The successful release of wild type and genetically modified micro-org
anisms into soil has been impeded because efficacy is unpredictable. E
vidence is presented showing that the plant-inoculum interaction can b
e strongly influenced by cropping system management or soil type. Rhiz
obacteria that inhibited the root growth of Poa annua and Lolium peren
ne were isolated from soil. The isolates, an inhibitory isolate (W7(+)
) and a noninhibitory mutant (G2Y(-)), were identified as Pseudomonas
putida. The rhizobacteria were used as soil inocula in a completely ra
ndomized block design that included Poa and Lolium, four soils, and th
ree inoculum carriers. Of the possible 15 interactions, 10 significant
ly affected both the root and shoot growth. These results suggest that
field trials conducted without consideration of soil quality factors
and carrier delivery systems will be of dubious value when evaluating
the performance of allochthonous micro-organisms in soil.