grown in the northern areas, farmers near Tracy apply 

 nitrogen fertilizer to most truck and field crops at 60 to 

 100 units per acre, which is very close to that applied in 

 the central and southern areas . 



Application rates of nitrogen fertilizer have generally 

 increased in the past ten yesurs . The rates for cotton, rice, 

 barley amd safflower have nearly doubled on many farms in 

 the central area since I960. This has been brought about by 

 recommendations of farm advisors and consultants for ferti- 

 lizer companies. Furthermore, an increased usage has also 

 resulted in some areas from plantings of lettuce, bell 

 peppers and other such high-fertilizer-use crops. 



Types and Methods of Use . Both commercial and natural ferti- 

 lizers were used in the tile-drained areas. The methods of 

 application depend a great deal on the types being used 

 which are discussed in the following subsection. 



Commercial Fertilizers . Various forms of nitrogen 

 fertilizer are popular in the study area; ammonium- 

 sulfate, urea, and aqua-aunmonia are the most 

 popular. Row crops are generally banded with 

 fertilizer, whereas some field crops, such as 

 barley and rice, have areal applications. Urea 

 fertilizer is used almost exclusively to reduce 

 denitrification losses in rice crops. 



Phosphate fertilizers have been used to a limited 

 extent on the west side; however, regular applica- 

 tions are made to large acreages of soil in the 

 Tulaj?e Lake area. 



Animal and Green Manures . The amounts of nitrogen 

 added from animal manure and green manure (cover 

 crops) were not determined because these materials 

 contain minimal amounts of nitrogen. Average fsLrm 

 manure generally contains less than 0.5 percent 

 nitrogen by weight, and the amounts provided by 

 green manure crops are extremely variable (12). 



No attempt was made to determine the sunounts of 

 nitrogen contributed b:' legume crops such as 

 alfalfa or beans. The amount of atmospheric 

 nitrogen "fixed" by legumes is quite variable; 

 also, the amount of nitrogen added to the soil 

 after several years growth of a leguminous crop is 

 questionable (13). 



Volatili z ation . Volatilization losses after fertilization 

 can be hTgh under certain conditions (lA). However, for 



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