Micronutrient deficiency commonly occurs in calcareous soils. We hypothesized that localized application of ammonium in the P-banding zone could improve micronutrient uptake and thus grain yield through inducing rhizosphere acidification and stimulating root proliferation. A two-year field experimentation with maize growing on a calcareous soil was conducted with localized application of P (superphosphate) only (P), P plus ammonium as ammonium sulfate (NH4-N+P) or urea plus P (urea+P) at sowing and jointing. Compared with localized urea+P, localized NH4-N+P significantly improved root dry weight, whole root length and first-order lateral root density/length at the seedling stage, as well as chlorophyll content and net photosynthetic rate at flowering. Shoot Zn content was 30-50% greater at the seedling stage and 22-36% higher at grain maturity in localized NH4-N+P compared with localized P and urea+P in 2012 and 2013. The Fe content at the seedling, flowering and harvest stages in localized NH4-N+P was higher than that in localized P, and also exhibited an increasing trend relative to localized urea+P. Compared with localized urea+P, grain Zn concentration in the NH4-N+P treatment increased by 17% in 2012, and exhibited an increasing trend in 2013. Grain Fe concentration in the NH4-N+P treatment increased in 2013 in comparison with the P and urea+P treatments. Maize grain yield in localized NH4-N+P increased by 10-14% in 2012 and 13-25% in 2013 compared with localized P and urea+P. The results indicate that localized application of NH4-N+P may be an efficient approach to improving maize growth, grain Zn and Fe accumulation and yield via modifying root traits (such as root length, lateral root proliferation and root length density), and intensifying rhizosphere acidification by ammonium uptake. © 2014 Elsevier B.V.