collecting soil for substrate: safe?

A recent posting on r.a.fw.plants mentioned the danger of
getting nematodes in your aquarium by using soil or manure
which is not sterile. My dictionary says nematodes are just
a class of worms which live in soil or mud under water. It didn't
say if a particular nematode can live in both environments.
I'm also curious as to how common these beasties are and if
they pose a problem to aquatic plants. I know one type of worm
that was a pest in gardens is called the cut-worm; I'm not sure
if this guy could survive underwater. Fortunately, it's quite
large so it's easy to remove (but eggs or small young might not

Another thing to consider, is that we don't want to completely
sterilize the substrate because there are many important
micro-organisms which need to be there to support the plants
esp. for resolubilization of Fe. In fact, if you just mixed
a little of an old substrate with a new one, it may take a
very long time for those bacteria to colonize the new
material to the extent that we need.

I can see that the chances of finding undesirable organisms
is much higher in manure than ordinary garden soil. Paul K.,
do you sterilize your manure if you add it for composting?

I should imagine that for biological experiments growing
aquatic plants in a controlled environment, there must be
established procedures for preparing a soil based substrate.
Can anyone describe some of these?

Steve Pushak    spush at hcsd_hac.com      Vancouver, BC, CANADA