Use efficiency of fertiliser nitrogen on vertisols in Kenya

The present study included laboratory experiments to identify and rank the factors that influence the N losses, and field trials to examine the interactive effects of drain depth, N source and time of fertiliser N application on yields, nutrient use efficiency and fertiliser N losses in Vertisols in Kenya. All together five soils were studied in the laboratory, and two in the field. In the denitrification study the effects of soil moisture level and addition of glicose were investigated in a Vertisol and a Phaoezem. The Phaoezem had higher denitrification rates than the vertisol which is attributed to the higher organic C content of the former. There was no denitrification at moisture levels below 60% of the water holding capacity, which is therefore considered critical for denitrification. Addition of glucose resulted in higher denitrification rates, which is ascribed to a decrease in 0 subscript of 3 supply to soil microsites. The effect of pH and fertiliser N source on NH subscript of 3 volatilistation was investigated using soils varying in pH, and ammonium sulphate, ammonium nitrate and control as variables. Volatilisation increased with soil pH and was investigated using soils varying in pH, and ammonium sulphate, ammonium nitrate and control as variables. Volatilisation increased with soil pH and was extremely low for the soils with pH 6, and was equal for both N sources when expressed as fraction of NH subscript 4-N applied. On vertisols with pH 7 it is safer to use NO subscript of 3 fertilisers, provided denitrification and bypass flow risks are considered. Effects of fertiliser N source and rate on N losses in bypass flow were investigated after the onset of simulated rains with an intensity of 30 mm/hr. Two rainstorms were applied per sample, before and after fertiliser application. NH subscript of 4 and NO subscript of 3 in the bypass flow following the 2nd rainstorm were determined. Only NO subscript of 3 was lost in the bypass flow and no NH subscript of 4. The fraction of fertiliser NO subscript of 3-N recovered in the bypass flow almost constant (11-13%)for all application rates. The treatments of the field trials comprised three draft depths, two N sources, and three times of N application, and maize tybrid H511 was used as rest crop. In fields without drains yields, harvest index and fertiliser N recovery were lower than in fields with drains. Only in the fields without drains, yields and fertiliser N recovery were lower with NO subscript of 3 than with NH subscript of 4 and application of NO subscript of 3 at planting resulted in lower yields than the other treatments. These effects were attributed mainly to denitrification and partly to bypass losses.