Response of soybean and cowpea to sources of 'starter' nitrogen in a low-nitrogen savanna soil

Abstract

Availability of nitrogen (N) to crops in most soils in sub-saharan Africa is low. However, grain legumes such as soybean (Glycine max L. Merr) and cowpea (Vigna unguiculata L. Walp) can fix atmospheric nitrogen to meet their N requirement when soil N is not adequate. For proper crop establishment and initiation of the biological nitrogen fixation process, some amount of N must be available to the crop at the early stages of growth.
In very low N soils, small amounts of N (referred to as 'starter' N) can be supplied to the crop for this purpose. This study examined the response of
soybean and cowpea to three sources of 'starter' N in a low-fertility savanna soil under greenhouse conditions for seven weeks.
The 'starter' N sources were potassium nitrate (KNO), ammonium 3 sulphate ((NH4)2SO4) and urea (NH2CONH2). The treatments including a control (i.e., without N addition) were replicated three times and arranged into a completely randomized design.
Growth parameters, yield attributes, and number and mass of nodules were determined. Results showed that soybean had few or no nodules but cowpea had relatively large number of nodules per plant which differed significantly among the N sources. The shoot and root dry matter yields were improved by the addition of 'starter' N. Both crops accumulated more N in their shoots when supplied with KNO3 than with the other sources. In addition, supplying 'starter' N as KNO3 increased P accumulation in cowpea shoots significantly. The study concluded that addition of small amounts of KNO3 as 'starter' N has the potential to improve the growth of soybean and cowpea in this savanna soil but further studies are necessary.