The influence of nitrogen sources on root-mediated changes in substrate pH

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Abstract
The relationship between nutritional nitrogen (nitrate, ammonium, and urea) and changes in rhizosphere pH was established using axenic cultures of seedlings grown on nutrient agar. A nutrient-related rhizosphere pH was obtained which was modulated by the source of nitrogen in the medium. After 3 weeks of growth, a pH of 8.5–9.5 was generated by wheat seedlings fertilized with Hoagland's nutrient solution No. 2. A comparable degree of alkalization was observed in nitrate-fertilized plants. A rhizosphere pH less than 4.0 was obtained in plants fertilized with ammonium salts while no significant change was detected in wheat seedlings provided with urea as the sole source of nitrogen. These results are consistent with the concept of cation–anion uptake balance, maintenance of electroneutrality across the root surface, uptake by cotransporters, and chemiosmotic principles.