Leaf stains

David R. Hershey dh321 at PGSTUMAIL.PG.CC.MD.US
Thu Nov 13 13:56:53 EST 1997

> =09Papers that I have read refer to both tannic and humic acids as being
> responsible
> for the stains. I=92m a plant ecologist not a physiologist so I=92m not s=
> what the difference
> between tannic and humic acids is--can anyone help me out in that area.
> I have assumed
> that tannic acid is one of many humic acids but I have been guessing and

> Ivan Williams
> Stafford High School
> Stafford, TX

Tannins are produced by plants. Humus consists of the products of the
microbial breakdown of plant molecules. Humus is the colloidal organic
fraction of the soil and is highly resistant to further decay. It is
biochemically complex. A lot of it is derived from plant lignin. Microbes
also synthesize various polysaccharides and polyuronides that are part of
humus. Humus can bind proteins and "protect" them from further attack.=20
This allows humus to act as the reservoir of soil nitrogen and to slowly
release the soil nitrogen as the humus degrades. Another important
characteristic of humus is its high cation exchange capacity, which
enables it to retain nutrient cations, such as zinc, copper, manganese,
etc. in a plant available form.

Based on solubility, humus can be divided into fulvic acid, soluble in=20
acid or base; humic acid, soluble in base, but not acid; and humin,
insoluble in acid or base. Color darkens as solubility decreases. =20

David R. Hershey

Snail mail: 6700 Belcrest Road #112, Hyattsville, MD 20782-1340

Adjunct Professor, Biology/Horticulture Dept.
Prince George's Community College, Largo, MD 20772-2199

Email: dh321 at pgstumail.pg.cc.md.us


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