Effect of Allium sativum (garlic) extract on the growth and nodulation of cowpea (Vigna unguiculata) and groundnut (Arachis hypogea)

It has been demonstrated that symbiotic N2 fixation enhances soil fertility and productivity as well as increases carbon sequestration and nutrient conservation. Green house experiments were conducted to ascertain the effect of garlic extract on the nodulation of cowpea and groundnut plants. Different concentrations of the extract (20, 40, 60 and 80%), were applied to the plant at 2 weeks after planting. Both legumes were adversely affected by the garlic extract, but the effect was more pronounced on the groundnut plants. Before application of the garlic extract to the soil, the soil pH was 6.5, phosphorus 8.3 mol/kg, calcium 2.51 mol/kg, potassium 0.45 mol/kg, and nitrogen 0.295 mol/kg, and these make the soil fertile enough for nodulation. However, after garlic extract application, there was reduced nodulation in the roots and a marked drop in plant height, leaf area and root development in comparison to the control. This adverse effect was more with increasing concentration of the extract.  It is recommended, that land for cowpea or groundnut product remains free of garlic and members of the families Allium and Cyperus plants. They should not be left in the soil or turned over in it, during land clearing and should be grown in inter-crop.  This is because their exudates during decomposition or growth could have inhibitory effects on the growth and nodulation of legumes.    Keywords: Garlic extract, nodulation, legumes, nitrogen fixation.