Agriculture is the art of cultivating the earth, so as to preserve and increase the natural fertility of the soil, as well as to render sterile tracts productive of vegetation useful to man; and the perfection of this art may be said to consist in obtaining the greatest quantity of the required product from a given quantity of land at the least possible expense of labour and material. Besides the production of grain and leguminous plants, which usually forms the chief business of the farmer, agriculture includes the culture of trees, and every description of plants; their planting, pruning, grafting, etc.; hence horticulture, or gardening, is a branch of this art. It includes also the breeding and management of cattle, since their manure and their labour are essential to the business of husbandry. Nothing would tend more to the successful practice of this the most important of all arts than the study of chemistry by the farmers. If a soil be unproductive, it must be owing to some defect in its constitution, which may not be apparent even to the eye of the most experienced husbandman. Not all his observation, nor all his practice, without the aid of chemical knowledge, will afford him any means either of ascertaining the cause, or removing the effect.
By chemical analysis, however, the cause is readily determined, and the remedy made obvious. If the salts of iron be present, they may be decomposed by lime; if an excess of silicious sand, the application or clay and calcareous matter will improve it; if vegetable matter be in excess liming, or paring and burning, will be advantageous. The excellent rules laid down by Sir Humphry Davy, in his Agricultural Chemistry, are particularly deserving of attention. In cases where a barren soil is examined with a view to its improvement it ought, if possible, to be compared with an extremely fertile soil in the neighbourhood, and in a similar situation. The difference given by their analyses would indicate the methods of cultivation, and thus the plan of improvement would be founded upon accurate scientific prin ciples. If the fertile soil contained a large quantity of sand in proportion to the barren soil, the process of melioration would depend simply upon a supply of this substance; and the method would be equally simple with regard to soils deficient in clay or calcareous matter.
In the application of clay, sand, loam, marl, or chalk, to lands, there are no particular chemical principles to be observed; but when quick-lime is used, great care must be taken that it is not obtained from the magnesian limestone, for in this case, as has been shown by Mr. Tennant, it is exceedingly injurious to land. The magnesian limestone may be distinguished from the common limestone by its greater hardness, and by the length of time that it requires for its solution in acids; and it may be analyzed by the process for carbonate of lime and magnesia. When the analytical comparison indicates an excess of vegetable matter as the cause of sterility, it may be destroyed by much pulverization and exposure to air, by paring and burning, or the agency of recently made quick-lime. The deficiency of either animal or vegetable matter must, of course, be supplied by animal or vegetable manure. The general indications of fertility and barrenness, as found by chemical experiments, must necessarily differ in different climates and under different circumstances.
The power of soils to absorb moisture, a principle essential to their productiveness, ought to be much greater in warm and dry countries than in cold and moist ones; and the quantity of fine aluminous earth they contain should be larger. Soils, likewise, that are situated on declivities, ought to be more absorbent than those in the same climate on plains or in valleys. The productiveness of soils must likewise be influenced by the nature of the subsoils, or the earthy or stony strata on which they rest; and this circumstance ought to be particularly attended to in considering their chemical nature and the system of improvement. Thus, a sandy soil may owe its fertility to the power of the subsoil to retain water; and an absorbent clayey soil may occasionally be prevented from being barren in a moist climate by the influence of a substratum of sand or gravel. Those soils that are most productive of corn contain always certain proportions of aluminous or calcareous earth in a finely-divided state, and a certain quantity of vegetable or animal matter. The quantity of calcareous earth is, however, very various, and, in some cases, exceedingly small.
A very fertile corn soil from Ormesten, in East Lothian, afforded in 100 parts only 11 parts of fine calcareous earth; the finely-divided clay amounting to 45 parts. It lost 9 parts in decomposed animal and vegetable matter, and 4 in water, and exhibited indications of a small quantity of phosphate of lime. This soil was of a very fine texture, and contained very few stones and vegetable fibres. It is not unlikely that its fertility was in some measure connected with the phosphate, for this substance is found in wheat, oats, and barley, and may be a part of their food. A soil from the lowlands of Somersetshire, celebrated for producing excellent crops of wheat and beans without manure, Sir Humphry Davy found to consist of one-ninth of sand, chiefly silicious, and eight-ninths of calcareous marl, tinged with iron, and containing about five parts in the hundred of vegetable matter. He could not detect in it any phosphate or sulphate of lime, so that its fertility must have depended principally upon its power of attracting principles of vegetable nourishment from water and the atmosphere. In some experiments made by Mr. Tillet on the composition of soils about Paris, he found that a soil, composed of three-eighths of clay, two-eighths of river sand, and three-eighths of the parings of limestone, was very proper for wheat. In general, bulbous roots require a soil much more sandy and absorbent than the grasses. A very good potato soil, from Varsel, in Cornwall, afforded seven-eighths of silicious sand, and its absorbent power was so small that 100 parts lost only 2 by drying at 400° Fahrenheit. Plants and trees, the roots of which are fibrous and hard, and capable of penetrating deeply into the earth, will vegetate to advantage in almost all common soils that are moderately dry, and do not contain a very great excess of vegetable matter. The soil taken from a field at Sheffield-place, in Sussex, remarkable for producing flourishing oats, was found to consist of 6 parts of sand, and 1 part of clay and finely divided matter; and 100 parts of the entire soil, submitted to analysis, produced water, 3; silex. 54; alumina, 28; carbonate of lime, 3; oxide of iron, 5; decomposing vegetable matter, 4; loss, 3.
From the great difference of the causes that influence the productiveness of lands, it is obvious, that in the present state of science no certain system can be devised for their improvement, independent of experiment; but there are few cases in which the labour of analytical trials will not be amply repaid by the certainty with which they denote the best methods of melioration; and this will particularly happen, when the defect of composition is found in the proportions of the primitive earths. In supplying animal or vegetable manure, a temporary food only is provided for plants, which is in all cases exhausted by means of a certain number of crops; but when a soil is rendered of the best possible constitution and texture, with regard to its earthy parts, its fertility may be considered as ermanently established. It becomes capable of attracting a very large portion of vegetable nourishment from the atmosphere, and of producing its crops with comparatively little labour and expense. For the mode of proceeding, and the instruments required in making analytical experiments on soils, we can refer the reader to the before-mentioned work of the late illustrious chemist; very accurate information on this subject will, however, be found in Dr. Ure's excellent Dictionary of Chemistry.
But to those whose business it may be to pursue this most important of studies in all its interesting details, we strongly recommend the perusal of the articles Agriculture in the Oxford Encyclopaedia and Supplement, which form together an elaborate compendium of the best works on the subject in the English language, enriched throughout by the judi-cious observations of its learned editors.