ROTATION OF CROPS. It has been observed in a former article [ARAOLE LAND] that a repetition of the same crops in succession has a peculiar effect on the soil, so that if grain of the same nature be sown year after year in the lame ground, it will not produce the same return of the seed, even when abundantly manured. The reason of this is not satisfactorily explained, but the experiments which have been made by men of science lead us to conclude that the real cause will be gradually discovered; and considerable advances have been made towards a rational solution of the question. It has been observed that it is the formation of the seed which principally causes the deterioration of the soil ; for if the crop be fed off in a green state, or mown before the seed is formed, the same may be safely repeated, and no diminution of the plants is apparent Thus grasses in a meadow which are mown before the blossom is faded or the seed formed, will spring up again vigorously; but if the seed be allowed to ripen, the roots die away, and the best grasses gradually disappear. It is thus that when a meadow is mown year after year for hay, and the earliest grasses are allowed to ripen their seed, the crop will be later and later, and all the earliest grasses will disappear. Irrigation prevents this, and seems to restore to the land whatever the grasses require for their continuance. Feeding off the meadows does the same; and this leads to the conclusion that water restores the power of production ; and that the gramme not being permitted to run to seed, the deteriorating effect is not produced.
If it had been a mere exhaustion of the nutritious particles in the soil which caused the deterioration of the subsequent crops, some kind of manure might restore the fertility ; but this is not the case. How ever judiciously the land may be manured, it is not practicable to raise a crop of wheat or clover, or of many other plants, on a soil which has shown that, as the farmers say, It is tired of that crop ; but clover grows well after wheat, and wheat after clover, so that the same effect is not produced in the soil by these two crops. A plant which has fibrous roots, and throws up a seed-stein with few leaves, thrives Inert after one which has a fleshy root and many succulent leaves on a ',ranching stem. Thum, wheat and oats thrive after beans or clover ; barley and oats after turnips, carrots, or potatoes. Independently of the manure which may be put into the ground, the crops will be better where the proper succession is attended to, than where plants of a 'similar kind are made to follow each other.
In all countries where peculiar attention has been paid to agriculture, the most advantageous succession of crops is generally known ; and if any deviation takes place, it is as an exception to the rule, and is not looked upon as a model for imitation, hut rather as an experiment of a doubtful result. Certain general principles are commonly admitted as
fully established; the chief of these is, that a plant with a naked stem and farinaceous seed should follow one with a branching stem and a fleshy root, which has been taken from the ground by mowing or feeding before the seed was ripe; or if all these conditions cannot be obtained, that some of thorn at least should bo complied with. Wheat sown after clover, which is allowed to be the best succession on light soils, fulfils all the conditions : when it is sown after beans, the con dition of the preceding crop not ripening its seed is given up; and consequently this succession is inferior to the other, but it is admirably effective nevertheless on all heavy soils. Potatoes, at first sight, appear to fulfil all the necessary conditions; but although they do not often ripen the seed above ground, in the formation of the tubers the soil is notoriously deteriorated.
In order to find the crops which may advantageously succeed each other in rotation, many circumstances must be taken into coneidera tion. First of all the quality of the soil, and its fitness for particular crops ; next the wants of the fanner and his family, and the mainte nance of the stock required to produce a sufficient supply of manure. It is unreasonable to expect poor light land to produce wheat and beans, although by high cultivation these crops may be forced. Rye, oats, and roots may give the fanner a better profit, by being raised at a less expense than more valuable crops, which must be forced with manure, and at best are precarious in soils not adapted to their growth. In moderate loans wheat may recur every fourth or fifth year whereas in very rich compact loans it may recur every third, and even every alternate year. Clover and many artificial grasses do not succeed well if they recur oftener than every sixth year, or with even a longer interval. Rape, flax, and potatoes require a still more distant recur rence on the same ground. All these considerations lead the farmer to the selection of the most advantageous rotation for the soil of his farm ; and where the land in a considerable district is nearly of an uniform quality experience soon establishes a course which no one finds it prudent to deviate from. It happens frequently however that a great variety of soils, very different in their nature and fertility, are intermixed; and then, unless the farmer can apply the true principles of rotations, be may greatly err by following the course, which may be very judicious for the prevailing soil of the district, but not at all suited to some of his fields. Hence a knowledge of the crops suited to any 'articular soil, and order in which these crops should succeed each other, is indispensable to the advantageous cultivation of a farm.