IOSR Journal of Agriculture and Veterinary Science (IOSR-JAVS) e-ISSN: 2319-2380, p-ISSN: 2319-2372. Volume 3, Issue 2 (May. - Jun. 2013), PP 64-71 www.iosrjournals.org www.iosrjournals.org 64 | Page Alteration of soil carbon and pH by selected common tree species on farms in Eastern Highlands of Kenya *Ndlovu E 1 , Ngamau K 1 , Muthuri CW 1,2 , Muriuki JK 2 1 Departmement of Horticulture, Jomo Kenyatta University of Agriculture and Technology, P.O Box 62000 Nairobi - Kenya 2 International Center for Agroforestry Research, P.O Box 30677-0100 Nairobi-Kenya Abstract: Lack of knowledge and evidence to farmers on the benefits of the tree and soil interactions which may halt land degradation on farms often results in the removal of the trees that grow on farms as scattered parklands. An on-farm study was carried out at four sites in Central highlands of Kenya to investigate the effects of selected common tree species on soil organic carbon and pH. Sampling of the top soil was done under the canopies of selected common tree species and from control plots (away from trees). Results revealed that Croton macrostachyus Hochst. Ex Delile plots had a significantly higher (P < 0.01) total organic carbon mean of 2.85% compared to 2.04% obtained in plots under Grevillea robusta A. Cunn. at Kyeni site. Senna spectabilis (DC.) H.S. Irwin & Barneby also had significantly higher (p < 0.05) total organic carbon mean of 2.89% compared to 2.33% obtained in control plots at Kanwaa site. In soil pH, control plots had significantly lower (P < 0.01) mean soil pH values than the plots under selected the trees at all the sites. Among the tree species only G. robusta revealed significantly lower (P < 0.01) soil pH than the C. macrostachyus, Cordia africana and control plots at Kyeni site. The study suggested that S. Specatabilis helps in increasing soil carbon and consequently soil fertility. S. specatabilis, C. africana, C. macrostachyus and Erythrina abyssinicca also showed some positive influence in reducing soil acidification on farms. It is recommended that an increase in the number of trees on farms may improve soil fertility and reduce land degradation. Key words: Agroforestry, Land degradation, Soil acidification, Soil fertility. Abbreviations: AEZ= Agro Ecological Zones; DBH= Diameter at Breast Height; LH= Lower Highlands; MASL= Meters above Sea Level; SOC= Soil Organic Carbon; TOC= Total Organic carbon. I. Introduction A significant increase in land degradation on farms in the Central highlands of Kenya due to continuous cultivation and low soil fertility replenishment has been widely reported (Woomer et al., 1994; Sanchez et al., 1997; Gitari and Friesen, 2001; Solomon et al., 2007). Land degradation refers to lowering of the land productive capacity through processes such as soil erosion, loss of soil fertility, soil salinity and acidification (Young, 1997). Soil fertility which is the capacity of the soils to provide essential elements for plant growth is one of the major limiting factors in crop production (Foth and Ellis, 1997). Soil organic carbon is one of the major factors influencing fertility and productivity of soils (Kucharik et al., 2001; USDA, 2002). Low soil fertility and poor crop husbandry among others have led to declining yields in Kenya (Murithi et al., 1994). The land degradation situation in Kenya has been exacerbated by the recent increase in human populations and consequent demand for farmland and wood products (Sekubyu and Mosango, 2012). The use of farm inputs like inorganic fertilizers and lime by smallholder farmers to replenish soil fertility is severely constrained by prohibitive costs associated with such solutions (Akinnifesi et al., 2007). Thus assessment and adoption of low-cost and sustainable technologies to address problems associated with land degradation is very crucial. Conservation agriculture complemented with agroforestry is now emerging as the promising land use option to sustain agricultural productivity and livelihoods of farmers (Syampunani et al., 2010). The integration of trees in farming systems provides environmental services and off-farm products that are either traded or used to confer multiple livelihood benefits; this can alleviate malnutrition, hunger and poverty in resource poor smallholder farmers (Leakey, 2010). One benefit of increasing trees on farms is an increase in soil organic carbon (SOC) pools which is a good indication of soil fertility (Konare et al., 2010). Increase in soil organic matter (SOM) and subsequently, SOC has the potential to improve several soil properties like structure, water retention capacity, nutrient availability, pH and microbial activity (Rashidi and Seilsepour, 2009; Jangra et al., 2010; Ch’ng et al, 2011; Sebukyu and Mosango, 2012). With such a rich soil status, cropping period can be prolonged; intensive cropping and higher crop yields are possible (Mosango, 1999; Palm et al., 2001; Sileshi, 2009; ICRAF, 2009). Research in agroforestry has typically focused on fast-growing tree species planted at high density which capture most of the available resources neglecting slow growing indigenous and probably adapted tree species that grow on farms (Ong and Leakey, 1999). Worldwide, much research has been focused on soil status