Re: soil vs water as source of nutrients
>> 6. I disagree with the statement from Dr. Dave about "the
>> chief benefits of the substrate being its anaerobic state and
>> not its Cation Exchange Capacity CEC". If this were true
>> plants (terestrial) would grow very well in all types of soil.
>> The cec of silt, clay, sand and organic matter are all different.
>> The best soils have a certian mix of these 4 things. The best
>> mix grows the most in the way of crop production. The sand
>> soil cannot "hold" (low to no cec) the nutrients long enought
>> for the plant to up take them. Ask someone from Flordia.
>> Paul L. "May the force be with you".
>You are thinking like an aggie here, Paul. Please dont relate the
>requirements of terrestrial plants to those of submerged aquatic plants.
>For one thing, aquatic plants can absorb nutrients from two vastly
>different environments ... the substrate and the water column. They have
>adapted to these conditions ... the aerobic water column generally supplies
>the cations Ca, Mg and K while the anaerobic sediment supplies NH4, PO4
>and ferrous iron (among other things). This is ideal since in many
>natural systems the cations are fairly stable in the water while the
>supplies of ammonia, phosphate and iron are much higher in the sediment.
>Another interesting benifit of an anaerobic sediment is the fact that
>some aquatic plant roots are inhibited in the presence of oxygen. Elodea,
>for instance, cannot produce root hairs in an oxygenated environment.
>I also never advocated using a sand substrate. A "good loam soil" was
>advocated as the best medium for growing rooted aquatic plants more than
>90 years ago by a man named Raymond Pond.
To add to what Dr. dave said, let's consider how important soil cation
exchange capacity is for aquatic plants in the long run. In an aquarium,
the plants have a limited amount of soil for their roots, and even if the
CEC of the soil is great, they are going to exhaust the soil of adsorbed
cation nutrients, such as K+, long before they get very big. Then, their
source will have to be the water. In nature they are growing in a soil
that has built up slowly and under water, usually, for a long time and has
had aquatic plant roots in it extracting what they can get out of it every
year. Again, I don't think that any aquatic substratum can have a CEC high
enough to support generations of plants. Logically, the most important
source of many cations will have to be the relativly huge and constantly
renewed supply of water surrounding the plants.
Raymond Pond's experiments showed unequivocally that aquatic plants did
much better rooted in soil, but a lot of subsequent work has showed that
iron is one of the most important things that they get from the soil.
There is a real possibility, needing more investigation, that aquatic
plants may be able to extract nutrients from the water more efficiently
when rooted than when floating. The speed at which my plants recover from
various deficiencies when the nutrients are added to the water has me
totally convinced that most if not all nutrients can be taken efficiently
out of the water by a well established and rooted aquatic plant, and that
you can't count on the soil as a long term source of any nutrients except
those which can exist in the soil as relativly insoluble precipitates.
Examples of the latter would include iron, as iron oxides hydroxides, etc.;
calcium, as calcium carbonate; and phosphorus, as calcium or iron
Paul Krombholz Tougaloo College, Tougaloo, MS 39174
Where our chances for decent winter weather are passing quickly. Soon there
will be nothing to look forward to but tornadoes and floods