The effect of no-tillage and mulch on soil properties and tuber yield of cassava was studied over a five year period on an infertile, add Ultisol in south-eastern Nigeria. Mulch was applied at a rate of 12 t ha' 1 as a split application once at planting and once six months after planting. No chemical fertilizers were used during the trial. Bulk density and proportion of micropores (pore radius, r, <1.4 ftm) were greater, and proportion of macropores (r>14.4 j»m) lower, with notillage. Water infiltration was increased by mulching, being greatest in tilled, mulched plots. Soil chemical properties were not significantly affected by tillage system, although levels of exchangeable calcium, magnesium and potassium were higher and acid saturation lower with mulching. Soil physical and chemical properties were best with tillage and mulching, and poorest when neither tillage nor mulch was used. Cassava tuber yield was unaffected by tillage system but increased by mulching. N. R. Hulugalle, R. Lai y M. Gichuru: Efecto de cinco anos sin arado con cobertura sobre las propiedades del suelo y el rendimiento de tubirculos de mandioca en un ultisol acido en al Sudeste de Nigeria. R E S U M E NSe estudio el efecto del no arado y la cobertura sobre las propiedades del suelo y el rendimiento de tubeVculos de mandioca a lo largo de un peri'odo de cinco anos en un ultisol acido e infertil en el sudeste de Nigeria. Se aplico material de cobertura en la proportion de 12 t ha" 1 en aplicacion repartida, una vez al plantar y una vez a los seis meses de la plantation. No se usaron fertilizantes qui'micos durante el ensayo. La densidad de volumen y proportion de microporos (radio de poro r < 1,4 iim) fueron mayores, y la proportion de macroporos (r > 14,1 ;um) menor, sin arado. La cobertura aumento la infiltration de agua, que fue mayor en las parcelas aradas y con cobertura. Las propiedades quimicas del suelo no se vieron afectadas de modo significativo por el sistema de arado, aunque los niveles de calcio, magnesio y potasio intercambiable fueron superiores y la saturation con acidos inferior con la cobertura. Las mejores propiedades fi'sicas y quimicas del suelo se obtuvieron con arado y cobertura, y peores cuando no se empleo ni el arado ni la cobertura. El rendimiento de tuberculos de mandioca no fue afectado por el sistema de arado pero aumento con la cobertura.

An experiment was established in 1986 to examine the contribution of Tephrosia candida and Cajanus cajan shrubs to improving the productivity of an acid soil. The main treatments were N levels (0 and 60kg ha -~) with subplots of maize/natural bush, maize/Tephrosia candida, maize/Cajanus cajan, maize + cassava/natural bush, maize + cassava/Tephrosia candida, and maize + cassava/Cajanus cajan.In 1988, all plots were cleared and maize uniformly planted to study the residual effects of the treatments. No residual effects of N application were observed. Tephrosia candida and Cajanus cajan increased surface soil organic carbon and total N levels over the natural bush. However, only Tephrosia candida plots produced improved maize grain and stover yield. Highly significant correlations were found between maize grain yield and earleaf N (r= 0.73"*), grain N (r = 0.51"*), and stover N (r = 0.54**) contents. These results suggest that Tephrosia candida increased N availability in the soil. Therefore, the shrub has potential for improving the productivity of acid soils under traditional systems, where N is limiting due to the absence of N2-fixing legumes in the natural bush fallow.

A major constraint to alley cropping is the competition of tree or shrub roots with those of companion food crops for available water and nutrients in the topsoil. Root distribution patterns of Acioa barteri, Alchornea cordifolia, Cassia siamea and Gmelina arborea grown on an acid Ultisol at Onne in the humid forest zone of southeastern Nigeria were examined to a depth of 120 cm and laterally to 200 cm from the tree trunk to study the suitability of the species for alley cropping. The four woody species have roots throughout the soil profile examined but differ in the concentration of roots both laterally and vertically. Alchornea cordifolia, Cassia siamea and Gmelina arborea, in spite of higher underground biomass production, most of their fine roots ( < 2 mm diameter) were in the top 20 cm of the soil. This soil layer had 73%, 76%, and 74% of the total Alchornea cordifolia, Cassia siamea, and Gmelina arborea fine roots in the profile examined, respectively. Such root systems would compete with food crops for nutrients and moisture in the surface soil. Alchornea cordifoIia and Gmelina arborea have many large woody roots in the surface soil which will make any tillage operation or seedbed preparation difficult. Acioa barteri in contrast, has the desirable rooting system with fewer fine roots in the surface soil (49%), and roots that are concentrated close to tree trunk and decrease markedly away from the tree base. In addition, Acioa barteri roots penetrate deeper soil horizons and can result in more efficient nutrient cycling from these layers, and reduced competition with shallow-rooted food crops. The rooting distribution patterns of Acioa barteri indicated that the species is a promising alley shrub in acid soils of the humid forest ecology. Therefore, consideration of rooting characteristics of potential tree/shrub species is recommended for the development of agroforestry systems such as alley cropping.

Making best use of available crop residues is an important component of integrated nutrient management. A field study was conducted over two seasons (1997 and 1998) in Kenya that examined use of wheat straw, soybean trash and nitrogen fertiliser as nutrient inputs for maize (Zea mays L.) production. The organic inputs were applied at the rate of 2 t ha-1 per season and urea was added at rates of 0, 20, 40, 80 and 100 kg N ha-1 in an incomplete factorial treatment structure that also included a complete control (no inputs) and 80 kg N ha-1 as urea without organic inputs. Maize grain yield ranged between 751 and 6836 kg ha-1 with lowest yields observed in the treatment receiving wheat straw alone and higher yields associated with soybean residue incorporation and during the second, wetter growing season. The 1998 crop benefited from more favourable rainfall, providing grain yield increase of 141% above control treatment as a result of combining 2 t ha-1 soybean trash and 100 kg N ha-1 urea. The generally high yields from soybean trash are explained in terms of its higher quality, faster decomposition and nutrient release compared to the lower quality wheat straw. A positive effect in increases of soil pH, C, N and P status as a result of cumulative use of crop residues was observed. Larger yields were obtained when organic and inorganic inputs were applied to soils, particularly when soil moistue was adequate and the organic inputs higher in mineralisable nutrients. Early indications of additional longer-term benefits through soil quality improvement were also measured. These findings suggest that better use may be made of crop residues than the burning following harvest as is currently practiced by many farmers in this area of western Kenya.

Alley cropping is increasingly becoming accepted as an appropriate technology with the potential to provide stable and sustainable food production in the tropics. However, only a few of the potential trees/shrubs have been tested. The performance of Calliandra calothyrsus (Meissn) as an alley cropping species was evaluated on an Oxic Paleustalf. The treatments were: prunings removal, prunings applicaton; and three N levels, 0,45, and 90 kg N ha -j , in a factorial arrangement. The cropping sequence was maize (main season) followed by cowpea (minor season). Nitrogen fertilizer treatments were applied to the maize crop only. Four annual prunings of Calliandra hedgerows produced a total of 6 t ha-~ of dry matter prunings containing about 200 kg ha-~ of N. Maize yields were increased by the application of prunings but no benefits were obtained by supplementing the prunings with inorganic N. An average maize grain yield of 3.1 t ha-~ per year was maintained without any chemical fertilizer input. However, without any prunings, maize yields were substantially increased by the application of inorganic N. Cowpea yield did not respond to application of prunings but plants grown adjacent to the hedgerows had reduced yield probably due to shading. Our results suggest that six rows (0.57 m inter-row spacing) between Calliandra hedgerows spaced at 4 m are optimum for this cowpea variety. The performance of Calliandra was comparable to that of Leucaena which has been widely shown to be effective in alley cropping systems of the region.