68 CULTIVATION AND TILLAGE 



packed down, and a large percentage of the water has been 

 lost by surface drainage. Again if spring plowing is prac- 

 tised the dry soil is turned under and the moist soil is exposed, 

 so that if a mulch is not immediately formed by cultiva- 

 tion a great deal of the moisture is lost by the exposure of 

 the soil to the sun and the drying winds. 



Bacterial Action of the Soil Influenced by Cultivation. — In 

 all soils there are two bacterial processes continually going 

 on, namely, nitrification and denitrification. Nitrification 

 is the process by which the nitrates and the nitrites are 

 produced in the soil by minute living organisms. These 



Fig. 20. — The Osborne sulky spring-tooth harrow. 



organisms are called bacteria and are very small, microscopic 

 plants. Nitrification results in the changing of the complex 

 organic nitrogen in the soil into other forms that can be 

 used by the plants. In order that this process can be 

 carried on successfully by the nitrifying bacteria, six funda- 

 mental conditions are necessary: (1) moisture, (2) oxygen, 

 (3) favorable temperature, (4) absence of sunlight, (5) 

 nitrifying bacteria, (6) some compound on which the bac- 

 teria can work. All of these conditions must be maintained. 

 Cultivation, particularly of the heavier types of soil, favor 

 nitrification by increasing the amount of air in the soil, 



