146 SOILS AND FERTILIZERS 



shallow plowing on new breaking gives better results 

 than deep plowing. Deep plowing at first causes nitri- 

 fication to take place to such an extent that the crop is 

 over-stimulated in growth, due to an excess of available 

 nitrogen. Deep plowing and thorough cultivation aid 

 nitrification. The longer a soil has been cultivated, the 

 deeper and more thorough must be the cultivation. 



Early fall plowing leaves more available nitrogen at 

 the disposal of the crop than late fall plowing. Nitrifi- 

 cation takes place only near the surface. I^ence when 

 late spring plowing is practiced there is brought to the 

 surface raw nitrogen, while the more active nitrogen has 

 been plowed under, and is beyond the reach of the young 

 plants when they require the most help in obtaining food. 

 The various methods of cultivation, as deep and shallow 

 plowing, spring and fall plowing, and surface cultivation, 

 have as much influence upon the available nitrogen 

 supply of crops as upon the water supply. The saying 

 that cultivation makes plant food available is particu- 

 larly true of the element nitrogen, the supply of which 

 is capable of being increased or decreased to a greater 

 extent than that of any other element. 



NITROGENOUS MANURES 



163. Sources of Nitrogenous Manures. The materials 

 used for enriching soils with nitrogen, to promote crop 

 growth, may be divided into two classes: (i) organic 

 nitrogenous manures, (2) mineral nitrogenous manures. 



