this he

will endeavour to accomplish by careful management and a liberal

treatment of the soil.

But while this system may be adopted to some extent, it must also be

borne in mind that the frequent repetition of some crops cannot be

practised with impunity, for plants are liable to certain diseases which

manifest themselves to the greatest extent when they have been too often

cultivated in the same soil. Clover sickness, which affects the plant

when frequently repeated on light soils, and the potatoe disease and

finger and toe have been attributed to the same cause. Whether this is

the sole origin of these diseases is questionable, but there is no doubt

that they are aggravated by frequent repetition, and hence a strong

argument in favour of rotation. It has been asserted by great

authorities in high farming, that with our present command of manures,

rotations may be done away with; but this is an opinion to which science

gives no countenance, and he would be a rash man who attempted to carry

it out in practice.