Project Topic On CONCRETIONARY NODULES AS NUTRIENTS SINKS AND THEIR EFFECT ON PHOSPHATE ADSORPTION IN A PLINTHIC HAPLUSTALF.
INTRODUCTION
A characteristic feature of weathered savannah soils is the abundance of iron (Fe), aluminium (Al) and manganese (Mn) oxides in the soil profiles deposited as nodules and concretions (Agbenin, 2003). The nature, distribution and abundance of these nodules and / or concretions significantly affect nutrient status and phosphate availability in the soil (Torrent, 1994; Agbenin, 2003). Zhang and Karathanasis,(1997), found that these concretions are reservoir for phosphorus (P) and other essential elements. Concretionary constituents of guinea savannah soils are yet to be researched fully to ascertain their potentials for nutrient retention especially phosphate sorption.
Plinthite (derived from Greek word plinthos which means stone) is defined as an Fe rich , humus poor mixture of clay with quartz and other diluent that, on exposure to repeated wetting and drying, changes irreversibly to an iron stone hard pan or an irregular soil aggregate (Soil Survey Staff, 2010). However, it can distinguished from other iron rich earth material that are not plinthites as other concretions do not irreversibly hardened upon exposure to oxygen rich environments. The irreversibly hard pan plinthite is generally referred to as petro-plinthite which is also known as ironstone. (Eswaran, et al., 1990; Soil Survey Staff, 2006). These plinthites are common in the alfsol with ustic moisture regime of northern Guinea savannah ecology according to (Yaro, 2005). Concretion is a mass or concentration of a chemical compound, such as calcium carbonate or iron oxide, in the form of a grain or nodule of varying size, shape, hardness and colour, found in soil or in rock. The term, concretion, is sometimes restricted to concentrations of chemical compounds having concentric fabric in soils (Rhoton et al., 1993). The main difference between a ferruginous nodule and concretion is the concentric internal fabric associated with the latter. Iron and Mn
concretions have been found in many soils, and especially in soils with restrictive internal drainage (Rhoton et al., 1991; Mc-Daniel et al., 1992). The effect of concretion accumulation on soil chemical properties could be significant at high concentrations, depending on the chemistry and mineralogy of the oxide components. In soils of the semi-arid tropics, the oxides, hydroxides and oxyhydroxides of Fe, Mn and Al are common. The best- known effect of the Fe-oxide surface is its high affinity towards phosphate retention.
The concretionary soils which comprise mostly lateritic ferruginous nodules are known to adsorb significant amounts of phosphate. Ferruginous nodules are abundant in concretionary soils. The nodules often have higher P content than the surrounding soils. They serve as effective sink for added soluble P (Zhang et al., 2005; Tiessen et al., 1991). Traces of Ca, Mg, P and Na are also noticed in the black and brown concretions thereby rendering those essential nutrients unavailable to plant. The trend in the formation of these concretion indicate that, the formation, development and accumulation of Fe-Mn concretion are dynamic and progressive translocation and transformation process of Fe and Mn in the soil profile (Manojlovic et al., 2007; Zhang and Karathanasis, 1997).
In acid soils, P sorption is generally attributed to hydrous oxides of Fe,Mn, and Al. Phosphorus sorption in acid soils was also considered to be a special case of precipitation, wherein Al or Fe remained as the constituent of the original phase while reacting with P by the use of residual force on the surface(Cornell, and Schwertmann, 2003).Lim and Jackson, (2003) considered sorption to prevail at low P concentrationsin soil solution, and precipitation at higher concentrations. Such a process of formation of new P compounds may be slow, and the attainment of equilibrium is likely to be disturbed by P uptake by plants, diffusion, and more importantly, by rapid adsorption of the soluble P by surface-reactive Fe and Al oxides. As a consequence, persistence of fertilizer reaction products in soils is unlikely (Sanyal and De Datta, 1991).
1.1 Problem Statement
The availability of P to crop in the savannah is more than just having P in the soil. It depends on the abundance of Fe and Al oxide and management factors that influence plant growth. The fate and efficiency of native and applied P is one of the biggest problems in arable crop production in the tropics. One problem is that fertilizer P can be largely fixed by oxides, hydro-oxides and oxy hydroxides of Fe, Mn, Al and clay minerals in acidic soils, which makes it less available or effectively unavailable to plants (Shen et al., 2011). Despite the high intensity of weathering and the abundance of free Fe oxides in savannah soils, their P sorption capacity is considered low (Agbenin, 1996; Abdu, 2009). Variable charge minerals are also major components of savannah soils that make P unavailable to plants because of the dominance of sesquioxides and low activity clay content especially the Oxisols, Ultisols and Alfisols.
This may seem to over simplify the problem of most savannah soils. There are, however, some soils with apparent P deficiency that have failed to respond to fertilizer P while others with apparent sufficient P level in the soil have responded to fertilizer P application (Mokwunye, 1995; Agbenin, 2003). This paradox has never been satisfactorily resolved for most savannah soils because sufficient attention has not been paid to the understanding of P retention mechanisms and the role of concretion and nodules on the soil‟s P retention and availability. Whereas, Agbenin, (2003) demonstrated the role of extractable Fe and Al oxides on P sorption in savannah soils while Tiessen et al. (1991) provided evidence that nodules/concretions are nutrients and P sink in some savannah soils of Ghana thereby curtailing crop response to fertilization.
1.2
Justification
With increasing demand of agricultural production, P is receiving more attention as non-renewable resource (Corddel andDangert, 2009) since P is a major limiting nutrient for agricultural and ecosystem productivity (Tiessen et al., 1990). One unique characteristic of P is its low availability due to its slow diffusion and high affinity for soil constituents. Maintaining a proper P supplying level at the root zone can enhance the efficiency of plant root to solubilize and acquire P from the rhizosphere. To avoid a future food related crises in West Africa, P deficiency and availability problems need to be addressed along with other nutrients, especially under intensive land use system. Demographic pressures have increased intensification of land use and the cultivation of marginal lands whose productivity even with inorganic P fertilization remains poor. Most of these lands are characterized by high pedogenic weathering with the abundance of Fe, Mn, and Al oxides deposited in the soil as nodules and concretions. Responses to fertilizer application in these soils can be described at best marginal with no significant increase in crop yield. More often than not, the main limitation to the productivity of the soil is the availability of P.
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