Fertilizer Value of Ambient Nitrogen in Irrigation Water for Vegetables.

Irrigation water from many wells on the central coast of California contains a significant amount of nitrate-nitrogen (NO₃-N), and recycled water, the main water source for approximately 16,000 acres of prime farmland on the coast, is high in both NO₃-N and NH₄-N.  Growers historically have been reluctant to modify N fertilization practices on the basis of ambient levels of N in irrigation water because it is unclear how to reliably calculate the ‘fertilizer value’ of this N.  A limited body of research documents the efficiency of crop uptake of N from irrigation water, upon which to base an estimate of ‘fertilizer value’ under normal irrigation and N management practices.  We conducted 2 replicated field trials to evaluate recovery efficiency of N for drip-irrigated lettuce using water of simulated concentrations of NO₃ and NH₄ ranging from 2 to 42 ppm of N.   The volume of applied water during the drip phase of the crop equaled either 110% and 160% of crop evapotranspiration (ETc) requirement for the summer harvested trial, and 120% and 220% of the ETc requirement for the fall harvested trial.  Results of both trials demonstrated that the concentration of N in the irrigation water significantly affected plant size, N content of tissue, and biomass yield.   Crop recovery of N from the water treatments averaged 86% during the summer trial and 41% during the fall trial, compared to 55% and 20% from standard fertilizer treatments for the summer and fall trials, respectively.   Nitrogen recovery efficiency (NRE) from NO₃ and NH₄ sources of N was equal.   The volume of water applied also did not affect NRE, and lettuce was able to recover N from water with NO₃-N concentrations as low as 10 ppm.