82 AGRICULTURAL BACTERIOLOGY 



of dairy products, live stock, wool, and the like, while the 

 manure is returned to the land, the tendency will be to 

 maintain the humus content of the soil. 



The crop grown on most cultivated lands is larger than 

 the land would produce under natural conditions. In other 

 words man by his tillage operations establishes more favor- 

 able conditions for the action of microorganisms in the soil 

 than obtains in nature. This results in the more rapid 

 change of the unavailable to available plant food, a process 

 that hastens the reduction of the humus content. 



The difference in the rate and completeness of the decom- 

 position of organic matter in the various types of soils is 

 shown when manure is added to them. An application of 

 barnyard manure to a sandy field may show its effect in an 

 increased crop only during the season in which it was ap- 

 plied, while in the case of a clay soil its effect may be noted 

 for two or three years. Again, when it is desired to use 

 land for the disposal of organic matter, as in the case of 

 sewage, the best conditions are found in an open, sandy 

 soil, in which the process will go on rapidly and completely 

 with little or no accumulation of humus. In denser soils, 

 a much larger area is required, as the applications can be 

 made with less frequency, or the soil becomes clogged with 

 the accumulation of organic matter which can not be decom- 

 posed by reason of the lack of growth of aerobic organisms. 

 The disposal of sewage by conducting it on to the land is 

 used only when areas of sandy land are available. 



