Fertilizer nitrogen use efficiency and nutrient uptake by maize (Zea mays L.) in vertisols in Kenya

The general objectives of this study were to increase the understanding of nitrogen (N) losses in maize cropping on Vertisols, and to develop management options to reduce such losses and to improve fertilizer N use efficiency. The specific objectives were. to quantify the effects of fertilizer N sources and management practices on (i) fertilizer N losses through denitrification, NH 3 volatilization and bypass flow, (ii) fertilizer N use efficiency by maize, considering agronomic, recovery, and physiological N efficiencies, and (iii) the uptake of nutrients other than N.Both laboratory- and field-based investigations were conducted. Laboratory experiments were carried out to identify and rank the factors influencing denitrification, NH 3 volatilization, and bypass flow. Field experiments were conducted to test various management options.It was found that the critical soil moisture content for denitrification to commence was 60% of the water holding capacity (WHC), but substantial denitrification occurred at ≥80% WHC. Denitrification rate depended primarily on soil moisture content and available C. The amount of N lost through denitrification was determined by both the rate and duration of denitrification. From the laboratory investigations it was confirmed that NH 3 volatilization depended primarily on soil pH and fertilizer properties. It was also found that Kenya Vertisols have pH ranging between 5.5 and 9.1, indicating different potentials for NH 3 volatilization. Incorporating fertilizer materials within the 0-5 cm soil layer significantly reduced NH 3 -N losses.Nitrate-N was the main N-form in which N was recovered in the bypass flow, and the amount of N recovered increased with increasing rate of N0 3 -N application. NH 4 -N treatment had no effect on N loss through bypass flow. The results showed that bypass flow can be an important avenue of N0 3 -N loss from Vertisols especially if applied early in the season when the characteristic cracks of Vertisols have not closed.Drains, 40 and 60 cm deep, led to deeper rooting depth and higher yields of maize than the 0 and 20 cm deep drains. Besides, the uptake of N, P, and K was higher on drained than undrained plots. The late maturing hybrid H614 was superior to early maturing H511 in terms of N uptake and nitrogen use efficiency. It is recommended that 40 cm deep drains with inter-drain spacing of 15 - 20 metres be provided as prerequisite step in the management of Vertisols for maize production.