Comparison of Cationic and Anionic Forms of Nitrogen Fertilization on Growth and Calcium, Zinc and Iron Content in Shoots of Two Varieties of Vegetable Amaranth

Nitrogen (N) is an essential macro nutrient for plant growth and development. Plants absorb nitrogen in either ammonium (NH4+) or nitrate (NO3-) forms, which influence plant growth and absorption of nutrients in higher plants. The aim of the study was to determine the influence of different N forms on growth and tissue content accumulation of mineral elements; calcium, zinc and iron in vegetable amaranth. The study was conducted in Agriculture Science and Technology Departmental farm of Kenyatta University between March and May, 2015. Three N forms used were; sole ammonium (NH4+) stabilized with Piadin® as nitrification inhibitor, sole nitrate (NO3-), ammonium nitrate (NH4NO3) (also stabilized with Piadin®) and control as subplots on two amaranth varieties were AB6 and AB7 as main plots in randomized complete block design (RCBD) replicated three times in a split plot arrangement. Results indicated that ammonium treatment led to acidification of the rhizosphere compared to nitrate treatment, while nitrate nutrition led to a rise in rhizosphere pH. Nitrogen forms significantly (P=0.05) affected shoot and root dry weights and leaf area. Nitrate-treated plants accumulated shoot dry weight 3.7 fold in case of AB6 variety and 2.5 fold for AB7 variety as compared to control, while that of sole ammonium treatment was twofold for AB6 and 1.3 fold for AB7. Root dry weight reduced by 52% in AB6 and 46.7% in AB7 variety under ammonium treatment compared to nitrate treatment. Compared to the control, plants treated with nitrate-N form had leaf area four times higher while those treated with ammonium form were twice superior in both AB6 and AB7 varieties. Amaranths treated with sole nitrate markedly increased plant tissue (calcium, zinc and iron) concentrations compared to ammonium treated plants. Ammonium/nitrate mixture enhanced plant growth and mineral element concentration compared to sole ammonium treatment. It was therefore concluded that ammonium-induced pH acidification restricted plant biomass accumulation as well as calcium and zinc concentration compared with nitrate treatment which exhibited better growth in vegetable amaranth irrespective of variety.