Interfan areas are those which are protected from repeated 

 flushing action of a nearby stream by their slightly higher 

 elevation. Interfan areas are interspersed with the many 

 coalescing alluvial fans on the west side. 



Different Geographical Sites . Soil profiles located in the 

 northern area tended to be lower in nitrogen than were the 

 soils investigated in the central area. Values ranged from 

 2 rag/l for one Rincon site to 59 mg/l at a site of Ambrose 

 soil; both sites were dry-farmed. Drainage from three tile 

 systems located in Ambrose soil never approached this value. 

 More sites are needed in the northern sirea to evaluate the 

 magnitudes of residual nitrogen in virgin and irrigated soil 

 profiles. 



Nitrogen in Soils and Tile Drainage 



VJhenever possible, compatrisons were made between the 



nitrogen found in soils (saturation extracts and field 



extracts) and that of tile drains, either in the same tiled 

 field, nearby field, or the same soil series located some 



distance away. However, much variability exists in soils, 



and soils data were somewhat limited; therefore, only a few 

 comparisons could be made. 



Saturation Extracts and Field Extracts . The most universally 

 accepted method for determining specific ions in soil samples 

 is the analysis of soil solutions known as the saturation 

 extract (28). The extracts are prepared by adding distilled 

 water to an oven-dried soil sample until a condition of 

 saturation is reached. The leachate then is extracted by 

 means of a suction filter device. The water content of a 

 saturated paste generally exceeds that found in the soil 

 under saturated field conditions because of the lower volume 

 weight and greater porosity. The saturation percentage (SP) 

 is expressed as grams of water per 100 grams of soil. 



Soil solutions extracted from saturated soil pastes have 

 been determined to be as neeir as possible to the water 

 content which occurs under field conditions. However, the 

 concentration of salts in tile drainage cannot be expected 

 to be exactly the same as that in saturation extracts. As a 

 soil approaches the permanent wilting point (PWP), the soil 

 solution becomes much more concentrated. The PWP varies 

 for different soils and is generally expressed as the 

 percent of moisture in a soil on a dry-weight basis at 

 which plants wilt and are unable to regain turgidity. 



75 



