this investigation, volatilization losses were considered 

 more than offset by the quantities of nitrogen added in 

 irrigation water. 



Data Sources . Records of commercial fertilizer applications 

 were collected along with the cropping data. Estimates of 

 missing or inadequate data for a given crop were based on 

 fertilizer recommendations of farm advisors or local ferti- 

 lizer companies for a particular year. 



Investigations of Native Nitrogen 



In addition to routine monitoring of tile drainage systems, 

 indigenous nitrogen in soil profiles was investigated in the 

 field. A literature review concerning residual nitrogen in 

 parent materials was also performed. 



Soil Profile Nitrogen Sampling 



Forty-five soil profiles representing different physiographic 

 positions and soil series were sampled for indigenous 

 nitrogen along the west side. Virgin sites were sought to 

 obtain the quantity of nitrogen in the soil before irriga- 

 tion. Several dry-farmed and irrigated sites were also 

 sampled. All profiles were logged for soils occupying recent 

 alluvial fan, older alluvial fan, basin rim, and basin 

 physiographic positions. Sites were hand-augered to a depth 

 of 10 feet or more and sanples were collected at l-l/2-foot 

 increments for nitrogen analyses. 



Parent Material Nitrogen Sampling 



Data on residual nitrogen concentrations of parent materials* 

 were obtained from investigations (15 ) conducted in 1959 

 along with deep boring investigations, to determine sources 

 of high nitrates observed in shallow ground water on the 

 east and west sides of the Valley. Alluvial parent sources 

 (cretaceous marine sediments of the Diablo Range) in 

 selected drainage ways (Arroyo Hondo and Arroyo Ciervo) were 

 sampled, including an array of different materials, such as 

 zones within the stratum, mudflows, cross sections of 

 arroyos, mud puddles, and streams. 



*Provided by the Agricultural Research Service 



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