The effect of DDT contamination on the productivity of a cultivated forest soil in the sub-humid tropics.
Seed yields of cowpea (Vigna unguiculata) were assessed in each of two annual growing seasons over the 4-year period 1973 to 1976 in experimental plots under continuous cultivation on land newly cleared from bush regrowth in Ibadan, southern Nigeria. No fertilizer was applied throughout the experimental period. One set of plots was sprayed weekly with DDT during both growing seasons of each year and one received DDT treatment only in the first growing season of each year. The yield from untreated controls was also monitored. Cowpea seed yields varied considerably from season to season and year to year in untreated plots due to differences in pest incidence and climatic conditions. The cowpea yield over the 4 years was considerably higher in the DDT treated plots but the decline of yield with time was faster than in untreated plots and by the seventh season of application DDT application did not significantly enhance cowpea yields. Maize was cultivated without pesticide protection during the first season of 1977 in all plots as an additional indicator of soil fertility before the plots were allowed to return to bush fallow. Crop production was lower in plots previously treated with DDT than in untreated plots, and although soil nutrients were depleted by the continous cultivation, there was no greater depletion in the plots with higher yields. It is suggested that the more rapid decline in fertility of plots with a history of DDT treatment may be associated with pesticide effects on the soil biota.