Testing of Commercial Inoculants to Enhance P Uptake and Grain Yield of Promiscuous Soybean in Kenya

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.

Analysis of Arbuscular Mycorrhizal Fungal Inoculant Benchmarks

“…Mycorrhiza inoculation especially in the field does not always produce positive effects on biomass and grain yield. For example, Faye et al (2020) [ 22 ] showed that inoculating soybean ( Glycine max (L.) Merr .) enhanced root nodulation but had no significant effect on grain yield.…”

“…Therefore, for AMF inoculant industry to thrive, rigorous research must be conducted to provide best practices to the inoculant companies regarding composition, quality, quantity, and application methods of the products [ 19 ]. Although lack of efficacy or negative impacts of AMF inoculation has been reported [ 20 , 21 , 22 ], recent studies conducted under field conditions have shown promising results [ 12 , 23 , 24 , 25 , 26 ].…”

Integrated Soil and Crop Management in Organic Agriculture: A Logical Framework to Ensure Food Quality and Human Health?

“…Rhizobium inocula are used in both conventional and organic farming, especially for grain legumes [67][68][69][70][71] and were shown to increase N-fixation and soil-N and N/crude protein in grain legume and cereal crops grown after inoculated grass-clover leys [69]. Rhizobium species are legume species-specific and when new legume crops (e.g., soya) are introduced into rotations, treatments of seed with Rhizobium inocula is recommended [71]. The use of Rhizobium inocula was also reported to increase mycorrhizal development from 'natural' mycorrhizal fungal inocula present in soil [70].…”

Potential Use of Beneficial Microorganisms for Soil Amelioration, Phytopathogen Biocontrol, and Sustainable Crop Production in Smallholder Agroecosystems

“…In Kenya, smallholder farmers utilize Rhizatech R inoculant (Table 1) containing a combination of three AMF species produced and distributed by Dudu Tech Ltd company (Faye et al, 2020). In South Africa, Mycoroot (Pty) Ltd produces a number of Mycoroot R branded AMF inoculants that solubilize P, K, Cu, Zn, and Fe, and improve plant tolerance against abiotic stresses.…”

“…This explains why microbial inoculants are recommended even in soils with less water and where most nutrients are immobile (Sindhu et al, 2020). In some instances, seed companies and researchers have solved this challenge by producing crop cultivars that customarily favor colonization of specific PGPMs under a wide range of soil conditions (Faye et al, 2020). Through this approach, Gitonga et al (2021) investigated how organic and conventional smallholder farming systems and soybean cultivars (promiscuous vs. non-promiscuous) affect native Bradyrhizobium spp.…”