Abstract
Leguminous plants are known to require phosphorus fertilizers and inoculation with nitrogen fixing rhizobia for optimum yield but other nutrients may also be lacking. In this study, the most limiting nutrients for legume growth were determined in soils where the crops had not responded to P and rhizobial inoculation in field trials, using the double pot technique. Soils were collected from 17 farmers' fields in West Kenya, Northern Nigeria, Eastern and Southern Rwanda, South-west and North-west Sierra Leone. Plant growth and mean biomass were measured on soils to which a full nutrient solution, containing phosphorus (P), potassium (K), magnesium (Mg), sulfur (S) and micronutrients (MN) were added, and which were compared to a control (no nutrient added), and individual omissions of each nutrient. The relationship between soil properties and nutrient deficiencies was explored. Nutrient limitations were found to differ between soils, both within and across countries. Generally, each soil was potentially deficient in at least one nutrient, with K, P, Mg, MN and S emerging as most limiting in 88, 65, 59, 18, and 12% of tested soils, respectively. While K was the most limiting nutrient in soils from Kenya and Rwanda, P was most limiting in soils from Nigeria. P and K were equally limiting in soils from Sierra Leone. Mg was found limiting in two soils from Kenya and three soils from Rwanda and one soil each in Nigeria and Sierra Leone. Micronutrients were found to be limiting in one soil from Nigeria and one soil from Rwanda. Estimates of nutrient deficiency using growth and mean biomass were found to be correlated with each other although the latter proved to be a more sensitive measure of deficiency. With few exceptions, the relation between soil parameters and nutrient deficiencies was weak and there were no significant relations between deficiency of specific nutrients and the soil content of these elements. Although our results cannot be translated directly to the field, they confirm that individual and multiple nutrient deficiencies were common in these “non-responsive” soils and may have contributed to reported low yields. This highlights the need for balanced nutrition in legume production in SSA.
Highlights
Grain legumes, cowpea (Vigna unguiculata (L.) Walp), groundnut (Arachis hypogaea L.), soybean
P deficient plants were observed across soils in Kenya and Sierra Leone with Control and -P treatments
K deficiency symptoms were evident at an early growth stage (10 to 12 days after emergence (DAE)) in the Control in Rwanda and across experimental soils in Kenya, Rwanda and Sierra Leone with the -K treatments
Summary
Cowpea (Vigna unguiculata (L.) Walp), groundnut (Arachis hypogaea L.), soybean Integrated soil fertility management (ISFM) interventions, including use of P fertilizers, animal manure, composts, inoculants and seeds of improved varieties can increase productivity of grain legumes (Giller et al, 2013) but yield responses measured in field experiments are known to vary considerably (Ronner et al, 2016; Van Heerwaarden et al, 2018). Such variation may partly reflect the effect of additional nutrient constraints, something which has been observed explicitly in crops such as maize (Franke et al, 2008). The transferability of results from such studies to natural conditions may be limited, because the sampled soil may not fully represent the field conditions, the technique has the advantage of being relatively quick and inexpensive, allowing soil limiting nutrients to be examined individually, with sufficient replication and in absence of complicating factors such as water stress and the occurrence of pests and diseases (Gibson et al, 1999)
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