Introduction
Wetland soils, otherwise designated hydromorphic soils, may be defined as soils whose properties are influenced by the presence, either seasonally or perennially of standing water within their profiles (Eshett, John, Omueti and Juo 1986),also considered soils, which are water saturated during most of the months (Onyekwere, Akpan-Idiok, Amalu, Asawalam and Eze, 2001). Wetland soils occur commonly in swamps, floodplains, inland valleys, and other areas of low topography and are usually underlain by impervious parent material which prevents or retards infiltration of water emanating from rainfall, irrigation, river inundation, surface run-off or seepage from surrounding highlands.
Wetland cultivation is now being emphasized (Okusami and Rust, 1992; Ogban and Babalola, 2003, 2009) to supplement the uplands where soil productivity and crop yields have declined due to severe accelerated erosion and degradation (Lal, 1988), accentuated by farmers mining the physical, chemical and biological fertility of their soils (IITA, 1990).
The wetlands are reputed to produce over 90% of crops grown on uplands, (Ogban, Effiong, Obi and Ibia, 2011) and are therefore as important as the upland farmlands in that they can be used in the wet season (mainly rice), dry season (cassava, yams, cocoyams, tomato and vegetables), and all year round for tree crop production. The wetlands thus forms an important component of the ecosystem in the State, therefore justifying the need to study them including their natural fertility with a view to recommending appropriate management options for crop production and putting them to other sustainable uses. Their fertility is important as nutrient depletion is one of the major constraints in tropical and sub-Saharan soils and P is one of the nutrients of concern or interest in agro-ecologies.
Phosphorus is one of major essential nutrient elements required by plants. It is the second most deficient plant nutrient element as more than ninety percent (90%) of the world soil require moderate to high phosphorus levels for optimum crop growth and development (Rashid, 1994). Phosphorus plays a vital role in energy transformation and photosynthesis. The quality of fruits, forages, vegetables and grain crops as well as disease resistance of crops are enhanced under adequate phosphorus availability (Sanyal and De Datta, 1991).
The major sources of phosphorus in the soil are the parent materials, inorganic fertilizers, plant and animal residues (Agbede, 2009). The availability of phosphorus to plants in soils is usually linked to the relative abundance of various forms of phosphorus, such as total phosphorus, organic phosphorus, inorganic phosphorus and available phosphorus.
Widespread deficiencies of phosphorus have been reported (Nnoke, 1980; Dyboh, 1999) as a major constraint to crop production in Southeastern Nigeria due to over cultivation, low total P and insufficient replenishment of lost P from soils. A major loss leading to phosphorus deficiency in these soils was attributed to phosphorus fixation by sesquioxides. According to Osodeke and Kamalu (1992), Ibia and Udo (1993), Sharawat, Abekoe, and Diatta (2001), Osodeke (2000), Abekoe and Sharawat (2001), (2003) and Osodeke and Ubah, (2005), highly weathered soils of the humid tropics rich in 1:1 clay minerals (kaolinitic clays) and high contents of sesquioxides are not only low in phosphorus but the applied soluble phosphorus is converted into forms not readily available to crops and as such limit crop production and productivity. This immobilization often described as fixation, precipitation, sorption or adsorption depends on the amount of phosphorus added, the nature of the fixation product and on the soil physical and chemical properties (Osodeke, 2000).
There is inadequate information on the phosphorus status of wetland soils in Akwa Ibom State. Therefore, the study seeks to investigate the phosphorus status of three wetland types in Akwa Ibom State.
The following were the specific objectives:
- To determine some physical and chemical properties of three wetland types.
- To assess the levels of total, organic, inorganic and available phosphorus in three wetland types.
- To determine the phosphorus fixing capacity of three wetland types.
- To assess the relationship between physical and chemical characteristics and phosphorus forms in the soil.
