A still clearer idea of these quantities will be obtained by a consideration of the fact, that if we carry off the entire produce, and return none of it again in the shape of manure, we must or ought in its stead, if the land is to be restored to its original condition, add to each acre every four years:—
| Pearl or Potash, | 390 | lbs. at a cost of l. | 3 | 10 | 0 |
| Crystallized carbonate of soda, | 440 | 2 | 5 | 0 | |
| Common salt, | 65 | 0 | 2 | 0 | |
| Quick (burned) lime, | 240 | 0 | 1 | 0 | |
| Epsom salts, | 250 | 1 | 5 | 0 | |
| Alum, | 84 | 0 | 8 | 0 | |
| Bone-dust, | 260 | 0 | 16 | 0 | |
| Total, | 1729 | l. | 8 | 7 | 0 |
Several observations suggest themselves from a consideration of the above statements: first, that if this inorganic matter be really necessary to the plant, the gradual and constant removal of it from the land ought by-and-by to impoverish the soil of this inorganic food; second, that the more of what grows upon the land we can again return to it in manure, the less will this deterioration be perceptible; third, that as many of these inorganic substances are readily soluble in water, the liquid manure of the farm-yard, so often allowed to run to waste, carries with it to the rivers much of the saline matter that ought to be returned to the land; and, lastly, that the utility and often indispensable necessity of certain artificial manures is owing, it may be, in some districts, to the natural poverty of the land in certain inorganic substances,—but more frequently to a want of acquaintance with the facts above stated, among practical men, and to the long continued neglect and waste which has been the natural consequence.
In certain districts, the soil and subsoil contain within themselves an almost unfailing supply of some of these inorganic substances, so that the waste is long in being felt; in others they become sooner exhausted, and hence call for more care, and, when exhausted, for a more expensive cultivation, in order to replace them.
One thing is of essential importance to be remembered by the practical farmer—that the deterioration of land is often an exceedingly slow process. In the hands of successive generations a field may so imperceptibly become less valuable, that a century even may elapse before the change prove such as to make a sensible diminution in the valued rental. Such slow changes, however, have been seldom recorded; and hence the practical man is occasionally led to despise the clearest theoretical principles, because he has not happened to see them verified in his own limited experience, and to neglect therefore the suggestions and the wise precautions which these principles lay before him.
The agricultural history of tracts of land of different qualities, shewing how they had been cropped and tilled, and the average produce in grain, hay, straw, and other crops, every five years, during an entire century, would be invaluable materials both to theoretical and to practical agriculture.
CHAPTER V.
Of Soils—their Organic and Inorganic Portions—Saline Matter in Soils—Examination and Classification of Soils—Diversities of Soils and Subsoils.
Soils consist of two parts,—of an organic part, which can readily be burnt away when the soil is heated to redness; and of an inorganic part, which is fixed in the fire, and which consists entirely of earthy and saline substances.