Effect of plant growth promoting rhizobacteria and rock phosphate enriched compost on growth, yield and P nutrition of cereals

Abstract

Relatively poor bioavailability of P in soil from the applied expensive phosphatic fertilizers due to fixation/precipitation is considered one of the most critical factors in limiting optimum crop yields. Thus use of cheap P sources and enhancing P bioavailability is considered viable approach/ strategy and is direly needed under the current prevailing situation for sustainable crop production. Rock phosphate (RP) is a cheap source of P but cannot be used directly as a soil amendment because of its extremely poor water solubility (0.1%). However, the bioavailability of RP-P can be enhanced by complexing it with compost and/ or through the use of specific bioinoculants. A series of studies were conducted to determine the effectiveness of various approaches to solubilize RP-P and the impact of bioavailable RP-P on growth and yield of wheat and maize under wire house and field conditions was assessed. Two soil incubation studies were conducted for 15 weeks to determine the potential of solubilization of RP. In first study soil was spiked with RP, RP + P-solubilizing bacteria (PSB), while in second study soil was amended either with RP, RP + compost, RP- enriched compost (RP-EC) and RP-EC + PSB to determine the release of bioavailable P from RP. Results of first incubation study revealed that RP + PSB, while in second study, RP-EC + PSB resulted in maximum release of plant available P in soil. In both the studies, addition of RP alone did not provide plant available P in soil. The results of these incubation studies were further investigated for their validity by conducting a series of pot and field trials on wheat and maize. Firstly, the efficacy of PSB and ACC-deaminase PGPR in solubilizing RP-P and uptake of P on wheat was investigated. The results verified the findings of incubation trial as bioinoculants (PSB or PSB + ACC-deaminase PGPR) plus RP significantly improved growth contributing parameters of wheat in comparison to RP alone. The results of growth parameters of wheat recorded in case of RP plus PSB + ACC deaminase PGPR were almost comparable to NPK control. Similarly RP-EC substitutions of 25% or 50% of the P requirements of the crop in the presence or absence of bioinoculants were also investigated for their impact on growth of test crops compared to NPK control. Results recorded that without inoculation, 25% replacement of P dose by RP-EC proved better than NPK control while substitution by 50% of P dose with RP-EC was relatively less effective than NPK control. However inoculation with PSB or PSB + ACC-deaminase PGPR proved effective under all the fertilizer treatments with different degree of efficacy. Application of 50% RP-EC with 24dual inoculation of PSB and ACC-deaminase PGPR proved to be the best combination in promoting growth and yield of wheat and maize in both pot and field trials. Similarly P contents as well as P-uptake of grain and straw improved substantially. Other treatments also produced higher yield contributing traits than uninoculated NPK control but with relatively less efficiency. The findings further verified the results of soil incubation studies with respect to their effectiveness of various treatments. It is very likely that both compost and PSB helped in solubilization of insoluble P while ACC-deaminase supported efficient uptake of P by improving root growth. The results of these studies may imply that RP-EC plus seed inoculation with novel PGPR proved to be a viable approach to use low grade RP and organic waste for sustained crop production as well as for promoting healthier environment.