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The FAO nitrogen limit of 5–30 mg/L corresponds to the range of N requirements for typical crops (Table 2, Doorenbos 1979). For adequate fertigation of, for example, tomatoes, total N should not exceed 25 mg/L. This limit is based on an N requirement of 125 kg N/ha per growing period and an irrigation demand of 5,000 m³/ha per growing period (125 kg N/ha ÷ 5,000 m³/ha = 25 mg/L) for two preconditions: irrigation only with reclaimed water and no leaching via excess irrigation or rainfall (Table 2).

Table 2

Irrigation and nutrient requirement for various crops (Doorenbos 1979) and water quality objectives for total N, P and K in irrigation water (when irrigated only with reclaimed water, no leaching, e.g. via excess irrigation or rainfall)

 Requirement per growing period
Required concentration
 WaterTotal NTotal PTotal KTotal NTotal PTotal K
Cropm³/hakg/hakg/hakg/hamg/Lmg/Lmg/L
Groundnut 6,000 15.0 27.5 32.5 2.5 4.6 5.4 
Bean 4,000 30.0 50.0 85.0 7.5 12.5 21.3 
Sunflower 8,000 75.0 32.5 92.5 9.4 4.1 11.6 
Safflower 9,000 85.0 22.5 32.5 9.4 2.5 3.6 
Banana 17,000 300 52.5 260 17.6 3.1 15.3 
Pepper 7,500 135 37.5 75.0 18.0 5.0 10.0 
Water melon 5,000 90.0 42.5 57.5 18.0 8.5 11.5 
Wheat 5,500 125 40.0 37.5 22.7 7.3 6.8 
Maize 6,500 150 65.0 80.0 23.1 10.0 12.3 
Sugarbeet 6,500 150 60.0 130 23.1 9.2 20.0 
Tomato 5,000 125 87.5 201 25.0 17.5 40.1 
Cabbage 4,400 125 57.5 115 28.4 13.1 26.1 
Olive 7,000 225 62.5 185 32.1 8.9 26.4 
 Requirement per growing period
Required concentration
 WaterTotal NTotal PTotal KTotal NTotal PTotal K
Cropm³/hakg/hakg/hakg/hamg/Lmg/Lmg/L
Groundnut 6,000 15.0 27.5 32.5 2.5 4.6 5.4 
Bean 4,000 30.0 50.0 85.0 7.5 12.5 21.3 
Sunflower 8,000 75.0 32.5 92.5 9.4 4.1 11.6 
Safflower 9,000 85.0 22.5 32.5 9.4 2.5 3.6 
Banana 17,000 300 52.5 260 17.6 3.1 15.3 
Pepper 7,500 135 37.5 75.0 18.0 5.0 10.0 
Water melon 5,000 90.0 42.5 57.5 18.0 8.5 11.5 
Wheat 5,500 125 40.0 37.5 22.7 7.3 6.8 
Maize 6,500 150 65.0 80.0 23.1 10.0 12.3 
Sugarbeet 6,500 150 60.0 130 23.1 9.2 20.0 
Tomato 5,000 125 87.5 201 25.0 17.5 40.1 
Cabbage 4,400 125 57.5 115 28.4 13.1 26.1 
Olive 7,000 225 62.5 185 32.1 8.9 26.4 

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