CHARACTERISTICS OF SOILS 15 



is present. The germs that bring about these changes 

 cannot work without oxygen. The nitrogen-fixation bac- 

 teria must have free nitrogen to work with and oxygen 

 to assist them. 



21. Prevention of food destruction. Probably most 

 soils contain certain kinds of bacteria that, while they 

 require oxygen for their well-being, are not entirely de- 



FIG. 3. The effect of ventilation on germination. The air was 

 excluded from the soil in right jar by flooding to a depth of f an inch 

 with water. Picture taken 7 days from planting. One hundred 

 per cent of the kernels in left jar germinated. 



pendent upon air for their oxygen supply. In the absence 

 of air oxygen, they have the power to destroy certain 

 chemical compounds for the oxygen these compounds 

 contain. The nitrates are among the compounds they 

 can destroy. Nitrates, it will be remembered, are the 

 forms of nitrogen that are most used by growing crops. 



When the air is excluded from a soil containing nitrates, 

 the germs attack the nitrates and remove the oxygen 

 from them, and by this act render the nitrogen of the 

 nitrates useless for the crops. The rate at which this 



