INFLUENCE OF SOIL PROPERTIES AND SOYA BEAN CROPPING HISTORY ON THE BRADYRHIZOBIUM-JAPONICUM POPULATION IN SOME FRENCH SOILS

As French soils are naturally free of Bradyrhizobium japonicum, soya b eans planted into new areas must be inoculated with this bacterium. Al though, the B. japonicum inoculum can survive in soils for long period s of time even in the absence of a soya bean crop, re-inoculation may increase nodulation and grain yield. Thus, populations of B. japonicum can fall below optimum for plant growth. To identify the soil propert ies controlling survival of the inoculated bacteria samples of soil we re collected from 52 sites from France that had previously grown soya beans. The samples were analysed for some physical and chemical charac teristics and the B. japonicum population counted. The soil's CaCO3 co ntent was the main factor affecting survival. The average B. japonicum numbers (per g soil) were 80 for calcareous soils and 15000 for non-c alcareous soils. In the latter, silt and sand contents were correlated with the numbers of B. japonicum. The cropping frequency of soya bean and the time since the crop was last grown were other factors affecti ng Bradyrhizobium populations. Thus, there is a probability of enhanci ng economic benefit for farmers with re-inoculation of soya bean in ca lcareous and sandy soils.