Thus the sandy lands (“sand hammocks”) of Florida, the (long-leaf) pine lands of the Gulf States, the “pine barrens” of New Jersey and of Michigan, are noted both for their sandy soils and their sterility after brief cultivation; necessitating fertilization within a few years from the time of occupation. In Europe, the “Heide” (heather) soils of northeastern Germany are of the same cultural character.
Sands of the Arid Regions.—The experience of arid countries however, has long ago shown that some very sandy lands—e. g., such as form the oases of the north African deserts—may be extremely productive when irrigated, and also of considerable durability. Actual experience and close investigation given this subject in the arid regions of the United States has fully demonstrated that lands appearing to the casual observer to be hopelessly sterile sandy deserts, very commonly prove to be even more productive than the more clayey lands of the same regions. Examination of the sand shows, in these cases, that instead of mere grains of quartz, the minerals of the parent rock, partially decomposed, themselves constitute a large proportion of the sandy mass. But in the regions of deficient rainfall, as has already been stated, ([p. 47]) the formation of clay (kaolinization) is exceedingly slow; hence the decomposition of the rock powder results in the production of predominantly pulverulent instead of clayey soils. But the mineral plant-food is not on that account less available, provided other physical conditions necessary for the success of plant growth are fulfilled. Among these moisture stands foremost; hence the relative proportions of the several grain-sizes are of vital importance, since upon this depends to a great extent the proper supply and distribution of moisture, without which no amount of plant-food will avail. Moreover, the finest and most highly decomposed powder is the portion from which the roots draw their chief food-supplies.
The point last mentioned is well shown in the results obtained by Dr. R. H. Loughridge, from the analysis of each of the several grain-sizes into which he had resolved a very generalized soil of the State of Mississippi, representing a very large land area in that State as well as in Tennessee and Louisiana. The details of this investigation are given farther on; but summarily it may be stated that he found practically the whole of the acid-soluble mineral plant-food accumulated within the portion of the soil the fineness of whose grains was below .025 millimeters (one-thousandth of an inch); ingredients so fine as to be wholly impalpable between the fingers. Moreover, two-thirds of the total amount was found in the portion described above as “clay.” It is thus readily understood why clay soils are in the regions of summer rains commonly designated as “strong” lands.
The corresponding later investigations of Rudzinski (Ann. Agr. Inst. Moscow, Vol. 9, No. 2, pp. 172-234; Exp. Sta. Record, Dec. 1904, p. 245) and of Mazurenko (Jour. Exp. Landw. 1904, pp. 73-75; Exp’t Stn. Record, Dec. 1904, p. 344) fully corroborate Loughridge’s conclusions, for typical soils of European Russia.
In the arid or irrigation regions, however, the case is different, for the reason that much of the decomposed rock-substance remains adherent to the surface of the larger grains, and plastic clay is formed to a much less extent. Much available plant-food may therefore, in arid lands, be present even in rather coarsely sandy soils almost devoid of clay; such as in humid climates would be likely to be found wholly barren. ([See chapt. 19]).
PHYSICAL ANALYSIS OF SOILS.
Use of Sieves.—Down to a certain point the separation of the soil into its several grain-sizes may be accomplished by means of sieves. We may thus separate coarse gravel from fine gravel and from sand; and the latter may itself be separated into several sizes by the same means. This presupposes, of course, that the soil has been previously prepared for the purpose by crushing the lumps consisting of aggregates of finer particles, that in the operation of tillage would again be resolved into their fine constituents, or be penetrated by roots. But this preparation of the soil for sifting must not be carried beyond the point mentioned, for a grain consisting of particles somewhat firmly cemented together will under ordinary conditions play in the soil precisely the same part as a solid sand-grain, and must not therefore be broken up, if the soil is to be examined in its natural condition. The pressure of the fingers or of a rubber pestle is as far as trituration should go. The disintegration of these compound particles by means of acids, as prescribed and practiced by the French soil chemists, may wholly change the physical nature of the soil by the breaking-up of mechanical aggregations which in the usual course of tillage would remain intact. This is especially true of strongly calcareous soils, and particularly those containing calcareous sand.
The sieves used for this purpose should not be ordinary wire sieves, but should have bottoms of sheet brass perforated by round holes of the various diameters desired, of fractions of inches, or preferably of millimeters. For the finer grain sizes, silk bolting cloth is used by the U. S. Bureau of Soils.
In the sifting process it will be found that so soon as the finer grain-sizes of the sand are approached, the sieve fails to act satisfactorily; the more so, the more clay was originally contained in the material. The fine particles flock together, forming little pellets, which refuse to be separated by the sieve. This difficulty can, of course, be partly overcome by previously separating the clay from the sand by means of water, as detailed above; but even then it will be found that so soon as the grain-sizes fall much below ¹/₅₀ of an inch (½ millimeter) the same difficulty is experienced, so long as the sand is dry. By playing a small stream of water upon the sieve, however, all the particles beyond the ¹/₅₀₀ of an inch may be successfully separated from the coarser portion; and for many practical purposes the separation need be carried no farther.
Use of Water for Separating Finest Grain-Sizes.—The scientific investigator, however, must of necessity proceed to separate the finer grain-sizes from each other, since, as will presently be shown, they influence the tilling qualities of the soil to a much greater degree than do the coarser particles. Such farther separation can be accomplished only by the aid of water.