Abundance and efficiency of garden pea nodulating rhizobia in central Kenya soils

Abstract

Garden pea is an important foreign exchange earner in the Kenyan economy and an excellent source of dietary proteins for its people. Its production is limited by the low soil nitrogen in major pea producing areas of Central Kenya. The high costs and the environmental concern over the use of nitrogen fertilizer prompted the need for the study. A field experiment was conducted at the University of Nairobi, Upper Kabete Campus Field station in the long and short rains seasons of 2007, to determine the effects of rhizobia inoculation and nitrogen fertilizer application on nodulation, growth and yield of garden pea. Response of pea to rhizobia inoculation is attained in soil with inefficient or low population of indigenous strains of rhizobia. A greenhouse experiment at the same site was therefore conducted in January 2008 to determine the abundance and efficiency of native garden pea nodulating rhizobia in Central Kenya soils. In the field experiment, two garden pea varieties, Plum and Ambassador, were tested. Each variety was either inoculated with a commercial strain of rhizobia or supplied with 30 or 60 kg N ha', and a control that had no treatment applied. In the greenhouse experiment, Pisum sativum cv Plum was used as the test legume. Soil samples were collected from 26 different sites in five garden pea growing areas of Central Kenya, and then subjected to a laboratory analysis. Each of the soils sampled then received the following treatments: 60 kg N ha', inoculation with a commercial strain of rhizobia and a control that had no treatment applied. In both experiments, treatments were laid out in a randomized complete block design (RCBD) with a factorial arrangement. Nodule numbers, nodule weights, biomass accumulated and yield of garden pea were determined. XIV In the field experiment, rhizobia inoculation enhanced pea emergence, nodule numbers, nodule dry mass and shoot dry matter. Variety Plum had more nodule numbers and weight compared to Ambassador when inoculated. Application of nitrogen fertilizer enhanced above ground biomass accumulated and photosynthetically active radiation (PAR) interception in garden pea. Plots supplied with 60 kg N ha' intercepted 18.7 % more PAR compared to the control. There was an interaction between variety Ambassador and 30 kg N ha" in increasing nodule numbers. Rhizobia inoculation and nitrogen fertilizer did not increase total fresh pod and seed yields of garden pea. Nodulation was depressed in the short rains compared to the long rains, due to moisture stress. In the greenhouse experiment, rhizobia inoculation enhanced nodule numbers, nodule weight and shoot biomass in some sites that had no history of cultivation. Some sites that had history of pea cultivation responded to inoculation possibly due to inefficiency of native rhizobia in nitrogen fixation. Sites that had low pH in Nyeri and Kabete responded the least to inoculation. Application of 60 kg N ha" depressed nodule formation in garden pea growing in soils from all sites. The numbers of active nodules in pea growing in untreated soil were high, and it translated into increased biomass accumulation. It was concluded that rhizobia inoculation and nitrogen fertilizer enhanced growth and nodulation but not yield of garden pea in Kabete soil. H-i-gh-rates of nitrogen fertilizer were found to depress nodule formation. It was also concluded that native garden pea rhizobia in Central Kenya soils are abundant and efficient in nitrogen fixation. It was therefore recommended that: a similar experiment involving several pea cultivars and a range of commercial strains of rhizobia be conducted. There is need to carry out a study to determine the effect of inoculation and liming on pea production in low pH soils.