soil, SESSION. 147 



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 oyer the sur- 

 face, and if the space occupied with 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, char- 

 acter and distribution of rain is such as to make maximum yields pos- 

 sible, 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 in- 

 cludes clovers and grasses. The roots themselves open the soil and, 

 carrying the organic matter into the subsoil, induce a deeper penetra- 

 tion of earthworms, ants and other burrowing animals. Moreover, the 

 deep incorporation of organic matter assists the action of the 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 or- 

 ganic matter is more efficient turned under than when left to decay 

 at the surface. 



I 



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 upon the rate at which the plant food elements present 

 in the soil are transformed into the condition available to crops. 



Taking Maxwell's complete analysis of a composite sample made up 

 of soils from the grounds of many of the Experiment Stations in the 



