Rhizosphere pH as Influenced by Plants, Soils, and Nitrogen Fertilizers

The rhizosphere pH (pH r ) of field and container‐grown wheat plants was measured and compared to the nonrhizosphere soil pH (pH b ). The predominant form of N supplied to the roots was a major factor influencing the pH r . The pH r was generally lower than pH b where NH 4 was supplied, higher where NO 3 was used, and relatively unchanged where both forms were added. Differences in pH r of up to 2.2 units were recorded for NH 4 ‐ vs NO 3 ‐fed wheat plants ( Triticum aestivum L.) in pots. Differences of up to 1.2 units occurred in the field. The pH b and pH r also differed by as much as 1.2 units. Changes in pH r were correlated with the concentrations of NH 4 in potted soils, although the acidifying influence of NH 4 decreased with time, as nitrification proceeded. Reductions in pH r values persisted longer in NH 4 treated, fumigated soils, and in non‐fumigated soils where NH 4 fertilizers had been treated with a nitrification suppressant. The pH r differences in NH 4 ‐ vs NO 3 ‐N treated soils were reduced by organic amendments. Difference with respect to pH r were found among wheat varieties and plant genera. The ability of plants to alter the pH r was related to assimilation of NO 3 primarily in the roots (monocots) or shoots (dicots).