Effects of moisture level and potassium on NH4+ nitrification and their influence on growth parameters and nutrient uptake of bread wheat (Triticum aestivum. L) grown on Typic Hapludert of central highlands of Ethiopia

Abstract

To understand the impact of moisture level and potassium on NH₄⁺ nitrification a greenhouse and laboratory studies were conducted using surface soil of Typic Hapludert (0–30 cm) of Ginchi, central Ethiopia. The treatments were two levels of moisture and three levels of fertilizer (six combinations replicated three times). The Typic Hapludert, which is dominated by smectite and contains relatively high illite, has 6.44% total potassium, high K-fixing (minimum of 4.89 and maximum of 69.88 mg/100 g soil) and buffering (45.05 mmol/) capacities and low potassium availability (2.26 cmol (+) kg soil). The soil contains significant amounts of potassium in the form of micaceous K. Eighty percent of the applied (NH₄)₂SO₄ was nitrified and caused sharp increase of NO₃ under both moisture statuses between the 2^nd and 5^th weeks. Maintaining moisture at constant level could only delay nitrification, but could not stop nitrification process completely. Under all treatments the oxidation of (NH₄)₂SO₄ into NO₃^- ions took place in a very short period. In treatments T₁ (without N and K fertilizers), T₃ (83 mg nitrogen) and T₅  (83 mg nitrogen, 37.5 mg K), which were watered every other day, 86-88% of (NH₄)₂SO₄ was converted to NO₃^-within 15 days where as in T₂ (without N and K fertilizers), T4 (83 mg nitrogen) and T₆ (83 mg nitrogen, 37.5 mg K), which were  moist to 60% water holding capacity of the soil every day, 34-44% of the applied (NH₄)₂SO₄  oxidized to NO₃^- within 22 days.  High moisture status has discouraged K assimilation without affecting nitrogen uptake significantly. However, Mg²⁺ uptake was not affected under both water treatments. Application of potassium was found to shorten the time required for flowering.