For proper plant growth, a regular supply of plant nutrients especially the essential ones, is necessary. Plants absorb a large number of elements from the soil, air and water during their growth period, but not all of these are essential.
Only 16 elements have been found to be essential for all plants and four others have been found to be essential for some plants.
Essential Elements for Plant Growth
For element to be so classified, it has to fulfill the following criteria:
- A deficiency of the element makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle
- The deficiency symptom of the element in question can be prevented or corrected only by supplying that element.
- The element must have a direct influence on the plant, and must be directly involved in the nutrition of the plant, quite apart from its possible effect in correcting some microbiological or chemical condition in the soil or culture medium
Essential element like carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur are the elements of which proteins and hence protoplasm are composed. The other ten elements which are essential for plants are potassium, calcium, magnesium, iron, manganese, molybdenum, copper, boron, zinc, and chlorine. The four elements which are essential only for some and not for all plants are sodium, cobalt, vanadium and silicon.
Sources of Nutrient for Plant Growth
The following elements are derived:
- From air; carbon (C) as CO₂ (carbon dioxide);
- From the water: hydrogen (H) and oxygen (O) as H₂O (water);
- From the water: fertilizer and animal manure : nitrogen (N) a considerable amount of nitrogen is also fixed by leguminous plants through root nodule bacteria
Macro and Micronutrients
Macronutrients are needed by the plants in large amount, and large quantities have to be applied if the soil is deficient in one or more of them. Within the group of macronutrients, which are needed for plant growth in large amounts, the primary nutrient such as nitrogen, phosphorus and potassium.
While calcium, magnesium and sulphur are sometimes called secondary nutrients due to their secondary importance In plant nutrition.
In contrast with macronutrients, micronutrients or trace elements are required in only minute amounts for correct plant growth and have to be added in very small quantities when they cannot be provided by the soil.
Functions of Nutrients in Plant Growth
- Nitrogen is the motor of plant growth. It makes up to 1 to 4 percent of dry matter of the plant. It is taken up from the soil in the form of nitrate (NO₃) or ammonium (NH₄⁺).
- In the plant it combines with compounds produced by carbohydrate metabolism to form amino acids and proteins.
- Being the essential constituent of proteins, it is involved in all the major process of plant development and yield formation.
- A good supply of nitrogen for the plant is important also for the uptake of the other nutrients
- It constitutes 0.1 to 0.4 percent of dry matter of the plant.
- It plays a key role in the transfer of energy.
- It is essential for photosynthesis and other chemical-physiological process in the plant.
- It is indispensable for cell differentiation for the development of the tissues, which form the growing points of the plant
- Potassium makes up 1 t0 4 percent of the dry matter of the plant.
- It activates more than 60 enzymes.
- It plays a vital part in carbohydrate and protein synthesis.
- Potassium improves the water regime of the plant and increase is tolerance to drought, frost and salinity.
- Plants well supplied with K are also less affected by diseases
- It is a central constituent of chlorophyll, the green pigment of the leaves which functions as acceptor of the energy from the sun thus, 15 to 20 percent of the magnesium found in plant is contained in the leaves.
- Mg is also involved in enzyme reactions related to the energy transfer of the plant.
- It is an essential constituent of protein and also involved in the formation of chlorophyll.
- In most plants it makes up to 0.2 to 0.3 percent of dry matter
- Calcium is essential for root growth
- It is a c0nstituent of cell wall materials
Micronutrients or Trace Elements
These are iron (Fe), manganese (Mg), Zinc (Zn), copper (Cu), molybdenum (Mo), chlorine (Cl) and boron (B). They are part of the key substances in plant growth and are comparable with vitamins in human nutrition. Being taken up in minute amounts, their range of optimal supply is very small.