Plant-affecting streptomycin-sensitive micro-organisms in barley monoculture soils

Soils from barley monoculture and from crop-rotation plots of three long-term field experiments were tested for their effects on barley growth in the glasshouse, using plant growth tubes. Test plants of young barley developed significantly shorter and thinner root axes when grown in soil from monoculture plots than when grown in soil from crop rotation plots, but the difference in shoot growth was only small. Addition of small doses of streptomycin sulphate neutralized the retarded root growth in soil from monoculture plots, whereas the fungicide metalaxyl induced no such effect. The difference in root growth of barley test plants could be transmitted into a soil-free root environment by inoculation with the microflora from roots grown in soil. These findings indicate that some component(s) of rhizosphere bacteria is the causal agent of the early monoculture effect measured.