A unique feature of the active system of ion absorption by plant roots is
that it exhibits ion competition, antagonism, and synergism. The competitive
effects restrict the absorption of some ions in favor of others. Examples of
enhanced uptake relationships include:
Potassium (K+) uptake is favored over calcium (Ca2+) and magnesium (Mg2+)
uptake.
Chloride (Cl–), sulfate (SO4
2–), and phosphate (H2PO4
–) uptake is stimulated
when nitrate (NO3
–) uptake is strongly depressed.
The rate of absorption is also different for various ions. The monovalent
ions (i.e., K+, Cl–, NO3
-) are more readily absorbed by roots than the divalent
(Ca2+, Mg2+, SO4
2–) ions are.
The uptake of certain ions is also enhanced in active uptake. If the NO3
–
anion is the major N source in the surrounding rooting environment, then
there tends to be a balancing effect marked by greater intake of the cations
K+, Ca2+, and Mg2+. If the NH4
+ cation is the major source of N, then uptake
of the cations K+, Ca2+, and Mg2+ is reduced. In addition, the presence of NH4
+
enhances NO3
– uptake. If Cl– ions are present in sizable concentrations, NO3
–
uptake is reduced.
These effects of ion competition, antagonism, and synergism are of considerable
importance to the hydroponic grower in order to avoid the hazard
of creating elemental imbalances in the nutrient solution that will, in turn,
affect plant growth and development. Therefore, the nutrient solution must
be properly and carefully balanced initially and then kept in balance during
its term of use. Imbalances arising from these ion effects will affect plant
growth. Steiner (1980) has discussed in considerable detail his concepts of
ion balance when constituting a nutrient solution. His concept is presented
in Chapter 7.
Unfortunately, many current systems of nutrient solution management do
not effectively deal with the problem of imbalance. This is true not only of
24 Hydroponics: A Practical Guide for the Soilless Grower
systems in which the nutrient solution is managed on the basis of weekly
dumping and reconstitution but also of constant-flow systems. Indeed, the
concept of rapid, constant-flow, low-concentration nutrient solution management
is made to look deceptively promising in minimizing the interacting
effects of ions in the nutrient solution on absorption and plant nutrition (more
about these problems in Chapter 7).
Finally, nonionic substances, mainly molecules dissolved in the soil water,
can also be taken into the root by mass flow. Substances such as amino acids,
simple proteins, carbohydrates, and urea can easily enter the plant and
contribute to its growth and development.
Metabolic transport across root structures to the xylem regulates the amount
of ions conveyed to the tops; interestingly, the amount is little affected by the
velocity of xylem sap flow. Once in the xylem, ions and other soluble solutes
move by mass flow, primarily to the leaf apoplast.