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Effect of Ammonium and Nitrate Fertilization on Phosphorus Uptake as Related to Root-Induced pH Changes at the Root-Soil Interface¹

ABSTRACT

Soybeans (Glycine max L.) fertilized with either NH₄-N or NO₃-N were grown in a growth chamber using soil with four different initial pH levels. Liming the soil used in this research to increase pH decreased the P level in solution. Fertilization of soybeans with NH₄-N decreased the pH of the rhizocylinder (root plus strongly adhering soil); fertilization with NO₃^- increased rhizocylinder pH. The difference between the rhizocylinder pH of the NH₄⁺ and NO₃^- treatments was as large as 1.9 pH units with an initial soil pH of 5.2 and as small as 0.2 units when soil pH prior to N application was 7.8.

Ammonium-fertilized soybeans absorbed more P and had a higher P concentration than NO₃-fertilized soybeans. The results for soybeans grown with NH₄⁺ and NO₃^- treatments at four initial soil pH levels showed that the P content of the shoots and roots was closely correlated with the pH of the rhizocylinder, but not the pH of the bulk soil. This suggests that the increased availability of P from the soil where NH₄⁺ was used was mainly due to the effect of the nitrogen source on the pH of the rhizosphere soil.

Soybean root length decreased from 180 to 120 m/gram of dry roots as the pH of the rhizocylinder increased from 4.7 to 7.5.

¹ Journal Paper no. 4001. Purdue Univ. Agr. Exp. Sta., Lafayette, Ind. 47907. Contribution from the Department of Agronomy. This research was supported in part by the Tennessee Valley Authority under cooperative project IND 1131-69.

² Research Associate and Professor of Agronomy, Purdue University, respectively. D. Riley is now with Plant Protection Ltd., Jealott's Hill, Berkshire RG 126EY, England.