Let us make in-depth study of the mineral nutrition and elements in plants.
Mineral Nutrition in Plants:
Under normal situation, all green plants are autotrophs. Hence, they require the supply of inorganic materials from outside for synthesis of their own organic material (viz., organic food).
Apart from the elements carbon, hydrogen and oxygen that may be absorbed as water, carbon dioxide or oxygen, and which together makeup a large part of the weight of a plant, all the inorganic materials are absorbed by the plants directly or indirectly from the soil with the help of their roots.
As the source of these inorganic materials in the soil are minerals, they are called as mineral elements or mineral nutrients. The process involving the absorption, distribution and utilization of mineral substances by the plants for their growth and development is called mineral nutrition.
Since these mineral elements enter the biosphere mainly through the root system of plants, the plants, in a way, act as the “miners of earth crust”. After absorption, inorganic mineral elements are transported to various parts of the plant either in the form of an anion or a cation, where they carry out specific biological functions.
In some cases, nitrogen fixing bacteria and mycorrhizal fungi in association with roots participate in the process of mineral nutrition. However, some epiphytes absorb the mineral ions from dust particles present in moisture.
Mineral Elements in Plants:
If a plant is burnt in a muffle furnace at 600° C, all organic components valorize leaving behind a white residue called plant ash that contains only the inorganic mineral elements in different concentrations. In fact, more than 60 elements of the 105 discovered so far are found in different plants. Now question arises, whether all the diverse mineral elements are really necessary for plants?
On the basis of their effects on plant, mineral elements are generally of two types:
i. Essential and
Only about 17-20 elements are found to be essential. The rest elements are called non-essential without which a plant can survive and reproduce. The non-essential elements may be beneficial or toxic. Beneficial elements improve growth or reduce disease susceptibility without which a plant can still complete its life cycle. For example, Silicon (Si) in grasses, Sodium (Na) in C4 plants and halophytes. Toxic elements impair growth either in low or high concentrations.
Any mineral ion concentration in tissues that reduce the dry weight of tissues by about 10% is considered toxic. Toxic level for any element also varies for different plants. For example, aluminum (Al) is always toxic in the acidic soil but acts as beneficial element for tea plant.
Na, Zn, B, Mo, Ma, Cu and Fe are toxic if present at high concentration is soil. It is very often seen that the uptake of one element inhibits the uptake of another element. For example, excess of magnesium uptake induce deficiency of iron, magnesium and calcium.
1. First observation in inorganic or mineral nutrition in plants was made by Van Helmont (1648). He observed that in five years the soil lost 56,7 gm. in nourishing a young seedling of Salix to form a small tree.
2. Mayhow (1656) observed that the plant growth increased when salt peter (KNO3) was added to the soil.
3. Woodward (1699) observed that plants grow better in muddy water than in clear rain water. He experimentally proved that water and minerals were essential for plants growth and development.
4. De Sassure (1804) observed that minerals present in the plant ash (i.e., the residue left after the dry matter of the plant has been burnt) are obtained from the soil through the root system of plant. He experimentally proved that nitrogen and mineral elements are essential for the growth and development of the plant.
5. Sachs and Knop (1830), using hydroponic cultures (a technique of growing plants in water cultures, or nutrient solution, without soil as a rooting medium) prepared a list of essential elements required by plants for normal growth and development.
6. Liebig (1940) demonstrated the essentiality of mineral nutrition in plants and proposed the law of minimum’ which states that “The yield of a crop plant is determined by the amount of the necessary element which is present in minimum, quantity in proportion to the demand of the plant.”