Plant nutrients — such as nitrogen, phosphorus, and potassium — often are present in water. This is especially true of recycled waters, which typically contain all three of these elements as a result of prior use.
Most laboratories test recycled water for nutrient content, reporting the nutrients in terms of "pounds per acre-foot of water applied."
If you receive a laboratory analysis that reports nutrients only as concentrations, either in mg/L or in ppm, and omits the acre-foot-based units, you can compute the latter yourself:
Nutrient content (in lb per acre-ft) = Nutrient content (in mg/L or ppm) × 2.72
The economic value of these nutrients can be substantial. In some cases irrigation water contains sufficient nutrient material to figure significantly in the fertilization program for a large turf landscape. Even in situations where the quantity of nutrient applied via irrigation is low, there's a benefit: plants are able to utilize the nutrients very efficiently because the nutrients arrive at each plant on a regular schedule, along with the plant's water.
Recycled waters that comply with California Title 22 regulations typically contain from 15 to 50 milligrams per liter of nitrogen, mostly in the form of nitrate and NH4 ions. That translates to 41 to 136 pounds of nitrogen per acre-foot of applied water. These nitrogen sources, if not accounted for as part of a nitrogen fertilization scheme, may cause excessive vegetative growth or may delay or reduce the blooming of flowers. In addition, excess application of nitrogen can lead to leaching of nitrogen below the root zone, potentially adding to the nitrate load of local groundwater.
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