Microbial Inoculants for Soil Quality and Plant Health

Agriculture is the major economic activity of most developing countries engaging more than 50 % of the population. Low world crop productivity due to low soil moisture, low nutrient capital, erosion risk, low pH, high phosphorus fixation, low levels of soil organic matter, aluminum toxicity pest and diseases, weeds and loss of soil biodiversity has induced the green revolution agriculture which involves high yielding varieties and agrochemicals. The continuous use of fertilizers, pesticides and herbicides has led to low agricultural productivity, low soil fertility, unfavourable economic returns, food poisoning, soil damage loss of biodiversity and serious environmental hazards. Microbial inoculants possess the capacity to enhance nutrient availability, uptake, and support the health of soil and plants to promote sustainable yield and has therefore gained attention of many agriculturist and researchers. We review the ability of soil through the use of microbial inoculants to supply nitrogen, phosphorus and potassium to crop plants and enhance structural stability. Microbial inoculants such as rhizobium, plant growth promoting rhizobacteria and arbuscular mycorrhizal fungi can be used as biofertilzer to improve soil nitrogen, phosphorus and potassium availability and uptake. Both bacteria and fungi inoculants show potential for use in soil aggregate formation and stabilization and hence, soil structure enhancement. The ability of microbial inoculants to ameliorate plant stress as a result of drought, soil contamination and salinity are also highlighted. The most commonly used microorganisms as biofertilizers, biocontrol and bioremediators include Bacillus spp, Pseudomonas spp, Streptomyces spp Trichoderma spp and Mycorrhizas. Microbial inoculants function through various mechanisms such as production of plant hormones, expansion and elongation of the root system, eliciting induced systemic resistance or systemic acquired resistance, production of lytic enzyme and antibiotic 4-hydroxyphenylactic acid, and production of 1-amino cyclopropane-1- carboxylate-deaminase (ACC-deaminase) in plants rhizosphere. These strategies are safe and sustainable in the long run. The use of appropriate carrier material determines the success of microbial inoculation techniques. Microbial inoculants could either be applied directly to the soil or as seed dressing. The fate of microbial inoculants under field application depends largely on both biotic and abiotic factors. The application of some microbial inoculants could cause a change (which could be a decrease or an increase) in the equilibrium of soil microbial communities while some produce no effect at all