ORGANIC MATTER 117 



sources of plant-food. The process of ammonification then is 

 one step in the formation of available plant-food. Therefore, 

 anything which will accelerate it during the growth of the plant 

 is beneficial, whereas anything which retards it is detrimental. 



Moreover, anything which would accelerate it during the wet 

 seasons when the plants are not on the soil is injurious, because as 

 the nitrogen changes into ammonia, and especially when trans- 

 formed into nitrates, it is soluble and can be leached from the soil. 



Since the process is beneficial, we would naturally expect to 

 £nd a relationship existing between the soil and the crop produced 

 upon such a soil. This is brought out in Figure 26. 



Other workers have observed a similar relationship between 

 ammonifying powers and productivity of a soil. There are 

 individual cases in which this relationship breaks down as ammoni- 

 fication is only one step in the complex process of nitrate forma- 

 tion. The quantity of ammonia in a good arable soil is about four 

 pounds per acre-foot. Occasionally ten times this amount may be 

 found in pasture or heavily manured soil. In special cases even 

 larger quantities may occur, but then abnormal plant growth 

 occurs. 



For these reasons the quantity of nitrates produced in a normal 

 soil in unit time is a much better index of the crop-producing 

 power of the soil than is the ammonifying power. Whereas in 

 an alkali soil the production of ammonia is a better criterion of 

 crop production than is the production of nitrates. 



