Some "cross-inoculation groups" tolerate acid soils
better than others. The cowpea group, can tolerate acidity
that the soybean Rhizobium cannot tolerate, for example.
So, there is a pH/cross-inoculation group interaction.
A second consideration, along these lines, is that
some cross-inoculation groups include some weedy species.
That is the case with the cowpea strain here in Florida.
Florida beggarweed and sicklepod share the same bacteria
as cowpea, pigeonpea and more. So, it is unlikely that there
is not going to be fresh bacteria of that particular group. The
Nitragin people have a brochure that lists crops. I don't recall
where one can find the weed listing. University botanists
I believe that soil type and climate have a great deal to
do with year-to-year survival of Rhizobium. Sandy, droughty
soils probably are not going to be as hospitable as
finer-textured soils, which are higher in organic matter and
with more moisture.
I am running out of comments. One more that is worth
mentioning is "what is the cost of the inoculum and what is
the value of the crop?" Last I checked, an acre's worth of
inoculum was about $3.00. If inoculating gives half a bushel
more of soybeans, one pays for the cost of the inoculum and
that is low-cost insurance.
I think it is very important that the Rhizobium be
well-taken-care-of. If it gets hot or dries out, it is not going to
have a positive effect. I remember seeing packages of
inoculum in displays and I wondered if there were ever any
Rules-of-thumb? Yes, but I don't know how good they
are. We used to say that if one plants the same crop once
every three years, it is highly unlikely that there will be a
yield response to inoculation. Again, if we are talking about
sandy soils in a hot environment, that may not be true.
One of the things that impressed me is we used to do
lab experiments back at Iowa State for teaching purposes.
We would buy a load of washed river sand and use that as
our planting medium. It would not be unusual to find
soybeans with nodules in those teaching labs. Where did
the Rhizobium come from? Probably blew in with the dust
(soil erosion!). Possibly, it could have come with the seed
(ours, not purchased).
It is rather hard to show a yield response to
inoculation, unless one plants a new legume in a whole new
area - soybeans in the Cerrado of Brazil, for example. We
normally think of it as "insurance."
All of this applies to annual crops. I don't have much
experience with inoculating perennials.
Ken Buhr, Agronomy, U of Florida,