Fate of applied nitrogen in along-term maize-bean croppling system in Kenya.
2005
Kibunja, C.N.(Kenya Agricultural Research Institute, Nairobi (Kenya). National Agricultural Research Laboratories)
Low soil fertility has been recognized as one of the major bio-physical constraints affecting agriculture in sub-sahara Africw. Nitrogen in particular is the most limiting element in crop production leading to low and declining yields as a result of continous crop harvest, leaching and soil erosion. One way of reversing this situation is by returning nutrients to the soil through ues of chemical and organic fertilizer. Dry matter yields, plant N uptake and 15N recovery was determined at the end of each season. Results showed that the 20 cm plough layer had a substantial amount of pre-season mineral N (10-25 mg N kg -I soil) that decreased as the crop matured. A bulge of mineral N (40-60 mg N kg .1 soil) occurred at depths below 100 cm. Combined use of organic and inorganic fertilizers gave the highest grain yields while the N content in the grain, stover and roots was highest in the plots supplied with chemical fertilizers. The fraction ofN in the grain derived from labelled fertilizer N (ndff %) was low in the range of 11-15%. The stover contained 12-19% while the roots took up 11-18%. Only about 10% was found in the top 20 cm soil layer at the end of the season while the rest (40%) was unaccounted for. The fertilizer use efficiency for all treatments was low (20-32%). This cropping system is not sustainable as it leads to a substantial loss of mineral N through leaching. Rotation of annual crops with deep¬rooted short fallows is a promising way of capturing sub-soil N.
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