Soil classification and phosphorus relationships as influenced by soil iron and manganese in soils of the Mbo Plain in Cameroon

Abstract

Decreasing yield of crops grown in soils of the Mbo Plain has been attributed to deficiencies of soil P. The objectives of this study were to classify these soils using the United States Soil Taxonomy System and to determine P, Fe, and Mn status at four sublocations (Nzock5, Mbomi4, Mbomi2 and Mbomi10) of the Mbo Plain. Two pedons were described at each sublocation. Phosphate rates (0, 25, 50, and 75 mg P kg^-1) and sources (triple superphosphate, TSP; dicalcium phosphate, DCP; and tricalcium phosphate, TCP) were evaluated in pots for rice (Oryza sativa L ) in Cameroon. Rice yields at varying rates of P using reagent grade monocalcium phosphate on soils of the Mbo Plain and two soils from the gulf coast of Texas was determined in the greenhouse at College Station. Phosphate fractionation and sorption isotherms were evaluated to determine P requirements of rice. Properties of the Mbo Plain soils were similar in all sublocations. The texture alternated between clay and clay loam. Base saturation was low and ranged from 4 to 52%. Soils are acidic with a water pH range of 3.8 to 5.0. Aluminum saturation percentage varied from 16 to 92%. Clay mineralogy was kaolinitic and ECEC ranged from 2.3 to 11.4 cmol kg^-1. Two taxonomic orders were identified. Pedons at Nzock5 were classified as inceptisols while pedons at Mbomi4, Mbomi2, and Mbomi10 were ultisols. The main P fractions were Al- and Fe-bound P. Sorption isotherms indicated 50 to 439 mg P kg^-1 were required to increase solution P to 0.1 mg L^-1. Triple superphosphate and DCP were suitable P sources for rice production in pots in the Mbo Plain at rates of 50 to 75 mg P kg^-1. Dry matter yield increased with P rate. Plant Ca and Mg were low for the Mbo Plain soils. Tissue concentrations were < 1.5 g Ca kg^-1 and < 1.0 g Mg kg^-1. Iron and Mn in plant tissue were present in apparent toxic concentrations (> 300 mg Fe kg^-1 and > 2500 mg Mn kg^-1). Extractable Mn correlated positively with plant Mn (r^2 = 0.99).