To utilize sewage sludge in agriculture, the decomposing process of dry sewage sludge and sludge compost after incorporation into soil and their effects on plant growth were investigated by the following methods : (1) Mineralization of nitrogen of sewage sludge was investigated by a kinetic study at three different temperatures (10, 20, 30℃). Dry sludge showed an nonlinear parallel equation having two fractions of rapidly decomposable and slowly decomposable nitrogen, but sludge compost showed a simple nonlinear equation having only slowly decomposable nitrogen fraction. The period in which 99% of rapidly decomposable nitrogen was decomposed was calculated by computer simulation from the obtained equations for nitrogen mineralization, and it was named a "dangerous period to cultivation". These periods for dry sludge were 33, 11 and 4 days at 10, 20 and 30℃, respectively. The dangerous periods did not exist in sludge compost. (2) Cultivation experiments using komatsuna indicated that safe amounts of an application of dry sludge and sludge compost were about 0.5 and 2% on a dry weight base, respectively. (3) From the analysis of thermograms measured with a microcalorimeter, it was found that heat generation of dry sewage sludge in soil was 81.0 kjoule g^<-1>, while that of sludge compost was only 7.7 kjoule g^<-1>. Thus, it was confirmed that the most part of organic matter in sewage sludge had been decomposed by composting and that harmful effects of decomposition in soil on plant growth could be avoided.
Five mutants of Xanthomonas oryzae pv. oryzae strain T7174R (a derivative of T7174 and tolerant to rifampicin), which express reduced virulence on compatible cultivars, were isolated by insertion of the transposon Tn4431. In the case that plants were inoculated with one of these mutants, the lesion length was decreased from 62 to 96%, of that of the parental strain T7174R. To learn the characteristics of these mutants, the in vitro and in planta multiplication abilities of the mutants were compared with those of T7174R. The mutant T7174R-5D4 was able to multiply as well as T7174R in vitro, but was not able to do as well as T7174R in planta, possibly due to a decline in mobility in xylem vessels. The mutant T7174 R-7H3 was not able to multiply as well as T7174R in vitro and in planta. In the case of this mutant, its limited ability in multiplication apparently caused reduced virulence. The mutant T7174R-12B12 could multiply almost as well as T7174R in vitro and in planta, but showed reduced virulence. Other factors for virulence would thus appear to be present in addition to multiplication ability and mobility in xylem vessels.