some samples. Nitrogen concentrations were found to be 

 higher for virgin than for irrigated soils of the same 

 series. Also, extreme differences in nitrogen concentrations 

 were observed between soils occupying different physiographic 

 positions; interfan areas were higher in nitrogen than was 

 the main body of the alluvial fan. 



Virgin Sites . The highest concentrations observed were 

 found in two virgin profiles of Panoche soil series; nitrogen 

 values averaged 233 and 234 mg/l in the tile zone. High 

 nitrogen concentrations were also found in soil extracts 

 taken from virgin profiles of the Lost Hills and Panhill 

 series, close relatives of the Panoche series. However, low 

 to moderate levels of nitrogen were also observed in some 

 virgin profiles of these soils where natural leaching 

 occurred. 



I rrigated Sites . Two irrigated sites of Lethent series 

 located in the basin rim physiographic position were nearly 

 as high in nitrogen as some of the virgin alluvial sites 

 mentioned above. One experimental tile system which was 

 mentioned previously In this report had nitrate-nitrogen 

 concentrations exceeding 2,000 mg/l. However, soil condi- 

 tions at these sites were not considered representative of 

 other known basin rim areas where tile systems have been 

 Installed. Four basin rim sites, which Included one site of 

 Rossi, one of Volta and two of the Orestlmba soil series, 

 averaged 4 mg/l, which correlated closely with tile drainage 

 from soils in similar physiographic positions, but from 

 different soil series . 



In several profiles of the Panoche series examined in irriga- 

 ted areas, nitrogen ranged from 6 to 9^ mg/l and averaged 

 about 37 mg/l in the saturation extracts. Only one Irrigated 

 site of Panhill soil was sampled; nitrogen averaged 35 mg/l. 



Table 22 compares the average nitrogen for virgin soils to 

 that of irrigated soils. The data show that nitrogen is 

 significantly higher in virgin soils. 



Alluvial Fan and Interfan Sites . Soil samples collected 

 from virgin sites within the main course of a depositing 

 stream are usually well leached and contain less nitrogen 

 than soils located in interfan areas. The extent of natural 

 leaching depends mainly upon the texture and stratigraphy of 

 the soil, precipitation, and relative proximity to a stream 

 or creek at one time or another. The data compiled in 

 Table 23 show that nitrogen is much more concentrated in the 

 soils sampled from alluvial Interfan areas than from fan 

 areas in the direct course of a depositing stream. 



73 



