Nitrate/ammonium ratio effects on nutrient solution pH,dry matter yield,and nitrogen uptake of sorghum 1

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.