AGRICULTURAL CHEMISTRY 195 bonic acid, and ammonia), beyond what the natural vegetation of any country needs. The soil is exceedingly variable in composition. When it can supply sufficient quantities of ash ingredients, it will produce most of the plants indigenous to its locality. It then is fertile. When there is a deficiency of ash ingredients in available form, or the absence of any one of them, the soil is barren. There is an important difference between natural, spon- taneous growth, and artificial, forced produc- tion. Natural growth in general is slow. Cul- tivated growth is rapid. For the former, nat- ural supplies are sufficient; for the latter, artificial supplies must be provided. For the former the supplies of atmospheric food are in excess compared to those of ash ingredients yielded hy the soil (telluric food), so that in forests and prairies the former accumulate on the surface of the soil as dead foliage, which in its decay becomes a telluric source of atmos- pheric food. In the latter the reverse most usually occurs, so that the organic matter of the soil diminishes and must be renewed by manures. To repeat, in artificial growth (in- tensive culture) the soil is made to perform not only its natural function of furnishing ash in- gredients, but also a part of the office naturally left to the atmosphere, viz. : the supply of car- bonic acid and ammonia. Soils consist of the more or less comminuted fragments of rocks, mixed with certain products of their chemical decomposition, and with some organic matter debris of vegetation. The composition of the soil varies according to the rocks from which it originates. It is rare that large tracts of soil are exclusively derived from the rocks that now underlie them. Most of the soils of our northern and middle states are partly composed of materials transported from the far north during what geologists term the drift period. The soils of valleys are constantly enriched from the rocks of surrounding hills, so that the composition of soils is thus more uniform in a general sense than it otherwise could be. We constantly meet, however, with limited areas having soils of peculiar characters. We find beds of sand, gravel, clay, marl, and peat or muck. The mechanically coarser parts of soil, the gravel and sand, consist of the still unde- composed fragments of the rocks from which it has been formed. A part of the finest (impal- pable) portion of every productive soil is usually made up of clay, which is a product of the chemical decomposition of certain minerals, and which possesses properties of the highest moment in agriculture. Under the general name humus is comprehended the organic mat- ter of the soil which has resulted from the partial decay of previous generations of plants. The mechanical texture and other physical characters of the soil have a controlling influ- ence on its fertility. Unless the soil be perme- able by the roots of plants, and preserve the proper degrees of warmth and moisture, vege- tation cannot attain its maximum development, no matter how favorable may be its chemical composition. Assuming then that the soil is physically adapted for a cultivated vegetation, its fertility depends upon its furnishing the growing plant with continuous and abundant supplies of the different bodies that have been named as the elements of vegetable nutrition. The quantity of ash ingredients that the heavi- est crop removes from a soil is small, compared with the whole weight of the soil taken to such a depth as is penetrated by the roots of plants. In average crops of the usually cultivated plants, those portions which are removed from the field as the valuable part of the crops do not carry off" more than 200 to 600 Ibs. of ash in- gredients per acre yearly, while the soil taken to the depth of one foot weighs three to four millions of pounds per acre. That part of the soil which is soluble in the water of rain repre- sents its available plant food. Large quanti- ties of water pass through the vegetation of every acre of highly cultivated ground. It is only needful, then, that this water should contain a few thousandths of ash ingredients in solution, in order to supply the mineral matters in an average crop, since even root crops, e. g. beets, remove but about 600 Ibs. of these substances from the acre. In culti- vated soils there is a constant removal of available ash ingredients, both by the harvests that are taken off, and by the rains which soak through or run over them. In a pro- ductive soil there is a constant renewal of available plant food, by the mechanical and chemical disintegration of the insoluble por- tion (the pulverization of the soil by the oper- ations of tillage), by the alternate contractions and expansions of water (frost), and by the affinities of oxygen and carbonic acid. In a few rare soils the disintegrating and solvent processes are so rapid (act on such finely di- vided or easily decomposable materials), that they always present a surplus of food to the plant. Such are certain soils of southern Rus- sia (tchernosem or black earth), and of the Scioto valley, Ohio. They yield successive crops for many years without manure. In most cases, however, the removal of a few crops exhausts the store of available plant food. Soils, when reduced in fertility, may be re- stored to productiveness by lying in fallow; mechanical and atmospheric agencies thus bring into solution enough of ash ingredients for a new crop. A soil consisting entirely of coarse sand is infertile, because it is too dry, and because there can occur hi it no sufficient accumulations of available plant, food. A soil consisting of fine sand may be highly produc- tive, especially if it originates from easily de- composable rocks, because the amount of sur- face that the grams expose, and the close tex- ture of the soil, maintain it in a proper degree of moisture (by capillarity), and allow a suffi- cient solution and accumulation of food for crops. Clay has a remarkable porosity and j retentiveness for water, for ammonia, and for
