Materials and methods
Study site
This study was carried out at Egerton University in Kenya, situated approximately 200 km northwest of Nairobi (0°22′S, 35°55′E, 2240 m above mean sea level). Mean annual rainfall is 1150 mm, distributed unimodally from March to September. Mean monthly temperature during the period of the study was 17 °C, with a minimum of 8.5 °C in January 2000 and a maximum of 26.4 °C in March 2000. The study site had a uniform slope of 8% running in the northeast direction and the field had been under Rhodes grass (Chloris gayana) for more than 3 years.
The soil was haplic Phaeozem and was well drained, very deep with topsoil colour varying from very dark gray to very dark brown (5YR 3/1 to 7.5YR 3/2). The subsoil varied from dark brown to dark reddish brown (7.5YR 3/1 to 5YR 3/2–3/4). Texture of top and subsoil was dominantly clay loam. Structure of both topsoil and subsoil was weakly to moderately developed sub-angular blocky. Organic C was 2.4% in the topsoil and 0.4% in the subsoil. Cation exchange capacity (CEC) varied from 22.8 me/100 g in the topsoil to 19.9 me/100 g in the subsoil. The pH varied from 5.6 to 6.1 increasing with depth.