PREPARATION OF SOILS FOR CROPS 3OI 



the land so as to break the force of the water. The 

 water should be encouraged to percolate through the 

 soil rather than to flow over the surface. (See Section 



25.) 



356. Bacterial Diseases of Soils. Many of the bac- 

 terial diseases to which crops are subject are caused 

 primarily by a diseased condition of the soil. These 

 diseases can often be checked by the right kind of culti- 

 vation, by securing good drainage, and by proper soil 

 ventilation supplemented with the application of alkaline 

 matter as wood ashes and land plaster. Undrained soils 

 are unsanitary; the products of decay of the organic 

 matter accumulate in the soil and produce toxic or poi- 

 sonous compounds which affect crops. When soils are 

 drained, air is admitted which prevents the formation 

 of these products. Both bacterial and fungous diseases 

 of soils may be controlled by cultivation, particularly 

 when it improves the general sanitary condition of the 

 soil. With improvement in sanitary condition, there 

 is less liability of bacterial diseases becoming established 

 and causing destruction of the crop. As a result of 

 some forms of bacterial action, chemical substances in- 

 jurious to plants are produced, and by controlling bac- 

 terial action the formation of these is prevented. Some 

 of the organisms propagated in the soil cause bacterial 

 diseases of dairy and other farm products. The use of 

 soil disinfectants is possible only where a small area is 

 involved ; they are not applicable to large tracts as they 



