Nutrient distribution in the profile of valley bottom soils cultivated to rice in Ghana

The consumption of rice has increased in Ghana recently, but supply cannot meet demand; hence, there is a large annual importation to make up for the deficit. In Ghana, rice is cultivated under three main systems, namely rain-fed upland conditions, irrigated conditions, and rain-fed lowland conditions. Due to unpredictable rainfall, production under rain-fed conditions has been very unstable. Fortunately, inland valley systems (IVs), which have been cited by many authors as the environment best suited for efficient and sustainable rice production, abound in this country. This study was therefore undertaken to determine the soil nutrient status, and to document a plant species distribution that is useful for nutrient cycling in the IVs of the Ashanti region of Ghana. The goal is to develop a sustainable sawah-rice production system.** Valley bottom soil samples for laboratory analyses were collected from standard soil profiles in areas under use for rice cultivation or capable of supporting rice. Results showed these soils to be slightly acidic (topsoil mean pH = 5.7) with low exchangeable acidity 0.10 cmol(+)/kg. Mean topsoil total carbon (C), nitrogen (N), and available phosphorus (P) (Bray 1) were 1.8%, 0.20%, and 4.4 mg/kg, respectively. Exchangeable cation cmol (+)/kg values of surface soils were 0.3 for potassium (K), 5.1 for calcium (Ca), 1.8 for magnesium (Mg), and 0.2 for sodium (Na), respectively. The concentrations of C, N, and most cations decreased down the profile. However, the overall fertility levels were well within the normal ranges for IVs of West Africa. The species dominating the valley bottoms were fairly similar in their carbon content. However, Manihot esculenta and Mallatus oppositifolium of the family Euphorbiaceae were higher than other species in N content. Species belonging to the family Euphorbiaceae were lower in P content (which ranged from 0.9 to 3.7 g/kg) while Physalis angulata and Manihot esculenta contained the highest (3.9 and 3.6 g/kg) P, respectively. The variable nutrient composition of these species has implications for nutrient cycling in the ecosystem. Knowledge of nutrient status of IVs will greatly assist in efforts at adopting the Asian type of sawah rice cultivation. If supported with appropriate farming practices that return crop residues to the soil and utilize inorganic fertilizer or organic manures, the inland valley ecology can contribute significantly to food security, especially of rice, and also conserve the environment.