42 SOILS 



a certain temperature in order to multiply and as a 

 corn plant needs water and hot weather in order to 

 bring forth its increase. The growth of these 

 microscopic soil plants is checked in very dry 

 weather as much as the growth of the larger plants 

 above ground. Furthermore, they do not thrive 

 in a very wet soil. The temperatures most favour- 

 able for their growth have been found to be 54 to 

 99 F. In the Southern States they grow the year 

 around. Another essential condition is a plenteous 

 supply of oxygen, such as would be had if the soil 

 were well drained and hence well ventilated. 



It will be seen, therefore, that the conditions that 

 favour the growth of these useful workers are 

 those that are most necessary for the growth of 

 farm crops a moist, well-drained soil and thor- 

 ough tillage. Given these conditions, a multitude 

 of the germs attack the rotten leaves, stems or 

 stubble lying in the soil, or the clover, rye or cow- 

 peas that have been plowed under, and soon 

 change the useless organic nitrogen into a nitrate. 

 In order to do this, however, the soil must contain 

 a sufficient quantity of some "base," as lime, to 

 combine with the nitrogen and so make it a nitrate. 

 If the soil is at all acid, or sour, (see Chapter XIV), 

 the germs cannot complete their work. 



Germs That Waste Nitrogen. It is interesting 

 to know that there are also at work in some soils 

 bacteria that accomplish a result exactly opposite 

 to that of the nitrogen-fixing germs. The process 

 is sometimes spoken of as "de-nitrification," and 

 the germs may be called "nitrogen- wasting" germs. 

 They feed upon the nitrates, and set free the nitro- 

 gen gas, which may then escape into the air and so 

 be lost to the soil. These germs are abundant in 

 wet soils; under-draining benefits the soil in more 



