Abstract
The aim of this study was to assess the potential of commercial mycorrhizal inoculants and a rhizobial inoculant to improve soybean yield in Kenya. A promiscuous soybean variety was grown in a greenhouse pot study with two representative soils amended with either water-soluble mineral P or rock P to assess product performance. The performance of selected mycorrhizal inoculants combined with a rhizobial inoculant (Legumefix) was then assessed with farmer groups in three agroecological zones using a small-plot, randomized complete block design to assess soybean root colonization by mycorrhiza, nodulation, and plant biomass production in comparison to rhizobial inoculant alone or with water-soluble mineral P. Greenhouse results showed highly significant root colonization by commercial mycorrhizal inoculant alone (p < 0.001) and in interaction with soil type (p < 0.0001) and P source (p < 0.0001). However, no significant effect was shown in plant P uptake, biomass production, or leaf chlorophyll index. In field conditions, the effects of mycorrhizal and rhizobial inoculants in combination or alone were highly context-specific and may induce either a significant increase or decrease in root mycorrhizal colonization and nodule formation. Mycorrhizal and rhizobial inoculants in combination or alone had limited effect on plant P uptake, biomass production, leaf chlorophyll index, and grain yield. Though some mycorrhizal inoculants induced significant root colonization by mycorrhizal inoculants, this did not lead to higher soybean yield, even in soils with limited P content. Our results are further evidence that inoculant type, soil type, and P source are critical factors to evaluate commercial inoculants on a context-specific basis. However, our results highlight the need for the identification of additional targeting criteria, as inoculant type, soil type, and P source alone were not enough to be predictive of the response. Without the identification of predictive criteria for improved targeting, the economic use of such inoculants will remain elusive.
Highlights
Soybean (Glycine max (L.) Merr.) is a rich source of plant protein (411 g kg−1) and vegetable oil (209 g kg−1)
Nodule formation is a P-demanding process [43,44] and the faster solubilizing mineral P source likely accounts for this observation, while improved P uptake due to Arbuscular mycorrhizal fungi (AMF) begins with the colonization of plant roots [17,45], which should have increased P uptake with increased mycorrhizal colonization; this was not observed
Foreign strain inoculants often fail to form mycorrhizal associations [28,52] and this likely accounts for our results showing that three inoculants significantly increased soybean root mycorrhizal colonization, whereas nine other inoculants were not significantly different (Table 4)
Summary
Soybean (Glycine max (L.) Merr.) is a rich source of plant protein (411 g kg−1) and vegetable oil (209 g kg−1) It is used in a wide range of industrial products, from hand lotions to biofuel [1,2]. The main reasons for low crop yields include poor soil fertility associated with poor organic content as a consequence of little or no nutrient inputs (both organic and inorganic) and P-deficient soils [4,5]. A recent comprehensive analysis [7,8] to understand the major limitations and opportunities to enhance soil fertility in sub-Saharan Africa, including Kenya, found that financially constrained farmers have limited incentive to use inorganic fertilizer alone or improved varieties without some improvement to soil health. Soil microbials that enhance nutrient pools and uptake efficiency are likely technologies that can be valuable
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
