Introductory Crop Physiology

Criteria of Essentiality of Elements

Criteria of Essentiality of Elements (Arnon and Stout, 1939)

In order to show that element is truly essential, it is necessary to show not only that:

i. A deficiency of the element makes it impossible for a plant to complete its vegetative and reproductive cycle
ii. It cannot be replaced by another element and
iii. The element should also have some part to play in metabolism of the organism etc. outside the plant.

Nutrient Availability in Soil Solution

• Soil serves as a main source of mineral salts in which clay crystals are present in colloidal form.
• These crystals have a central nucleus called 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 soluble fraction of soil solution or as adsorbed ions on the surface of colloidal particles.
• It is believed that the uninterrupted supply of mineral nutrient from the absorbed fractions is possibly done by ionic exchange.
Ion Exchange

• 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 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.
TOP