Nutrient Availability in Soil Solution

 Soil serves as the main source of mineral salts in which clay crystals are present in colloidal form.

These crystals have a central nucleus called a micelle.

The micelles are negatively charged and in order to maintain a balance, they attract and hold

positively charged ions on the surface of the colloidal clay crystals. Thus, the balance is always

maintained.

The available minerals of soil occur in ionic forms. The common cationic forms are: K, Mg, Ca,

Fe, Mn, Cu, Zn, and Co while anionic forms and N, P, B, S, and Cl. These ions are found either in the form of

loosely absorbed ions or firmly absorbed ions on the colloidal particles. The order of cation retentive capacity

of colloids is as follows :

                                         H+ > Ca++ > Mg++ > K+ > NH4+ > Na+

The loosely absorbed ions can be easily displaced by decreasing their own concentration in the soil

solution while the firmly absorbed ions can be replaced by other ions which have more affinity for the colloid

or ion exchange. The ion exchange may be either cation exchange or anion exchange.

Availability of Mineral Salts

Mineral salts are found either as a soluble fraction of soil solution or as adsorbed ions on the surface

of colloidal particles. It is believed that the uninterrupted supply of mineral nutrients from the absorbed

fractions is possibly done by ionic exchange.

Ion Exchange :

The first step in the absorption of mineral salts is the process of Ion-Exchange which does

not require metabolic energy but greatly facilitates mineral salt absorption.

The ions adsorbed on the surface of the wall or membranes of root cells may be exchanged with the

ions of the same sign from the external solution. For example, the cation K+ of the external soil solution may be

exchanged with H+ ion adsorbed on the surface of the root cells. Similarly, an anion may be exchanged with

OH- ion.

There are two theories proposed to explain the mechanism of ion exchange:]

a. CO2 hypothesis (or) Carbonic Acid Exchange Theory

b. Cation Exchange hypothesis (or) Contact Exchange Theory.

a. Cos hypothesis :

According to this theory, CO2 released by the roots during respiration combines with water to produce

carbonic acid (H2Co3). The carbonic acid dissociates into hydrogen ions(H+) and bicarbonate ions (HCO3).

These hydrogen ions may be exchanged for cations adsorbed on clay particles. The cations thus released

into the soil solution from the clay particles may be absorbed on root cells in exchange for H+ ions. While the

dissociated bicarbonate ions release the adsorbed anions (Fig.9). Thus, both cations and anions are made

available to the closeness of the roots of plants. Thus, soil solution plays an important role in carbonic acid

exchange theory.

Fig.9. Figure explaining CO2 hypothesis

b. Cation Exchange hypothesis

This theory states that the ions adsorbed on the surface of root cells and clay particles (micelles)

are not held tightly but always oscillate within a small volume of space. If the roots and clay particles are

in close contact with each other, the oscillation volume of ions adsorbed on the root-surface may overlap

the volume of ions adsorbed on clay particles. (Fig.10). Then, the ions adsorbed on clay particle may be

exchanged with the ions adsorbed on the root surface directly without first being dissolved in soil solution.