Organic matter is no doubt essential to great fertility in a soil, but some soils require more of it than others. In every stage of its spon taneous decomposition it keeps the pores of the soil open, and admits, if it does not even attract, air and moisture to the fibres of the roots. In all rich soils which have been long cultivated, especially in gardens, there are particles of a dark colour and fibrous texture, which in the microscope appear like minute loge of charred wood. These keep the soil open, and supply carbonic acid, when the air reaches them. A proper texture seems an indispensable condition of fertility. It is much easier to supply the deficiency of vegetable matter in a soil, which at best forms but a very small portion of it, than of silica or clay, which should enter into its composition in the proportion of one half or a third of the whole. It is practicable to carry lime or chalk upon soils which do not contain calcareous matter ; clay may also be carried upon loose sandy soils, where it can be found below the surface, or at a moderate distance; but if a soil is very deficient in silica, it requires so large a proportion of this earth to give porosity to stiff clay, that it very seldom can repay the trouble and expense. Hence the difficulty of bringing poor wet clay soils into a ifertile state, except where an abundance of chalk and vegetable manures can be easily procured. In this case the perfect draining of the land, and exposure of the ploughed surface to the frosts of winter, with the addition of chalk and manure, produce such an alteration in the texture of the clay, that by continuing the improving process it is entirely changed into a mellow and fertile loam. The burning of a portion of the retentive subsoil into a brick-like earth gives it a porosity which renders it mechanically similar to eilicious sand, and converting the iron which all these clays contain into a peroxide, the soil is thereby greatly improved in fertility; for it seems that iron, in a state of slight oxidation, or combined with any acid, is hurtful to vegetation, whereas the red peroxide is not only innocuous, but seems to have fertilising properties.
In ascertaining the value of a soil for the purposes of agriculture, two circumstances should be carefully noticed. The first is the permea bility of the soil to water; and the second is its power of absorbing moisture from the atmosphere. To ascertain the first, it is only required to place an equal weight of different soils in glass tubes of equal diameter, pressing them so that they shall occupy equal spacers, but not filling the tubes. Then pour an equal quantity of water over each soil, and place them upright with cups under them. Examiuo which has the surface first dry, and how much water runs through each in a given time. That which presents a dry surface, while it holds most water in its pores, is probably the best. To ascertain the comparative absorption of moisture, the soils are dried in pairs on a plate of metal heated by steam, or at a heat of 212°, to expel the water. They are then placed in equal quantities in similar fiat cups or dishes, and placed in opposite scales of a balance, and poised. The apparatus is exposed to a moist atmosphere out of doors, or is a cellar, and occa sionally examined. That which is heaviest is, in general, the most fertile.
It is often useful to ascertain roughly the composition of a soil, without having time or opportunity to make accurate experiments. A graduated glass tube, which can be carried in the pocket, and a small phial with a ground stopper, containing diluted muriatic acid, and secured in a wooden case for fear of accident, is all the apparatus required. A little of the soil is taken and moistened with water ; a few drops of the acid are poured on; and by the greater or less dis engagement of bubbles the proportion of calcareous matter is guessed at, and its presence proved. The soil mixed with water is poured into the glass tube and well shaken. In a few minutes the coarse sand is deposited : shortly after the finer sand, and, lastly, the clay and impalpable matter, of which the lightest remains longest suspended. Distinct rings can be observed in the deposits, and the graduated tube shows their proportion. A person accustomed to this method will guess with great precision the general qualities of the soil; and when the geological structure of the neighbourhood and the nature of the subsoil are taken into consideration, the value of the land for pasture or cultivation is guessed with little danger of making very glaring mistakes. To surveyors and valuers this method is of very great help, when other means are not at hand. Among the properties of soils of the greatest agricultural importance must be named the absorptive powers which they possess over ammoniacal and other valuable in gredients of manures, either volatile or soluble, which would otherwise be wasted when applied to the land. This fact, first observed by Mr. Thompson, M.P., has since been investigated by Professor Way, and explains the economy of autumnal manurings and top-dressings in the winter season, when vegetation is inactive and unable at once to turn to account artificial stores of food for plants.
In practice, soils are usually divided into light, mellow, and stiff; but this gives very little information, there being every imaginable variety in each of these. There are still minute circumstances which produce great fertility or the reverse, and which it is difficult to investigate. An accurate chemical analysis, which, however, is a process requiring the service of the educated chemist, joined to a careful mechanical examination, and very correct accounts of the average produce under different systems of cultivation, can alone give us a scale according to which the natural fertility of different soils can be classed; and this must be the work of time and industry joined to science and practical knowledge. Directed as it. is to the detection and estimation of ingredients, many of which occur in very small pro portion, the process of chemical analysis is one which cannot be undertaken by the farmer. It is sometimes useful to him, however, to know the exact composition of his soil, and the chemist is thus some times able to point out the causes of infertility, and so enable him to remove them. When, therefore, he is at a loss, such an analysis may be the guide he needs ; and in a deficiency of phosphate of lime, or an excess of chloride of sodium, or a deficiency of organic matter, or in the presence of iron salt which is thus detected, he may read the cause of his failure, and so be able to remove it.