a) All plants use nitrogen (N) to sustain themselves and grow. To encourage plant growth, farmers apply manure or fertilizer to supply necessary amounts of nitrogen. The amount of nitrogen needed to reach a desired crop yield varies with the crop grown. Table 2, for example, lists the nitrogen needs of corn. The nitrogen needs of other crops are listed in the Ohio Agronomy Guide, 13th Edition, Bulletin 472, available from local Ohio State University Extension offices.
A nitrogen balance for the irrigation site should be developed to insure that groundwater contamination will not result. The objective is to keep nitrate levels below 10 mg/l in the groundwater beneath the site.
Nitrogen applied - nitrogen used by crop = nitrogen available for leaching
Water applied + precipitation - water evaporated = water available for dilution
All of the nitrate and ammonia in the wastewater is available for plant uptake and any excess can leach into groundwater. Organic nitrogen in the wastewater becomes a part of the soil organic matter and is mineralized at a rate of less than 5 percent per year.
| Table 2. Annual nitrogen application (expressed as lb N/acre/year) recommended for corn | |||
|---|---|---|---|
| Yield goal (bushels/acre) | |||
| 120 | 150 | 180 | |
| Previous Crop | Annual application (lb N/acre) | ||
| Forage legume | 60 | 110 | 150 |
| Grass crop | 65 | 170 | 200 |
| Soybeans | 85 | 190 | 200 |
| Continuous corn or other crops | 115 | 200 | 200 |
b) Calculate:
Nitrogen available for leaching (lb/acre) =
(Inorganic N content of wastewater (mg/l) x annual wastewater application(inch) x
0.226) + N fertilizer (lb/acre) - N crop needs (lb/acre)
c) Calculate:
Water available for dilution (inches) = Wastewater applied (inch) + annual precipitation (inch) - annual evaporation (inch)
d) Calculate:
Nitrate content of water leaching from site (mg/l) = [available N (lb/acre)/ dilution water (inch)] x 4.42
This calculation is a conservative approach in that the nitrate leaching from the site can be further diluted in the groundwater. It also does not take into account denitrification losses. Consider working with a hydrogeologist to conduct a more comprehensive analysis if designing a large system, applying high nitrogen content wastewater, or working in problem areas with elevated groundwater nitrate levels.
(30 mg/l x 38.7 acre-inches x 0.226) + 0 lb/acre - 200 lb/acre = 62 lb/acre available for leaching
38.7 inches applied + 38 inches precipitation - 32 inches evaporation = 44.7 inches water available for dilution
[(62 lb/acre)/44.7 inches] x 4.42 = 6.1 mg/l
Annual application of 38.7 acre-inches of treated wastewater per year will not pose a problem in elevating the nitrate levels in the infiltration water above 10 mg/l. Fertilizer and /or manure application to the site may need to be adjusted and agreed upon to account for nitrogen applied through reused wastewater.