Soils 49 



that is in need of surface drainage will not decay for several 

 years, due to the absence of air, but sod turned under in a well- 

 aerated soil will decay in a few months. 



The carbon dioxide of the soil air gets into the water of the 

 soil, where it aids in making plant-food more readily soluble. 

 Water containing carbon dioxide is a much better solvent of 

 plant-food than is pure water. 



The nitrogen of the soil air is needed in the life process of 

 the bacteria that live in the soil and on the roots of the 

 legumes clover, peas, and the like. Some of these bacteria 

 take free nitrogen from the air and convert it into soluble forms 

 that are available as plant-food. Free nitrogen cannot be taken 

 up as food by roots until it has been combined with certain 

 elements to form a soluble compound. 



All the tillage practices aerate the soil. Thus one of the 

 benefits of tillage is to increase the quantity of air. 



25. Bacteria in soU. The minute organisms known as 

 bacteria form a very essential contituent of soil. In fact if 

 there were no bacteria in soil, there would be no plant growth. 

 A very important effect of bacteria in soil is the decay of or- 

 ganic matter to form humus. Bacteria also have an effect on 

 the amount of nitrogen that is available in soils. Nitrogen 

 of organic matter is made available by the process of nitrifica- 

 tion. In this process the work of three forms of bacteria is 

 necessary; the first form changes the organic nitrogenous 

 compounds into ammonia, the second, changes the ammonia 

 into compounds called nitrites, and the third, changes the 

 nitrites into nitrates. These compounds are available as plant- 

 food and in this form the nitrogen is useful to most crops. 



What is termed denitrification sometimes takes place in soils 

 This is an undesirable process, the reverse of nitrification, and 

 is the work of bacteria that change the nitrates into nitrites. 

 From nitrites other bacteria are likely to change the compounds 

 into ammonia and finally into free nitrogen, which means a 

 loss of nitrogen that had been available as plant-food. It has 



