Experiments were conducted in both the greenhouse and the field on highly weathered (Orthic Ferralsol) soils to evaluate the effect of combining on-farm organic residues (chicken manure, farmyard manure, sugar bagasse) with Minjingu phosphate rock (MPR) on soil pH, extractable (available) and sorbed phosphorus (P), and to assess the cost benefit for maize production. The greenhouse study results indicated that, in the first eight weeks of incubation, soil pH increased linearly with increase in MPR rates and decreased thereafter. The available P also increased linearly. Chicken manure and sugar bagasse were most and least effective respectively in reducing P sorption, while there was a significant negative relationship between P adsorption maxima and extractable P. The field experiment data showed that the available P values increased significantly above the control in all the treatments where MPR and organic materials were applied separately or combined. Treatment effects on Langmuir sorption maxima (S max ) in the field were variable. An increase in S max with an increase in MPR rates was noted. However, there was a positive relationship between S max and available P (r = 0.52 to 0.69), suggesting the diversity of factors affecting the complex nature of P dynamics under field conditions. MPR applied alone in the first season gave a significant residual positive effect on maize grain yield for two additional seasons. The economic analysis revealed that a single application of chicken manure (2 t ha −1 ) combined with MPR at 60 kg P ha −1 gave the highest incremental net benefit equivalent to US$ 657 ha −1 during the three maize cropping seasons. I N T RO D U C T I O NPhosphorus (P) is an essential element for plant growth and development. However, among the nutrient elements, P is the most susceptible to fixation (sorption), particularly in acid or calcareous soils so that crops usually recover less P in the short term than other nutrients. Fixation occurs by sorption or precipitation reactions with Fe and or Al oxides in acid soils or Ca in calcareous soils (Chien and Hammond, 1988). Liming acid soils to reduce acidity is one way of overcoming P fixation; alternatively direct application of reactive phosphate rock (PR) or manure can reduce P fixation, or large quantities of P fertilizer can satisfy P fixation sites and thus increase

Lepidopteran stem borers are the key pests of maize in Sub-Saharan Africa. In the lowland tropics, dry mid-altitude, dry transitional and the moist mid-altitude zones of Kenya, the invasive crambid Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) causes up to 73% yield loss. The International Centre of Insect Physiology and Ecology (ICIPE) started a biological control (BC) program in 1991 to control stem borers in subsistence agriculture in Africa with emphasis on classical BC of C. partellus. The project released the braconid larval parasitoid Cotesia fl avipes Cameron (Hymenoptera: Braconidae) in 1993 in coastal Kenya, where it got established and spread to other regions. This study assesses the economic impact of the introduced parasitoid. Temporal data on percentage parasitism by the introduced parasitoid and on stem borer density were collected between 1995 and 2004. Socioeconomic data was collected through administration of questionnaires to 300 farmers. Economic impact of the project was calculated as the value of the yield loss abated by the parasitoid based on a model of expected stem borer density and parasitism level. Average annual parasitism increased linearly from the time of introduction to reach 20% parasitism by 2004. The net reduction in total stem borer density over the last 10 years was 33.7%, thus abating 47.3% of yield loss. The region will accumulate a net present value of US $ 183 million in economic benefi ts in 20 years since release of the parasitoid. Introduction of other parasitoid species targeting the egg and pupal stages of the stem borer life cycle stages would be required for biological control to push yield loss by stem borers to an insignifi cant level. Résumé. Contrôle biologique des foreurs des céréales au Kenya : une approche économique. Les lépidoptères foreurs de graminées sont des ravageurs importants du maïs en Afrique subsaharienne. Dans les zones de faibles et moyennes altitudes du Kenya, le foreur exotique Chilo partellus (Swinhoe) (Lepidoptera : Crambidae) a causé jusqu'à 73% de perte de rendement. Le Centre international pour l'étude de la physiologie et de l'écologie des insectes (ICIPE, son abréviation anglo-saxonne) a initié en 1991 un programme de lutte biologique classique pour lutter contre ce ravageur. Ce programme a permis la libération en 1993 d'un parasitoïde braconide, Cotesia fl avipes Cameron (Hymenoptera : Braconidae), dans la région côtière du Kenya, où il s'est établi et à partir de laquelle il a colonisé d'autres régions. Cette étude détermine l'impact économique de l'introduction de ce parasitoïde. Des données temporelles sur le pourcentage de parasitisme et sur la densité de foreurs ont été collectées entre 1995 et 2004. Les données socio-économiques ont été obtenues à l'aide d'un questionnaire diffusé auprès de 300 fermiers. L'impact économique a été calculé à partir des données de perte de rendement inférées par un modèle d'estimation de la densité de foreurs et du taux de parasitisme. Le niveau moyen de parasitisme a augmenté linéairement au ...

This paper provides the economic evaluation of different soil fertility replenishing technologies (use of inorganic fertilizers, organic manure, and rhizobium inoculant) that were tested during field studies and recommended to groundnut farmers. Data on soil fertility technologies used by households, groundnut yields, and resource use and farm and farmers characteristics were collected through administration of a questionnaire to a sample of 332 farmers from three districts of western Kenya. The data was analyzed to determine whether adoption of the technologies would increase household incomes and production efficiency. Benefits and costs of each technology were computed through the use of budgets. Technical and allocative inefficiencies are investigated by fitting a Cobb-Douglas production function. The technical efficiency of the farmers varied between 0.56 and 0.69 while labor allocative efficiency varied between 0.81 and 0.93. Farmers applying organic fertilizers only were technically more efficient but had lower potential yield compared to farmers who applied inorganic or a combination of organic and inorganic fertilizers. Use of inorganic fertilizers lead to a benefit cost ratio of up to 3:1. Organic manure had the lowest benefit-cost ratio (2.2:1) even when compared with that obtained when farmers did not apply any fertility replenishing input resulting from high cost of labor required to use this technology. There is a high potential for farmers to increase their groundnut yields and incomes by improving on production efficiency and by fertilizing their groundnut farms.

In western Kenya, severe nutrient depletion, especially that of nitrogen and phosphorus, has drastically reduced crop yields over the past two to three decades. The potential effects of P application, with a single direct application of Minjingu phosphate rock (MPR) at 20, 40 and 60 kg P ha −1 , and fallow biomass in terms of nutrient replenishment on maize and bean yields were investigated on N and P deficient soils of western Kenya for six cropping seasons (three years). The agro-forestry shrubs tested were one season (six months) Crotalaria grahamiana and Tephrosia vogelii followed by maize monocrop. These fallow species were intercropped with beans in the 2000 long rains season to provide a food benefit to the farmer. Soil samples were taken at the end of each season and analyzed for soil pH and extractable P. The results showed that soil available P increased significantly following application of MPR. Significant increases in bean yields were obtained when 60 kg P ha −1 as MPR were applied, and contributed to a 260 % increase above the control. Significant maize grain yield increases were obtained when MPR was applied alone or in combination with fallow biomass as compared with treatments with either no external nutrient addition (control) or with fallow biomass alone in all seasons. The 60 kg P ha −1 MPR rate gave the highest cumulative maize grain yields (9.6 t ha −1 ) over the five consecutive maize growing seasons, followed by 40 kg P ha −1 (8.8 t ha −1 ). The residual benefits of MPR at modest rates of application (60 kg P ha −1 ) were found to persist in the soil for only three cropping seasons. Thereafter, there was a steady decline in soil chemical properties (pH and available P), grain yields and net benefits. The study demonstrated the need for frequent additions of P especially in the fourth consecutive season to ensure sustained availability of P, favorable pH, and increased crop yields and net benefits on the nutrient-depleted soils of western Kenya. I N T RO D U C T I O NIn the highly populated areas of western Kenya, where continuous cropping with minimal external nutrient addition has been practiced for decades, nutrient depletion of soil is widespread and seasonal grain yields of the staple maize crop rarely exceed 1 t ha −1 . Phosphorus deficiency is of primary concern: more than 90 % of farm soils tested had less than 5 mg P kg −1 of bicarbonate-extractable P (Lijzenga, 1998). As a