11 THE MODERN THEORY OE AGRICULTURE
of the heath-plants that grow upon it. Those ashes contain soda and potash, conveyed to the growing furze or gorse by rain-water. The soil of one district consists of sandstone; certain trees find in it a quantity of alkaline earths sufficient for their own sustenance. When felled, and burnt a nd sprinkled upon the soil, oats will grow and thrive that without such aid would not vegetate. The most decisive proof of the absurdity of the indiscriminate use of any strong manure was obtained at Bingen, a town on the Rhine, where the produce and development of vines were highly increased by manuring them with animal matters such as shavings of horn. After some years, the formation of the wood and leaves decreased perceptibly Such ma nure had too much hastened the growth of the vines: in two or three years they had exhausted the potash in the formation of their fruit leaves and wood; so that none remained for the future crops, as shavings of horn contain no potash. Cow-dung would have been better, and is known to be better. Conditions of Vegetation.
The sun's heat and light, air, water, and the common elements of the earth are necessary to the existence of plants. But a greater or less abundance of certain elements, and their existence in more or less favorable states of combination, determines the magnitude and fertility or, in a word, the whole productiveness, of the vegetable growth.
The rules of agriculture should then, if rationally perfected, enable us to give to each plant what it requires for the attainment of the special object of its culture, namely the increase of certain parts which are used as food for men and animals.
One instance may illustrate this idea. The means to be resorted to for the production of fine pliable straw for hats and bonnets are the very opposite to those which would tend to produce the greatest possible a mount of seed or grain from the same plant.
Sa nd, clay and lime, as has been said are the principal constituents of soils. Clay and marl always contain potash a nd soda. Pure sa nd, or pure limestone, would alone constitute absolutely barren soils. All arable land contains a n ad mixture of clay although an excess of it, in proportion, is of course disadvantageous. Rotation of Crops.
The exhaustion of alkalies in a soil by successive crops is the true reason why practical farmers suppose themselves compelled to suffer land to lie fallow. lt is the 15
greatest possible mistake to think that the temporary diminution of fertility in a field is chiefly owing to the loss of the decaying vegetable matter it previously contained: it is principally the consequence of the exhaustion of potash a nd soda, which are restored by the slow process of the more complete disintegration of the materials of the soil. lt is evident that the careful tilling of fallow land must accelerate a nd increase this further breaking up of its mineral ingredients. Nor is this repose of the soil always necessary. A field, which has become unfitted for a certain kind of produce, may not, on that account, be unsuitable for another; and upon this observation a system of agriculture has been gradually formed, the principal object of which is to obtain the greatest possible produce in a succession of years, with the least outlay for manure. Because plants require for their growth different constituents of soil, changing the crop from year to year will maintain the fertility of that soil (provided it be done with judgment) quite as well as leaving it at rest or fallow. ln this we but imitate nature. The oak, after thriving for long generations on a particular spot, gradually sickens; its entire race dies out; other trees and shrubs succeed it, till, at length, the surface becomes so charged with an excess of dead vegetable matter, that the forest becomes a peat moss, or a surface upon which no large tree will grow. Generally long before this ca n occur, the operation of natural causes has gradually removed from the soil substances, essential to the growth of oak leaving others favorable and necessary to the growth of beech or pine. So, in practical farming, one crop, in artificial rotation with others, extracts from the soil a certain quantity of necessary materials; a second carries off, in preference, those which the former has left.
One hundred parts of wheat straw yield 15 1/2 of ashes; the same quantity of barley straw, 8 1 / 2; of oat straw, only 4; a nd the ashes of the three are chemically of about the same composition. Upon the same field, which will yield only one harvest of wheat, two successive crops of barley may be raised, a nd three of oats. We have in these facts a clear proof of what is abstracted from the soil and the key to the rational mode of supplying the deficiency Since wheat consumes a large amount of silicate of potassa from the soil, the plants which should succeed or alternate with it must be such as require but little potassa, as potatoes or turnips. After three or four years the same lands may well bear wheat, because, during the interval, the soil will have been, by the action of the atmosphere, a nd the solution of vegetable and a nimal substances decaying upon or in it, again rendered capable of yielding what the wheat requires. Whetlier this process can be artificially anticipated, by supplying the