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

Soybeans ( Glycine max L.) fertilized with either NH 4 ‐N or NO 3 ‐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 NH 4 ‐N decreased the pH of the rhizocylinder (root plus strongly adhering soil); fertilization with NO 3 ‐ increased rhizocylinder pH. The difference between the rhizocylinder pH of the NH 4 + and NO 3 ‐ 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 NO 3 ‐fertilized soybeans. The results for soybeans grown with NH 4 + and NO 3 ‐ 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 NH 4 + 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.