Mineral elements and Plant Nutrition

Elements derived from soil are termed as mineral elements. Mineral nutrition is the study of source, mode of absorption, distribution and metabolism of various inorganic substances or minerals by plants for their growth, development, structure, physiology and reproduction.

Nutrition in plants may be defined as a process of synthesis of food, its breakdown and utilization for various functions in the body. The chemical substances that provide nourishment to the plants are termed as plant nutrients. The two types of nutrients are organic and inorganic nutrient. Carbon di oxide is an inorganic nutrient for plants.

The first study in inorganic or mineral solution was carried out by Van Helmont in 1648.

Essentiality of minerals:

Arnon and Stout (1939) proposed criteria for knowing the essentiality of an element. Following are the criteria for knowing the essentiality of an element:

It is indispensable for the growth of plants.
The element is directly involved in the nutrition of plants. It becomes a component of either a structural or functional molecule. The element may additionally have a corrective effect on mineral balance and other soil conditions.
A plant is unable to complete its vegetative or reproductive phase in the absence of the element.
The element cannot be replaced by any other element.
The absence or deficiency of the element produces disorders.
The element alone can correct the disorders produced by its absence or deficiency.

Essential and non-essential elements:

Seventeen elements have been found to be essential. They are C, H, O, N, P, K, S, Mg, Ca, Fe, Zn, Mn, B, Cu, Mo, Cl and Ni. Others are non-essential elements. However, some of the non-essential elements are required in metabolic activities of certain plants. They include cobalt, silicon, sodium, vanadium, aluminium etc.

Macroelements (Macronutrients):

It comprises of those essential elements which are present in easily detectable quantities, at least one milligram per gram of dry matter. They are involved in the synthesis of organic molecules and development of osmotic potential. They are nine in number-C, H, O, N, P, K, S, Mg and Ca. Non essential functional elements which belong to the category of macroelements are silicon and sodium.

Microelements (Micronutients):

It comprises of those essential elements which we use in traces only, equal or less than 0.1 mg /gm of dry matter. They are essential in functioning of enzymes, as co-factors or metal activators. They are eight in number-Fe, Zn, Mn, B, Cu, Mo, Cl and Ni. Non essential functional elements which belong to the category of microelements are cobalt, vanadium and aluminium etc.

Difference between Macroelements and Microelements:

Macroelements	Microelements
1. They are present in plants in easily detectable quantities.	1. They occur in plants in traces only.
2. The concentration of macroelement per gm of dry matter is at least 1mg.	2. The concentration of microelement is less than 1mg/gm of dry matter.
3. They build up the plant body and different protoplasmic constituents.	3. Microelements do not have such a role.
4. Some macroelements accumulate in cell sap and take part in developing osmotic potential.	4. They have no significant role in the development of osmotic potential
5. Turgor movements occur mostly by the influx and efflux of potassium, a macroelement.	5. None of the microelements have any such function.
6. They do not become toxic in slight excess.	6. They become toxic in slight excess.

Mineral and non-mineral elements:

Essential elements derived from soil are termed as mineral elements. Essential elements obtained from air or water are termed as non-mineral elements e.g., C, H, O. They are building blocks of macromolecules that form the bulk of plant body.

Methods to study the mineral requirements of plants:

In 1860, Julius von Sachs, a prominent German botanist, demonstrated for the first time, that plants could be grown to maturity in a defined nutrient solution in complete absence of soil.

Solution culture (Hydroponics):

Solution culture methods are very useful in determining the essentiality of a particular element. The method of growing plants in nutrient solution is termed as hydroponics.

It consists of growing plants with their roots immersed in nutrient solution containing only inorganic salts. Nutrient solution usually contains all the essential mineral elements and is termed as stock solution or complete solution.

The solution culture apparatus consists of a seedling with its roots immersed in the nutrient solution. It also consists of an aerating tube to compensate for the depleting oxygen in the solution.

Absence of oxygen in the solution may lead to anoxia (a condition which inhibits respiration in the cells of the roots). Continuous exposure of solution to light may also promote growth of algae in the container. So to avoid it we need to wrap the container with opaque material.

The next step is to grow several similar plants in different containers. The nutrient solution of each of these containers lack one of the essential elements. Such solutions are termed as deficient solutions. Effects of deficiency of each element on normal growth of plant are observed. These symptoms are termed as deficiency symptoms.

If the plant shows deviation from its normal growth, it indicates that the mineral which is either lacking or in short supply is essential for the growth and development of that plant.

Fig: Mineral nutrition ( A simple apparatus to demonstrate solution culture)

Significance of hydroponics:

Following are the significance of hydroponics:

Use of soil less culture for the commercial production of many fruits and vegetables is frequent now-a-days.
The quality of crop obtained in hydroponic system is better than obtained by normal soil farming.
In many industrial areas where soil is not so suitable for normal farming, hydroponics is a good alternative.
Hydroponics can regulate _PH optimum for a particular crop. It can control soil borne pathogens, avoid problems of weeding and therefore can obtain consistently better yield.

Role of mineral elements in plant nutrition

Plants use the mineral elements in following ways:

The elements may form part of structural unit, such as carbon in cellulose or nitrogen in protein.
They may also be incorporated into organic molecules important in metabolism, like magnesium in chlorophyll molecule or phosphorous in ATP.
Elements may function as enzyme activators, i.e., they act as catalyst in certain enzymatic reactions.
Elements in ionic form, help to maintain osmotic balance, such as potassium in guard cells.