Effects of High Ammonium/Nitrate Ratios on Nitrification and Growth of Wheat in Hydroponic Culture
Nitrogen is the only plant nutrient taken up as both a cation (NH4+) and anion (N03-). Nitrate is considered the "safe" form of N and NH4+ is generally thought to be toxic, especially at high levels. High NH4+/N03- ratios are thought to be toxic because they result in a rhizosphere pH low enough to damage root membranes, induced cation deficiencies, and build-up of NH3 caused by delayed NH4+ assimilation. These factors can be minimized in hydroponic culture. The objective of these studies was to quantify lll the effects of high NH4+/N03- ratios on nitrification and growth of wheat in hydroponics.
Two cultivars of wheat (Triticum aestivum L.) were grown to maturity with either 15% or 80% of the N supplied as NH4+. The effect of using Cl- versus SO42- as counter ions to NH4+ was also examined. Yield was not significantly affected by NH4+ ratio or counter ion. Seed protein was increased from 15 to 19% with high NH4+. Harvest index was reduced from 52 to 48% with 80% NH4+, but was unaffected by counter ion.
Rates of nitrification in hydroponic culture are not well quantified and could result in significant conversion of NH4+ to NO3- before plant uptake. An isotopic dilution study was conducted to quantify rates of nitrification in hydroponic culture. A 2 x 2 x 2 factorial design was used to examine the effect of pH (5.8 or 7.0), inoculation with nitrifying bacteria, and the presence of plants. This study was done with wheat grown in vigorously-aerated, 2-L bottles. Each bottle contained 10 g of diatomaceous earth to provide surface area for microbial growth. Nitrate began to accumulate in 5 d in unplanted, inoculated bottles at pH 7.0; in 20 d at pH 5.8 with inoculation; but did not begin to accumulate in non-inoculated bottles (pH 5.8 or 7.0) until day 30. Nitrate never accumulated in any of the planted bottles, most likely because plants consumed the NO3- that was produced. Calculations from the isotopic dilution measurements indicated that the rate of nitrification averaged 58 μmol NO3- L-1d-1 in the planted bottles, and averaged 270 μmol NO3- L-1d-1 in unplanted bottles. Nitrification was likely reduced in the planted bottles because the reduced concentration of NH4+ limited nitrification.
To provide rapid, inexpensive measurement of nutrient concentration in hydroponic solution, five colorimetric tests were evaluated. Tests for NO3- and PO4 were accurate and reliable, but the tests for SO42- SiO2, and Fe need additional refinement.