Effects of Eucalyptus Plantation on Soil Physico-Chemical Properties in Thiririka Sub-Catchment, Kiambu County, Kenya.
Mensah, Albert Kobina
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Trees are very important in Thiririka sub-catchment of Kiambu County in Kenya for reasons such as income generation, soil and water conservation, biodiversity conservation, charcoal production, erosion control, among many others. Eucalyptus spp. (E. grandis, E. saligna and E.globulus) remain part of the dominant tree species planted in the Thiririka sub-catchment.Although these species are planted in various spatial patterns in the sub-catchment to meet the demand for fuel wood, building construction, timber, electricity poles, water conservation,charcoal production and erosion control, and cushion farmers when the markets for their agricultural produce fail or are low, reports such as drying up of water courses, affecting the soil physico-chemical properties, tendency to deplete soil nutrients and fertility, suppression of other vegetation, reduction of forest biodiversity and reducing crop yield in agro-forestry systems havebeen made. Although a number of reports have been made in other countries concerning the negative impact of Eucalyptus spp. plantation on edaphic characteristics of soils, few studies have been done in Kenya on the implications of Eucalyptus spp. on the soil physico-chemical properties. This study was therefore, carried out to assess the effects of Eucalyptus spp.plantations on selected soil physical and chemical properties in the Thiririka sub-catchment. Two experimental plots (monoculture Eucalyptus spp. plantation and native forest of mixed vegetation) of 100 m x 100 m (1 ha) each were used for the study. Each experimental plot was sub-divided into five sub-plots of 20 m x 20 m for soil and litter sampling. All soil samples were collected at a depth of 0-20 cm using screw soil auger. Soil analyses were done using standard methods. Statistical analysis of the data collected was done using SPSS 17.0 statistical software and Microsoft Excel 2010. The results from the Eucalyptus spp. plantation were compared to those from the native forest within the same catchment and conclusions regarding the soil fertility level were drawn based on internationally acceptable standards. The results obtained in the study indicated that Eucalyptus spp. plantation significantly affects the soil moisture (26.6%). Soil bulk density under Eucalyptus spp. plantation (1.16 g cm-3) was within acceptable range. Also, the results showed that cultivation of Eucalyptus spp. significantly lowered the soil pH (4.8), leading to a significant decline in soil total nitrogen (0.09 %) and soil total organic carbon (0.83 %) concentrations. Decomposition of the litter of Eucalyptus spp. also caused increase in concentration of soil exchangeable acidity (0.32 c mol Kg-1), soil exchangeable sodium (0.52 c mol Kg-1), Fe concentration (95.28 mg kg-1), immobilization of soil available phosphorus (concentration of P was 23.2 mg kg-1), rendering it unavailable for plant use.However, in the study, no significant differences were found between the Eucalyptus spp.plantation and the control native forest in terms of soil exchangeable Mg, Ca and K, cation exchange capacity and effective cation exchange capacity although, concentrations of the exchangeable Mg, Ca and K and effective cation exchange capacity were low, which possibly contributed to the increased soil acidity, exchangeable acidity and Mn and Fe concentrations observed in the soils under the Eucalyptus sp. plantation. Furthermore, the Eucalyptus spp. plant litter fall concentrations of total N (1.05 %) and total P (0.15 %) were lower compared with the native forest, where concentrations of total N and total P were 2.11 and 0.17 %, respectively. The study demonstrated that sole cultivation of Eucalyptus spp. had the tendency to lower soil fertility and so it is advisable to interplant Eucalyptus spp. with other leguminous species, in addition to good agronomic practices such as nitrogenous and phosphate fertilizer application to replenish loss of the essential soil nutrients such as N and P associated with Eucalyptus spp.planting and also to ensure sustainability of Eucalyptus spp. cultivation on the soil resources.