Corn Root Growth and Nitrogen Uptake as Affected by Ammonium Placement

Stimulation of root growth in the N-fertilized soil volume of placed N fertilizer has been observed, but no quantitative data are available. The objective of this research was to obtain quantitative data on the influence of the fractional soil volume fertilized with NH⁺₄ on corn (Zea mays L.) root distribution between NH⁺₄-fertilized and unfertilized soil, and to determine the effect of N placement on N uptake and plant growth. A pot experiment with seven treatments and three replicates was conducted using Raub silt loam (fine silty, mixed, mesic Aquic Argiudolls). The treatments were six volume fractions of 1.00, 0.50, 0.20, 0.10, 0.05, and 0.02 of soil fertilized with NH⁺₄ plus a no-N treatment. Before use, the soil was leached to remove NO⁻₃ and a nitrification inhibitor was added. The NH⁺₄ rate was 90 mg N kg⁻¹ of soil in the pot. Six-day-old corn plants were transplanted into pots with 3 kg of soil and grown for 14 d, and into pots with 15 kg of soil and grown for 40 d. Corn was grown for 14 d with all treatments and for 40 d with the 0.2 placement. Root length density (km m ⁻³) was greater in the NH⁺₄-fertilized soil. Root distribution between the NH⁺₄-fertilized and unfertilized soil volumes, the same at 14 d and 40 d, can be described by the equation y = x⁰.⁷⁰ (r²=0.99), where y is the fraction of total root length in the NH⁺₄ -fertilized soil when NH⁺₄ is mixed with x fraction of the soil. The effect of NH⁺₄ on root distribution was similar to that observed in earlier studies with P. Shoot N concentration was not affected by NH⁺₄ placement; however, plant weight and N uptake were less with the 0.5 and 0.2 placement treatments. Contribution from the Dep. of Agronomy, Purdue Univ. Journal Paper no. 11 277 of the Purdue Univ. Agric. Exp. Stn., West Lafayettte. Research supported in part by a grant from Dow Chemical Co.