130 THE BOOK OF WHEAT 



by diversified farming. To keep the soils in the best physical 

 and chemical condition, such a system of rotation should be 

 practiced as will include both humus producing and humus con- 

 suming crops. Leguminous crops seem to have a marked effect 

 in increasing the organic matter in the soil. 



Soil Moisture. A considerable amount of work was done on 

 every type of soil during 1902 by the bureau of soils. As a 

 result, soil moisture is now looked upon as a great nutritive 

 solution which has approximately the same composition every- 

 where, and can vary only within narrow limits if plant de- 

 velopment is to be successful. It is thought that the kind of 

 crop adapted to a soil is largely determined by its physical 

 characteristics, while yield is more influenced by chemical 

 characteristics. The dissolved salt content of soils seems to 

 be only a minor factor in determining the yield and quality 

 of crops, the wide differences observed on different soils being 

 mainly due to other factors. "It appears, further, that prac- 

 tically all soils contain sufficient plant food for a good crop 

 yield, that this supply will be indefinitely maintained, and 

 that this actual yield of plants adapted to the soil depends 

 mainly, under favorable climatic conditions, upon the cultural 

 methods and suitable crop rotation, a conclusion strictly in 

 accord with the experience of good farm practice in all coun- 

 tries. 7 ' It seems that a chemical analysis of a soil, even if 

 made by extremely delicate and sensitive methods, will in itself 

 give no indication of soil fertility. If the probable yield of a 

 crop can be determined at all, it is likely to be by physical 

 methods. 1 



BACTERIA. 



Bacteria and Nitrification. For cereal crops a previous legu- 

 minous crop is practically equivalent to the application of a 

 nitrogenous fertilizer. In effect this was known by the Romans 

 2,000 years ago. Many theories were advanced to explain the 

 beneficial effects of a leguminous crop, but the true explanation 

 was not found until 1886, when Hellriegel convinced the entire 

 scientific world that bacteria cause and inhabit the root nodules 

 of leguminous plants, and that the symbiotic relation between 

 * U. S. Dept. Agr., Bu. of Soils, Bui. 22 (1903), p. 64* 



