Soils and fertilizers . ted, but the occurrence ofanything like needle shape is not common. Soils formedby erosion and wave action are likely to have roundedparticles, as are also soils formed from limestone. 30. Space occupied by particles. — The number of par-ticles in a given volume of soil can only be estimated, theirminute size precludes an actual enumeration. It has beenestimated that the number of particles in a gram of soilof certain different kinds is as follows: Early truck 1,955,000,000 Truck and small fruit 3,955,000,000 Tobacco 6,786,000,000 Wheat 10,228,000,000 Grass and wheat 14
Soils and fertilizers . ted, but the occurrence ofanything like needle shape is not common. Soils formedby erosion and wave action are likely to have roundedparticles, as are also soils formed from limestone. 30. Space occupied by particles. — The number of par-ticles in a given volume of soil can only be estimated, theirminute size precludes an actual enumeration. It has beenestimated that the number of particles in a gram of soilof certain different kinds is as follows: Early truck 1,955,000,000 Truck and small fruit 3,955,000,000 Tobacco 6,786,000,000 Wheat 10,228,000,000 Grass and wheat 14,735,000,000 Limestone 19,638,000,000 30 TEXTURE AND STRUCTURE OF SOILS 31 If all the particles were spheres, it is estimated that eachcubic foot of soil would have a surface area on its particlesamounting to from two to three and one-half acres. 31. Mechanical analysis of soils. — A separation of theparticles of a soil into groups, each of which comprisesparticles whose sizes fall within certain definite limits, is. FIWE GRAVEL COAESE aAHD MEDIUM -FHIE VERTRHE SILT CLAY Fio. 2. — Relative sizes of soil particles in the various grades into whicha mechanical analysis separates a soil. All are enlarged many times. Par-ticles of fine gravel may vary in size from the largest circle to the next largest;coarse sand from the second to the third; medium sand from the thirdto the fourth, and so on. The dot in the center represents the largest clayparticles; the smallest cannot be shown in a figure of this magnification. called a mechanical analysis of the soil. The size limitof these groups is a purely arbitrary matter, consequentlyit is desirable that a universal system shall be classification in general use in this country is one pro-posed by members of the Bureau of Soils of the United StatesDepartment of Agriculture. It provides for groups of thefollowing sizes: 32 SOILS AND FERTILIZERS Diameters op Particles Millimeters Inches Fine gravel .Coarse sand .Medium sand
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