No. 6. DEPARTMENT OF AGRICULTURE. 165 



Figure 2, suggested by the illus- 

 tration of Professor King, of the 

 Wisconsin Experiment Station, is 

 given in abbreviated form, to show 

 how reducing the size increases 

 the area of surface in a given 



quantity of soil. A marble one ^^ 



inch in diameter that will slip in- Figure z 



side of a cubic inch box presents 3.1416 square inches of surface; 

 reduce the size of a marble to one-tenth of an inch in diameter, and 

 then lOUO of them will be required to fill the same box. The surfaces 

 of these 1000 marbles will be 31.416 square inches. Eeduce the mar- 

 bles again to one-hundredth of an inch in diameter, and 1,000.000 

 marbles will be needed; and reduce to one-thousandth of an inch in 

 diameter, 1,000,000,000 will be required to till the cubic inch box, the 

 surface of which, if spread out flat, would equal 31,416 square inches, 

 or in a cubic foot 37,700 square feet, or seven-eighths of an acre! In 

 a soil with grains one-thousandth of an inch in diameter (this would 

 be the equivalent of a medium silt) there will be nearly three and 

 one-half acres of exposed surface in a section one foot square and 

 four feet deep. A verj^ thin film of moisture spread over this extent 

 of surface provides a large quantity of water within the reach of 

 plant roots. 



Clay soils divide up very much finer than this, yet a finer divison 

 into separate grains is not desirable under field conditions. A much 

 closer arrangement of the soil grains that Ave find in fine silt would 

 retard the circulation of the air and the movement of the surplus rain 

 water down into the subsoil. The soil must be kept just poious enough 

 to take in all the rain as it falls, and store it for the future as well 

 as the present use. Too fine a division may make the soil imper- 

 vious to water, and cause surface washing, always a serious loss. 

 The 1000 spheres in the cube in Figure 2 occupy 52.36 per cent, of 

 the entire space in the box; the volume of the empty air spaces 

 between is 47.64 per cent. If the diameter of these spheres is 

 reduced to one-tenth, the number will be increased to one million, 

 and, arranged in the same way, they will just fill the cubic inch 

 box. But the space in the box occupied by the million spheres will 

 be just the same, and the volume of the empty space between will 

 remain unchanged. We can change the volume of space, however, 

 by changing the arrangement of the spheres to the pyramid form, 

 which packs them more closely together and reduces the volume 

 of space to 26.95 per cent. The variation in the volume of space in 

 dry soils is from 25 to 75 per cent., according to whether it is very 

 sandy, or a rich loam, or peaty soil. In the great mass of tillable 

 land in a friable condition, the range is from 40 to 60 per cent, and 

 the average about 50 per cent., i. e., the soil occupies about half the 

 space in a cubic foot box, the air filling the remaining half. The 

 amount of space in any soil is increased or decreased, according to 

 its physical condition or degree of moistness at the time of cultiva- 

 tion, and the method of tillage employed. If the soil is quite moist 

 when stirred, so that particles stick together, then the space is 

 increased ; but if dry the particles will settle closer together and the 

 gpace be decreased. 



