Effect of nitrogen fertilizer form on ph of the bulk soil and rhizosphere, and on the growth, phosphorus, and micronutrient uptake of bean

In a pot experiment, the effects of NO3-N and NH4-N fertilizer were examined on the pH of the bulk soil and rhizosphere, and on the growth and nutrient uptake of 18-35-d old bean plants (Phaseolus vulgaris L.) supplied with KH2PO4 or rock phosphate (Hyperphos). Prior to sowing, the soil was incubated for 16 d to ensure complete nitrification of NH4-N which decreased bulk soil pH from 6.8 to 5.5. In other pots, a nitrification inhibitor, N-Serve, was added together with the ammonium fertilizer and after 18 d growth, the pH of the bulk soil was 6.6 while the pH of the rhizosphere decreased to 4.5. Shoot and root dry matter yield was significally greater for plants supplied with KH2PO4 and fertilized with NH4-N compared with N03-N. This increased growth by NH4-N fed plants was presumably due to a increased nutrient availability caused by the acidification of the bulk soil. Shoot concentrations of P and micronutrients, such as Fe, Mn, Zn, and Cu, were higher for plants supplied with NH4-N, and more strikingly were higher for plats supplied with NH4-N+N-Serve when expressed on a root length basis. In this latter case, the increased nutrient acquisition by plants could only be due to acidification of the rhizopshere. The inhibitory effect of NH₄-N+Nz-Serve, particularly on root growth, was not caused by NH₄ ⁺ toxicity, but was due to a direct effect of N-Serve as shown by growth comparisons with another nitrification inhibitor, dicyanodiamide (DCD).