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 2nd and 5th 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