P hosphorous Determinations . A modified stannous chloride 

 method was used to determine phosphates (orthophosphate), 

 using the same colorimeter as that used In the nitrate tests. 

 Standaj:'dlzatlon curves were established and checked In the 

 same manner as In the nitrate tests In order to maintain 

 optimum control and the same high degree of accuracy. 



In standard phosphate solutions ranging from 0.1 to 6.0 mg/l, 

 the average standard deviation was 3.6? percent of the mean. 



A detailed account of the methods used In determining 

 nutrients, the techniques used for maintaining accuracy and 

 precision, and statistical comparisons made during the 

 course of the Investigation Is In the Department's files. 



Electrical Conductivity . A conductivity bridge was used to 

 determine the electrical conductivity (EC) of the tile 

 drainage water and soil extracts. These tests were run In 

 the laboratory along with the nitrogen and phosphate deter- 

 minations. A factor of 0.7 was used In converting EC values 

 (mlcromhos x 10"") to total dissolved solids (TDS, mg/l). 

 This Is an average factor determined from mineral laboratory 

 reports . 



S oil Extrac ts . Nitrate analyses (cadmium reduction method) 

 of soil sampTes were made on extracts that were suction- 

 filtered from a saturated soil paste. Soil extracts were 

 refrigerated prior to analysis to minimize possible 

 denltrlf Icatlon . 



Collection of Agricultural Data 



Agricultural practices data which Include the type of crops 

 grown and the quantities of nitrogen fertilizer applied were 

 recorded for every tile system Investigated during 1966 and 

 1967. Irrigation application rates and quantities of runoff 

 were determined for several tile systems from 1959 to I963. 



Crops 



Cropping data were collected from Individual farm operators 

 and Irrigation district offices of the actual crops grown 

 for any tile system between the years 1959 through I967. 



Irrigation 



Actual amounts of applied Irrigation water were determined 

 for a number of tlle-dralned fields from 1959 to 1963. 



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