Pedological characteristics and implication on soil fertility of selected soils of Mbeya Region, Tanzania

Understanding the soil origin and its fertility in a given climatic conditions is important for efficient and sustainable utilization of soils. The composition of parent material determines the mineral nutrient content (Nube and Voortman, 2006), and sorption of nutrients, other chemicals and soil organic matter. A recent study in China demonstrated that available Fe and Zn in Xichang city soils differed depending on the nature of the parent rock (Zhang et al., 2012). Time is also essential in determining the stage of weathering, and hence soil properties. The soils of the surveyed areas are derived from volcanic tephra deposits of which are reported to be from relatively old (Upper Miocene to Lower Pleistocene Kitulo and Ngozi volcanoes) to most recent eruptions (Middle Pleistocene to Holocene Rungwe and Kyejo Volcano) (Fontijn et al., 2010). Information on the differences in chemical and other characteristics of the soils derived from these chronologically variable deposits of the area is limited. Mbeya region is among the four regions of Southern Highlands of Tanzania with high agricultural production, supplying major staple food in the country. However, crop yields are still low despite use of NPK fertilizers, contributing to food insecurity in Tanzania. Copper and Zn deficiencies were reported in this area’s volcanic soils about three decades ago, which was associated with nutrient mining (Kamasho, 1980). Yet, to date there is no follow-up studies on the status of these micronutrients in soils. Therefore, there is a need to determine the properties of soils of Mbeya region in relation to their origin and genesis. Mineral malnutrition is also wide spread and it is estimated that about 66% of world population is Fe deficient, over 30% is Zn deficient, 30% is iodine deficient and 15% is selenium deficient (White and Broadley, 2009). In Tanzania about 65% of children under 5 years of age are iron deficient (Fortifying African’s Future, 2009), 58.2% of pregnant women are anemic and 23% of the population are at risk of Zn deficiency (TFNC, 2009). These nutritional health problems can be attributed to low soil mineral content and/or availability to crops (Welch and Graham, 2002). Adequate levels of essential minerals in food crops to satisfy human health requirement is referred to as crop nutritive quality. One way to curb mineral malnutrition is fortification of some staple food with these essential minerals. However, food fortification cannot be expected to reach all populations deficient in essential micronutrients, especially in developing countries, and have challenge of low bioavailability (Hurrell, 1997; Mehasho, 2006). Therefore, there is a need to investigate, systematically, the relationship between soil nutrient contents (both macro- and micro-nutrients) and crop quality as might be related to the potential for agronomic biofortification to improve human nutrition and health. This study was therefore conducted as a contribution to efforts by national and international programs and strategies to attain not only high yields but also nutritional health for poverty alleviation. Therefore, the objective of this study was to determine and assess the pedologicalcharacteristics of soils and their influence on soil fertility and crop quality in selected physiographic units of Mbeya region. Specifically, i) to determine the morphological and physico-chemical characteristics of the soils ii) to determine the mineralogical composition of the soils iii) to determine the extent of weathering, and iv) to determine the implication of soil characteristics on nutrient availability, agricultural productivity and crop nutritive quality.

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