Changes in Rhizosphere Concentration of Mineral Elements as Affected by Differences in Root Uptake and Plant Growth of Five Cowpea Genotypes Grown in Mixed Culture and at Different Densities with Sorghum

Aims: To evaluate the effect of planting density and cropping systems on the changes in rhizosphere concentration and uptake of mineral elements of five cowpea genotypes (i.e. Bensogla, ITH98-46, Sanzie, TVu1509 and Omondaw).

Study Design: 3-factorial randomized complete block design.

Place and Duration of Study: Nietvoorbij (33º54S, 18º14E), Stellenbosch, South Africa during 2005 and 2006 summer seasons.

Methodology: A field experiment involving two cowpea plant densities (83,333 and 166,666 plants.ha-1), two cropping systems (monocropping and intercropping) and five cowpea genotypes (i.e. Bensogla, ITH98-46, Sanzie, TVu1509 and Omondaw).

Results: The data for 2005 and 2006 were similar, and therefore pooled for statistical analysis. The concentrations of P, K, S, Na, Cu, and Zn were lower in rhizosphere of cowpea relative to bulk soil, while those of Ca and Mg were greater in the rhizosphere compared with bulk soil. With sorghum, only K, S, and Na were lower in the rhizosphere, in contrast to P, Ca, Mg, Cu, and Zn, which were higher in the rhizosphere. These differences in mineral concentration were due to alteration in rhizosphere pH, which was increased by cowpea but unchanged by sorghum. The data also showed that high plant density (166,666 plants.ha-1) and mixed culture significantly decreased rhizosphere soil pH, resulting in low availability of P, K, Ca, Mg, Na, S, Fe, Cu, Zn, Mn and B in the rhizosphere of cowpea and sorghum compared with low plant density (83,333 plants.ha-1) or monocropping. The results also showed significant differences in rhizosphere concentration of minerals between and among the five cowpea genotypes, with cv. Sanzie consistently indicating much lower levels of P and Ca as a result of higher root uptake, which was evidenced by the higher tissue content of P, K, Ca, Mg, Na, S, Fe, Zn, Mn and B in cv. Sanzie.