Nitrate/ammonium ratio effects on nutrient solution pH, dry matter yield, and nitrogen uptake of sorghum1

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Abstract
Sorghum [Sorghum bicolor (L.) Moench] seedlings were grown in nutrient solutions in a growth chamber to investigate the effects of different ratios of NO3- and NH4+ on nutrient solution pH, dry matter yield and N uptake. Nutrient solutions and plant tissues were assayed throughout the time plants grew in the nutrient solutions. Nutrient solution pH depended on source of N. The pH rose to near 8 with NO3- as the sole source of N and decreased to near or below 4 with NH4+ added to the solutions. Upon depletion of NH4+ from solution, pH values rose abruptly to near 8 and remained near this value throughout the duration of the experiments. Dry matter yield was generally higher for plants grown with some NH4+ compared to plants grown with NO3- alone. Nitrogen uptake was generally higher in plants grown with the higher proportions of NH4+. Nitrogen concentrations remained unchanged with plant age as NO3-/NH4+ ratio varied. For solutions low in NH4+, N concentrations in roots increased with plant age. Severe Fe deficiency appeared in plants when solution pH reached and remained above 7.