CULTURAL METHODS IN ORCHARDS 151 



nitrogen compounds and change the nitrogen to avaihible 

 forms. This process is known as nitrification, and the nitro- 

 gen which the legume organisms took from the air is not 

 available to other plants until these nitrifying organisms 

 have had an opportunity to do their work. Incidentally 

 it should be mentioned that there are other groups of ni- 

 trogen-fixing organisms in the soil capable of taking nitrogen 

 from the soil-air in considerable quantities when supplied 



Nitrogen as Nifrate 



Nitrogen as Organic Matter 



Fig. 26. — A graphic representation of the cycle of nitrogen through 

 the plant and the soil. 



with an abundance of phosphate, limestone, and organic 

 matter. These organisms are not related to legumes but 

 are present in all normal soils. The nitrogen left be- 

 hind in their bodies also becomes available to plants in the 

 process of nitrification. The nitrification process is graph- 

 ically illustrated in Fig. 26. 



133. Value of cover-crops in California.— In semi-arid 

 regions, such as are found within the agricultural section of 

 California, there is a notable deficiency in the organic mat- 



