INCREASED CROP PRODUCTION 115- 



The only advice one can give is to abstain from 

 growing the crop oftener than once in eight years or 

 so, but this is unsatisfactory, because one would like 

 to grow it every third or fourth year. Some varieties 

 suffer less than others, e.g. Alsike less than Red 

 Clover. 



Another important disease is finger-and-toe, an 

 organism that enters turnip or swede roots and causes 

 malformation, and very soon decay. Fortunately this 

 cannot tolerate a neutral or alkaline medium. It is 

 possible that the hydrogen ion concentration of the 

 soil solution may furnish a valuable indication as 

 to whether particular pests are or are not to be ex- 

 pected. An interesting investigation was made in 

 North Maine, 1 U.S.A., of two soil types, both ex- 

 tensively cropped with potatoes: on one the Wash- 

 burn loam --potato scab is common; on the other 

 -the Caribou loam the scab is rare. In the latter 

 case the exponent is 5.2, in the former it is 5.9; the 

 more intense acidity of the Caribou loam apparently 

 being beyond the limits of tolerance of the organism. 



But disease organisms are only part of the trouble. 

 One of the most efficient ways of building up soil 

 fertility is to leave the land in grass for a period of 

 years. Considerable quantities of plant residues are 

 added to the soil, and nitrogen-fixing organisms, both 

 free-living and symbiotic, carry on their wonderful 

 work of taking up gaseous nitrogen from the air and 

 synthesizing it into protein. Of all bacterial pro- 

 cesses this is perhaps the most fascinating and the 

 least understood; it well deserves the attention of a 

 vigorous biochemist who is looking fora big problem. 



The practical importance of the process is extra- 

 ordinarily great. It is in this way that much of the 

 fertility of our soils was built up, and it is this store 



1 L. J. Gillespie and L. A. Hurst, Soil Sci., 1917, 4, 313. 



