Geoscience Reference
In-Depth Information
R-Group or Side Chain
R
O
a
Carbon
H
3
N
C
C
−
Carboxyl
Group
+
a
Amino
Group
O
H
a
Hydrogen
R
H
O
C
H
N
C
H
O
H
Fig. 5.13
The structure of a humic molecule. The molecule has the positive charge on the side of
amino group and the negative charge on the side of carboxyl group. The molecule can be attracted
either to negative charges in the soil or to positive charges in the soil in accordance to our simpli-
fi ed description of the humic molecule
environment like pH, aeration, etc. also infl uence the type of each of the mentioned
processes.
Because of the outside portion of humic substances that carry negative charge,
they attract and hold positively charged cations like potassium, calcium, and mag-
nesium that belong to important plant nutrients. Their fi xation to negatively charged
parts of humic substances is not as strong as the bonds inside of the humic
compounds. We already described this property in a former section of this chapter
as cation exchange capacity. Plant roots are active in this exchange game in order
that their roots gain the nutrients needed for their metabolism and growth. But at the
same time, nutrients not absorbed by roots are still kept in their same exchange posi-
tions on the humic substance and prevented from leaching out of the soil by perco-
lating water. Everything is similar to our shopping for groceries. If we behave
reasonably, we usually buy only enough food that we plan to eat at home today and
during the next several days in spite of the supermarket overfl owing with food.
