86 BACTERIA AND SOIL FERTILITY 



soil, but the structure can be quite nicely controlled by proper 

 methods of tillage and the addition of organic manures either in 

 the form of barnyard or green manures. Lime can be added to 

 an acid soil and gypsum to a black alkali soil. Fall plowing 

 greatly modifies the soil structure by the permitting of weathering 

 due to frost. 



The bacterial flora of a soil is related to soil structure and the 

 modification of structure modifies bacterial activity. The addi- 

 tion of lime to an acid soil makes of it a fit abode for the nitrify- 

 ing and nitrogen-fixing bacteria. Manure contains bacteria, and 

 when it is added to the soil these and the soil organisms break 

 down the organic constituents of the manure with the formation 

 of acids. These in turn liberate more plant-food. Beneficial 

 bacteria do not function in alkali soil; hence, anything that will 

 reduce the alkali also increases the number and kind of bacteria. 

 All the beneficial microbes are air-loving; therefore, proper cul- 

 tivation and even fall plowing will increase the aeration of the 

 soil and with it the bacterial activities. 



The moisture content of a soil is frequently the limiting factor 

 of crop production. The amount of water held by a soil is di- 

 rectly related to the soil texture. Light soils part with their mois- 

 ture rapidly. On the other hand heavy soils hold it more tenac- 

 iously. Most beneficial bacteria require for maximum activity, 

 that the water content of the soil be 60 per cent of its water 

 holding capacity. It is, therefore, evident that the principles of 

 maintaining good structure and good bacterial activities are the 

 same. It is quite probable that the increased yields which come 

 from keeping a soil in good tilth are directly due to increased 

 beneficial bacterial activities. 



Plant Requirements and Soil Composition. — ^The duration 

 of a bank account is measured by the rate with which it is drawn 

 from the bank and the capital on deposit. The same is true of 

 fertility. The time it will last varies with the total fertility in 

 the soil and the rate at which it is drawn from the soil. Hence, 

 we must consider the plant requirements and the total quantity 

 in the soil. 



