180 Agricultural Bacteriology. 



accumulate. On the other hand in land under grass, in 

 woodland and in wet lands the anaerobic bacteria grow 

 best. Here the decomposition of the vegetable residue 

 is not complete and the humus accumulates in the soil. 

 When the lowland is drained, or the prairie broken, the 

 aerobic bacteria begin to act on the humus, and grad- 

 ually destroy it. The land now produces more than in 

 its wild state, but unless organic matter is returned to it 

 as manure, etc., it gradually diminishes in fertility, as 

 has been so well shown in the eastern part of the United 

 States. The burning of the straw on the prairies of Miij- 

 nesota and Dakota is one way in which the fertility of 

 the land is destroyed. The straw, if it had been plowed 

 under, would have helped to keep up the humus content 

 of the soil. It would have furnished the bacteria food, 

 and the result would have been a much less rapid de- 

 crease in fertility in those fields, the soil of which the 

 settlers thought could never be exhausted. 



In a sandy soil, the aerobic bacteria grow rapidly and 

 any organic matter added is soon completely destroyed; 

 while in a close soil like a clay, the decomposition pro- 

 cesses go on much more slowly. The effect of a heavy 

 coating of manure is .often to be noted only during the 

 season in which it is applied to a sandy soil, while on a 

 clay soil it may manifest its effect in the second and 

 even in the third year. 



In some kinds of farming, the farmer cares little for 

 the fertility of the soil, but wishes one that will allow 

 the rapid growth of bacteria which are to decompose the 

 manure that is added. The market gardener, with an 

 abundant supply of manure from the city, wishes such 

 a soil, simply a place for the bacteria to work on the raw 

 material he adds to the soil. 



