Applying silicon in the form of metasilicic acid (H4SiO3) or silicic acid (H4SiO3) to Bradyrhizobium-infected,
hydroponically grown cowpea seedlings resulted in a significant (P[les ]0.05) increase in the number of nodules,
nodule dry matter, and nitrogen fixed on a per plant basis. Total dry matter of plants increased with silicon supply,
and the differences were significant (P[les ]0.05) at the higher silicon concentrations. Cowpea plants cultured in sand
were also assessed for their response to silicic acid. Nodule number and nodule mass increased with silicon supply
to sand cultured plants, though nitrogen fixation was unaltered. Although silicon is not essential for growth of
cowpea, it is important for nodule formation and nodule functioning in hydroponically grown plants.
Consequently, data collected and conclusions drawn from earlier glasshouse experiments, which have excluded
silicon from nutrient solutions, may be flawed. Future studies on nodulation and nitrogen fixation using legumes
in liquid culture must therefore include silicon as a nutrient element.