pcirtly by denltrlflcation . The mid-summer decline of 

 nutrients in the composited drainage cannot be attributed 

 solely to the Influence of drainage from rice fields. Similar 

 decreases in nutrient and TDS concentrations also occurred 

 in composited drainage from the central area when rice field 

 data were eliminated. 



Nitrogen in Irrigation Water . One important aspect of Irri- 

 gation is the amount of nutrients added during application. 

 Nitrogen in irrigation water is usually found in the nitrate 

 form; organic nitrogen is nearly always found to be less 

 than 1 mg/l . The nitrate form of nitrogen is readily suita- 

 ble for uptake by most plants, although the ammonium ion can 

 be assimilated by some crops. However, more than an acre- 

 foot of water is applied to fields for preirrigation, when 

 no crop is being grown, and during early stages of growth 

 when the root development is not great, which could result 

 in losses to the ground water. Lyslmeter studies (19) have 

 shown exceptionally high leaching losses when soils are 

 fallow. Nitrate-nitrogen concentrations in irrigation water 

 applied to tiled fields in the study aj'ea were determined to 

 be 4.0 mg/l, which amounts to approximately 11 pounds of 

 nitrogen added for every acre-foot of irrigation water 

 applied. Nitrogen contributed in this manner could exceed 

 40 pounds per acre for certain heavily irrigated crops for 

 even one year, discounting other possible losses. 



For this investigation, nitrogen contributed to the soil in 

 applied water or rainfall was assumed to be about the same 

 in all areas, considering crop diversification and water 

 requirements of such large areas. Gains in nitrogen due to 

 irrigation application were considered more than "offset" by 

 the amounts lost due to deep percolation, volatilization, 

 and denltrlflcation. 



Fertilization 



Much has been written regarding the leaching of nitrogen 

 from soils. However, data to show the contribution of ferti- 

 lizer nitrogen to shallow ground water are scarce. Although 

 specific studies were not conducted to determine the actual 

 amounts leaching to the water table, agricultural data 

 showing the amounts of nitrogen fertilizer applied in major 

 tiled areas over a long period are discussed in relation to 

 to the concentrations of nitrogen observed in the drainage. 

 Leaching of fertilizers is also discussed in the subsection 

 that follows. 



Leach ing. Lyslmeter investigations (19) have shown that the 

 amount of nitrogen leached through soils during irrigation 



58 



