The nature of soil type is an important factor in the bioaccumulation of substances in plants. Application of nitrogen fertilizer is a common practice in vegetable farming. This study was therefore conducted to obtain information on the effect of soil nitrogen on the levels of oxalate, nitrate, cyanide, vitamins C and β-carotene (vitamin A precursor), and some mineral elements. Pot experiments were conducted to study the effect of soil nitrogen levels on the above substances including Fe, Mg, Zn, Cu, Ca, Na and K in Corchorus olitorius. Leaves of the vegetable were harvested at market maturity and fruiting and subjected to chemical analysis. Results obtained showed that the applied nitrogen fertilizer significantly (p < 0.05) elevated the nitrate content both at market maturity and fruiting. This is of particular interest because of its public health significance, nitrate being able to lead to human cancer. The cyanide, soluble and total oxalate contents were not significantly affected. The concentration of β-carotene increased significantly (p < 0.05) at market maturity, but not so at fruiting. Vitamin C content was reduced significantly (p < 0.05) at both stages of the plant development. There was significant (p < 0.05) reduction in Fe content at market maturity. Copper content was increased at market maturity but decreased at fruiting. The K content decreased significantly at both stages of plant development.
Key words: Corchorus olitorius, nitrogen level, antinutrients, toxic substances, micronutrients, market maturity, fruiting.
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