Specific gravity is also one of the tests of a good soil, since it will show if the soil be fmely comminuted the presence of a. good proportion of day. The percentage of clay and sand in various soils from pure clay (pipe clay), to humus soils and peaty soils is as follows: Pure clay, pipe clay, sixty per cent. silica, forty per cent. alumina, oxide of iron chemically combined. . Sfrongest clay soil, brick clay, pure clay with five to ten per cent. of sand which can be separated. Clay loam fifteen to thirty per cent. fine sand and pure clay. Loamy clay thirty to sixty per cent. sand and pure clay. Sandy loam sixty to ninety per cent. sand and pure clay. Sandy soil containing no more than ten per cent. of pure clay. Marly soils, in which the proportion of lime is more than five, but does not exceed twenty per cent. of the whole weight of the dry soil. Calcareous soils, in which the lime exceeding twenty per cent. becomes the distinguishing constituent. Vegetable soils from garden mold, which con tains from five to ten per cent., to the peaty soil in which the organic matter may amount to sixty or seventy per cent. These soils again may be clayey, loamy, or sandy, according to the pre dominant character of the earthy admixtures. A fertile soil, therefore, consists of three earths, clay, sand, and Male mechanically 'combined, organic matter from the decay of plants, etc., -which may be resolved into carbon, oxygen, hydrogen and nitrogen, and the inorganic ele ments, in combination, chemically, with metals, hydrogen, oxygen, chlorine, and sulphur. These are in small quantities. Thus chlorine produces the chlorides, iodine, the iodides, sulphur, the sulphurets, and sulphuric acid; phosphorus, the phosphoric acid; potassium, the potash; sodiinn, the soda and common salt; calcium, the lime; aluminum, the clay; silicon, the sand; and iron and manganese, oxides and sulphurets. The mechanical texture of the soil has a strong influ ence upon its practical fertility, very heavy clay, and very light sands, being both, for opposite reasons, apt to produce badly. The soil in which the particles are the finest, so that the air can enter, and the roots spread without difficulty, is, other things being equal, the best. In clay soils this division of the particles must be pro duced by the plow and other mechanical means; while in loose sands it is too great, and must be amended by an admixture of clay and other sub stances. The cause of the great and lasting fertility of prairie soils, is, first, its extreme com minution—fineness—and its large proportion of clay, sand, and organic and inorganic elements. The color of the soil is not always an indication of its fertility. As a rule, however, dark soils are the most fertile. The absorbing power of soils is another indication of fertility, since a porous, or other soil that will easily absorb water and hold its vapor, is generally a fertile soil. The following table from Schubler, shows the rela tive absorbing power of the soils named: Thus while sandy lands may suffer from long continuance of dry weather, a neighboring field abounding in humus may absorb sufficient mois ture from the air to serve all the requirements of vegetation. The power of saturation by water,
and the retention of moisture, vary in the same manner, and nearly in the same degrees. An other important property of soils is their power to absorb oxygen from the air. According to Schubler: Soils lose, in drying, the property of absorbing oxygen from the air, but regain it in the same proportion as before, on being moistened. The action of organic manures, and the production of carbonic acid, depend on the existence of oxygen in the soil. Capillary attraction or power, tneans the power by which a liquid ascends in the interior of a capillary tube, or tube of small bore, above the surface of the liquid which sur rounds it. The phenomenon occurs in solid bodies which are capable of being wetted. Thus, when water is poured into the basin of a flower pot, the soil gradually sucks it in, and becomes moist even to the surface. The same takes place in the soil in the open fields. The water from beneath—that contained in the subsoil—is gradually sucked up to the surface. Where water is present in excess, this capillary action keeps the soil always moist and cold. Evapora tion takes place from the surface of the land, and as each atom of moisture is taken up into the • atmosphere, its place is supplied by another atom, communicated by the contact of the particles of soil, the more superficial acting on the deeper particles like so many pumps, to elevate the water, and supply the loss. Thus a naturally porous soil may be kept injuriously wet by an impervious subsoil several feet below. Drainage counteracts this. The capillary action of the soil, however, is an important action of the soil, since thus the vapor of water is con stantly passed upwards from below, to supply that lost by evaporation at the surface during droughts. A moderately compact, and yet por ous soil, has strong capillary power; and hence, again, another reason for its fertility, since a soil, when abundantly dry, is always unpro ductive; and for one principal reason, that it is only in a solnble form, that is in combination with water, that the elements of plant food can become available. Thus, given a virgin and fer tile soil, capable of producing those crops nat ural to a climate, it would seem to be of the utmost importauce that the farmer keep it up to the original standard. This is accomplished by fallowing, manuring, or by a proper succession of crops, or, better, by a combination of the three. (See articles Fallow, Manure, and Rota tion.) There is another agent fully as impor tant, and that is disintegration. This enables natural agents to act promptly in restoring lost fertility, both summer and winter. These are heat, electricity, moisture, carbonic and other acids formed in the soil, and the complex chem ical changes constantly going on in a soil in which heat, moisture, and porosity are present in normal conditions. This being the case and the elements of fertility present in due propor tion we have a fertile soil.