water may be stored to become charged with dissolved plant food materials, over which organic matter may widely spread to constitute the feeding ground of those microscopic forms of life which turn its nitrogen and other plant food elements into forms available to crops, and these internal surfaces far enough apart to give strong and deep ventilation and ample space into which the roots may spread and find abundant opportunity to set the soil grains aside to make room for due enlargement.

Upon the accompanying chart there has been represented, by drawing to proper scale, the volume of the component portions of the surface foot of a soil possessing about the medium amount of room which is found in a soil of average productive capacity. The bottom cube in the diagram stands for a cubic foot of undisturbed surface soil, in its normal field condition when well settled toward the end of the growing season after having been plowed 6 to 7 inches deep in the spring of the same season. The second cube represents the volume the dry soil would

Fig. 2 Showing the relative amounts of dry soil, soil air, total plant food and plant food materials soluble in water, contained in one cubic foot of average surface soil in the United States.

occupy with the moisture and the air removed and with the soil so consolidated that all open spaces are obliterated. In this condition its volume would be 764.5 cubic inches, or 44.24 per cent of the whole cubic foot. The dry weight of this soil is 73.176 pounds, while the dimension gives a cube a little more than 9 inches on each edge. The third cube on the chart represents the volume of the space in the foot of surface soil which is occupied by air when the amount of moisture present is very near the best amount for good crop conditions. In it there are 581.1 cubic inches of space occupied by air, comprising 33.63 per cent. That is, the surface foot of soil in good moisture condition possesses an amount of room through which air may circulate and in which the roots of crops may develop, which is rather more than one-third of the entire volume, expressed by a cube 8.3 inches on each edge. The volume of water carried in a cubic foot of surface soil is represented relatively by the fourth cube on the chart, which contains 382.5 cubic inches, or 22.13

per cent of the whole volume, and contained by a cube a little less than 7.3 inches on each edge. This amount of water represents nearly 2.66 inches in depth on the level over the surface, and if the space occupied by air were also filled with water the combined amount would overspread the surface to a depth of 6.69 inches.

There are few places in the United States where the amount, character and distribution of rain are such as to make maximum yields possible, and hence it is a matter of great importance that the roominess of the soil be maintained of such an amount and of such a character that whatever rain falls during the growing season may be quickly taken in without puddling the surface and without so completely filling the soil as to seriously check soil ventilation during any long interval of time.

Thorough underdrainage is the first requisite for developing and maintaining roominess and openness deep in the soil. Next comes a deep incorporation of organic matter through a rotation which includes

clovers and grasses. The roots themselves open the soil and, carrying the organic matter into the subsoil, induce a deeper penetration of earthworms, ants and other burrowing animals. Moreover, the deep incorporation of organic matter assists the action of frost and of shrinkage, due to drying, in developing the crumb structure which renders the openness more efficient. The occasional deep turning under of stable and green manures and of roughage of all kinds is extremely helpful; and it is not sufficiently appreciated that most organic matter is more efficient turned under than when left to decay at the surface.

AMOUNT OF PLANT FOOD CARRIED BY SOILS

It is very important to understand that there is present in the soil a very large amount of the different plant food elements; that only a small portion relatively of that present exists in immediately available form; but that good and bad methods of soil management exert a very marked influence