Sulfur and zinc levels as limiting factors to rice production in West Africa lowlands

A total of 172 soil samples from 85 locations within river flood plains and 201 samples from 78 locations within inland valley swamps were collected and analyzed to determine their sulfur (sulfate-S) and micronutrients (Zn, Fe, Mn, Cu, Ni) supplying capacities. The soils were observed to be very deficient in both sulfate-S and available Zn. Mean topsoil (0-15) cm sulfate-S levels were 3.41 mg kg(-1) for river flood plains and 4.88 mg kg(-1) for inland valley swamps. Even though mean topsoil available Zn levels were 1.23 and 1.56 mg kg(-1) for river flood plains and inland valley swamps, respectively, over 66% of West Africa lowlands had available Zn below the critical soil level of 0.83 mg kg(-1) necessary for rice cultivation. Observed levels of topsoil mean available Fe (163 and 220), Mn (66.6 and 57.6), Cu (3.38 and 2.49), and Ni (1.68 and 1.36) mg kg(-1) for river flood plains and inland valley swamps, respectively, were moderate across the sub-region. However, isolated areas within both lowlands showed very high levels of available Fe, more especially within the inland valley swamps. A correlation analysis with some selected nutrient parameters showed that topsoil sulfate-S correlated positively and significantly with total C (r = 0.690), available P (r = 0.939), eCEC (r = 0.867) and Clay (r = 0.859) for river flood plains but only with total C (r = 0.874), available P (r = 0.873) and eCEC (r = 0.612) for inland valley swamps. Available Zn showed a similar relationship with total C (r = 0.867 and 0.800) and available P (r = 0.690 and 0.850) but a negative correlation with eCEC (r = -0.675 and -0.544) for both river flood plains and inland valley swamps, respectively. Available Ni showed a similar relation to Zn. Available Mn significantly correlated positively with total C and available P but showed a negative correlation with pH for both lowland types. Nutrient availability and distribution were more influenced by agro-ecology within the inland valley swamps than the river flood plains. (C) 2000 Elsevier Science B.V. All rights reserved.