Soil Fertility Management Investigations on Benchmark Soils in the Humid Low Altitude Tropics of West Africa: Investigations on Egbeda Soil Series1
Authors: Kang B. T. , Donkoh F. , Moody K. 
Published: July 2010

Information on productivity and on suitable soil fertility management practices for sustained crop production for the major soils in the humid low altitude tropics of West Africa is still very limited. Investigations were therefore initiated on the Egbeda soil series (Oxic Paleustalf) from 1970 till 1975 in the forest zone of southern Nigeria, to determine its potential productivity and, also the effects of cultural factors on crop yield and soil properties.

During 1971 and 1972 the effects of four cultural factors each at two levels (with and without fertilizer application; with and without weeding; with and without disease and insect protections; and plant population densities of 26,600 and 53,200 plants/ha) were investigated using a 24 factorial design with four replications. A maize (Zea mays L.)‐maize rotation was used. From 1973 till 1975, only the fertilizer and weeding treatments each at two levels were continued in a 22 factorial design with four replications. A maize‐sweet potato (Ipomoea batatas L.) rotation was used. Fertilizer was applied at a rate of 120 N — 66 P — 30 K in kg/ha to each maize crop. Each sweet potato crop received 40 kg N/ha. Four kg Zn/ha was applied to the 1972 main season maize crop. Each of the maize and sweet potato crops were hand weeded twice.

Increasing maize plant density from 26,600 to 53,200 plants/ha significantly increased grain yield. The effect of crop protection on maize yield was inconsistent and varies with season. Fertilizer application and weeding significantly affected maize and sweet potato yields. The maize and sweet potato crops compete better with the weeds under high soil fertility conditions. Weeding appears to have more effect on sweet potato yield than fertilizer application. Maize and sweet potato yields without fertilizer application declined faster with no weeding than with weeding, partly due to nutrient competition from weeds. In the 5th cropping year without fertilizer application and weedings, maize grain yield was 450 kg/ha while sweet potato tuber yield was 4,900 kg/ha. In the same year with a weeding treatment only, maize yield was 2,500 kg/ha and sweet potato yield was 11,600 kg/ha, partly reflecting the high productivity of the soil. With fertilizer application and weeding, the maize and sweet potato yields were maintained at high levels at about 6,000 and ≥ 18,000 kg/ha respectively.

Maize ear leaf composition indicated that for continuous cropping N and P appear to be the limiting nutrients during the first 6 years of cropping after clearing the secondary forest. Under low fertility conditions, sweet potato appears to be more efficient than maize in utilizing the available plant nutrients. Weeds showed higher percentage of N, P, K, Ca, and Mn than maize. The weeds also removed substantial amounts of N and particularly K, Ca, and Mg.

After 4 years of application of N, P, and K, soil pH decreased from 5.80 to 5.30. The soil pH decrease was less with no weeding. Higher soil organic C content was observed with fertilizer application. Addition of 330 kg P/ha during a 4‐year period raised the level of extractable P from 11 to 70 ppm P.