The impact of adding raw
and composted dairy manure on crop, weed, disease, nematode, soil, and microbial
variables are being examined in a vegetable rotation that is transitioning to
organic production. The addition of two forms of organic amendments
differentially affected crop and microbial variables. Compost additions preferentially impacted plant variables
such as crop yield and weed emergence. Manure
additions, with a higher C:N ratio, more profoundly affected soil microbial
populations. Significant increases
in the numbers of pseudomonads, including those with biocontrol activity, were
observed. In addition, significant
changes in bacterial and fungal communities inhabiting the rhizosphere were
detected in the amended plots using DNA-based methods at mid-season.
However, following harvest, only minor differences in soil microbial
population structure were observed among the treatments, although some residual
effects in microbial biomass N and substrate-induced respiration were also
detected. These results indicate that while the effects of organic
amendments may be varied and transient, direct effects on plant growth
correlated with nitrogen availability while effects on microbial variables were
more greatly affected by carbon availability in the organic amendments.
Nonetheless, differences in plant disease pressure in the tomato plots
indicated that compost amendments may contribute to enhanced host resistance.