58 UNIVERSITY OF ILLINOIS 



is true, it looks like an enormous undertaking to restore to the soils 

 the net loss of a million tons annually by the growth of legumes. It 

 would seem a difficult matter even to maintain a nitrogen supply in 

 soils having a large productive capacity, and well nigh impossible to 

 build up a poor soil to a very high productive capacity. To pass the 

 answer to legumes does not solve the problem of adequate nitrogen 

 restoration. On a soil rich enough to give up sufficient nitrogen for a 

 hundred-bushel crop of corn, how much nitrogen will be fixed by a 

 four-ton crop of clover, when the nitrogen requirements are about the 

 same for both crops ? Or on fifty-bushel corn land, how much will be 

 fixed when only two tons of clover are grown ? 



To keep any type of soil up to a high productiveness it is neces- 

 sary to grow more or less of the deep-rooting legume crops for other 

 purposes than the fixation of nitrogen ; and it seems that the amount 

 of legumes necessary for these other purposes is considerably less than 

 the amount required to maintain a nitrogen supply. Will it be found 

 profitable, then, to supplement legume nitrogen with artificially fixed 

 nitrogen? If the promise to fix artificial nitrogen at about five cents 

 per pound at the Muscle Shoals plant be realized, what effect will it 

 have on the future of agriculture? Will the tendency be to replace 

 legume nitrogen entirely, or will it still find a profitable use ? With 

 this cheap nitrogen, would the farmers be able to produce fixed carbon 

 cheaply enough for it to be converted into motor fuel without eco- 

 nomic ruin to the farming business? 



MAINTAINING FERTILITY BELOW THE PLOW LINE 



A problem of great importance in the future will be to maintain 

 the fertility in that part of the soil which lies below the plowed por- 

 tion. The productiveness of the corn-belt soils has been due in great 

 measure to the depth of their fertility; and especially in seasons of 

 drouth is deep fertility a factor, because the deeper crop roots not only 

 secure more food but are able to secure more moisture when needed. 

 Plant food is constantly removed from the surface soil and below by 

 ordinary cropping ; it is moved upward and stored in the mature crop, 

 which is taken from the land. The surface, or plowed soil, may be 

 kept rich by plowing under organic matter in various forms and by 

 adding other forms of plant food ; but with the exception of limestone, 

 this method does not materially improve the soil below the line of 

 plowing. At Rothamsted where 15.7 tons of manure per acre were 

 added annually for fifty years, the soil analyses at the beginning and 

 the end of the period showed no material gain in any element of plant 



