In this article we will discuss about the essential elements for healthy growth of a plant.
It is found that thirteen elements are constantly present in the plant body, as revealed by ash analysis. The mere presence in the composition does not necessarily mean that they are all essential for the growth and nutrition of plants. What elements are essential and what are not, can be determined by water-culture experiment.
As plants absorb most of the elements from soil, a standard solution is prepared with those elements, and plants are grown there. By conducting a series of experiments it has been found that ten elements are very important for the nutrition of plants and the remaining three, referred to as non-essential elements, are not directly concerned with nutrition.
It has been found that minute traces of the elements like boron, zinc, copper, manganese, and molybdenum are equally necessary for the normal development of the plants. Thus essential elements are put into two groups.
(1) Major elements are macro-elements—C, H, O, N, S, P, K, Mg, Ca, Fe.
(2) Trace elements or micro-elements—B, Zn, Cu, Mn, Mo.
That the above elements are essential for the healthy growth of a plant can be proved by water-culture experiment (Fig. 162). It is undoubtedly a laborious one. A few wide mouthed jars are taken, thoroughly washed and rinsed in nitric acid. Seeds are germinated on sterilised saw dust.
A few healthy seedlings of more or less same size are selected for experiment. Jars are labelled as 1, 2, 3, 4 and so on. The first jar 1 is filled with normal culture solution, i.e. a solution with all the elements essential for growth of plants. Culture solution should not be alkaline. Culture solutions of various compositions are used. Knop’s normal culture solution is a quite suitable one.
Knop’s normal culture solution:
Potassium nitrate, KNO3—1–0 g.
Potassium phosphate KH2PO4—1’O g.
Magnesium sulphate MgSO4—1 ‘0 g.
Calcium nitrate, Ca(NO3)2—3’0 g.
Ferric chloride, FeCl3—a few drops.
Jar No. 1 is filled with normal culture solution; a healthy seedling is selected and the roots after proper washing are inserted in the jar through the cork fitted at the mouth. A bent glass tube is put for aeration of roots and the jar is covered by black paper to cut off light.
Now a series of jars are similarly filled with plants but one element, in turn, is eliminated from the solution poured in every jar; thus jar 2 has no potassium salt, jar 3, no calcium salt and so on. Thus the experiment is set up.
After a few days it is observed that only the first one is having normal healthy growth and the rest are all weak, deficient, and stunted. Plant in jar 2 without potassium salt is weak and bears unhealthy leaves; in jar 3 without calcium salt the plant has poorly developed roots and spotted deformed leaves; leaves are yellow (chlorotic) in that without iron salts, and so on.
That shows beyond doubt that the elements used in water culture solution and hydrogen, oxygen and carbon are absolutely necessary for the healthy growth of the plant.
The essential elements are absorbed either as salts and compounds from the soil or as gas from the air by the two main absorbing organs, roots and leaves. Recent researches have, however, shown that minute traces of manganese, boron, zinc, copper and molybdenum are also essential for normal growth.
Unlike the essential elements, they do not enter into the composition of plant body but they are absolutely essential as catalysts or growth regulators.
Deficiency symptoms of these trace elements are not always observed for they are generally present in the soil or in the water culture where even the very pure analar reagents have traces of these elements. The requirements of these trace elements are fantastically small, not more than 1-3 parts per 100 million parts.
Carbon is absorbed from the air as carbon dioxide gas. Though the percentage of this gas in atmosphere is 0–03, that is the only source of carbon for the plants which really forms nearly half of the dry weight of the plant and is the primary constituent of protoplasm, cell wall, food matters, etc.
Hydrogen and oxygen are taken mainly as water from the soil. Free oxygen is also absorbed from air during respiration. They enter into the composition of cell wall, reserve materials, and the protoplasm itself. Water is indispensable for protoplasmic activities and also for translocation of dissolved food materials.
Nitrogen forms an important element of protein and a considerable part of protoplasm. Though atmosphere has 78% of molecular nitrogen, it is not available to plants as such. It enters all visible plants and animals and comes out again without taking part in the life processes. Nitrogen mainly comes from soil as nitrates and ammonia salts.
The soil bacteria play important roles in converting nitrogenous matters into nitrates for ready absorption. Atmospheric nitrogen can be fixed by some free nitrogen-fixing bacteria and also by some bacteria which inhabit the roots of leguminous plants forming characteristic nodules.
The legume bacteria lead a life of mutual friendship (symbiosis) with the plants. The amount of nitrogen fixed by the bacteria is not very high—about 10% of the total nitrogen fixed in the soil.
Certain blue green algae can also fix nitrogen, which has considerable economic importance in increasing fertility of the rice fields, particularly in the tropics. During thunder storm free nitrogen becomes available to some extent. The nitrogen of the air combines with oxygen to form nitric oxide—Na-+ O2= 2NO. This nitric oxide unites with O4 of form nitrogen peroxide—2NO+O2=2NO2.
The nitrogen peroxide dissolves in falling rain water and forms nitrous acid and nitric acid—2NO2+H2O = HNO2+ HNO3, and reach the soil, where they combine with metals like calcium and potassium to form nitrites and nitrates.
The nitrates are directly absorbed by the plants, whereas the nitrites are further oxidised to nitrates. Insectivorous plants get part of their nitrogen requirement from the bodies of the insects they capture.
Nitrogen occurs in the soil in organic form as complex proteins, and in inorganic form as nitrates and nitrites, and also ammonia and ammonium ions. The proteins of plant and animal bodies are broken down to simpler substances by the action of decomposers, the soil micro-organisms and fungi.
Ammonia is such a simple substance which readily dissolves in water. It reacts chemically with H-ions to form Ammonia ions NH4+. Only some plants can utilise nitrogen in this form for building up complex proteins. In case of most of the plants a group of micro-organism called nitrifying bacteria of the soil are responsible for converting the ammonium compounds into nitrates.
The process nitrification, as it is called, occurs in two stages—first, by the action of nitrite-bacteria ammonium compounds are oxidised into nitrites (NO2–), and then nitrites are acted upon by the nitrate-bacteria to be finally oxidised into nitrates (NO3–).
Nitrates thus formed are readily absorbed by the plants and utilised. Nitrifying bacteria work only under aerobic conditions, i.e. when oxygen is available. Under anaerobic condition another group of micro-organism, the denitrifying bacteria change the remaining nitrates into free nitrogen which gradually escapes to the atmosphere.
The whole picture, referred to as nitrogen cycle is represented by the following schematic diagram:
Sulphur is absorbed as sulphate from the soil. It is present in proteins and protoplasm.
Phosphorus is also obtained as phosphates from the soil. It is richly present in nucleo-proteins and has direct influence on cell-division and growth.
Potassium is absorbed as salts from the soil. It is important for the formation of healthy vegetative organs and maturation of fruits and seeds. Potassium is present abundantly in the meristematic region. It helps in the synthesis of reserve materials—carbohydrates, proteins, and fats.
Magnesium is also taken as salts. It is present in chlorophyll and has influence on the formation of proteins.
Calcium is essential for healthy growth. In absence of cium, plants get diseased. It is abundantly present in the middle lamella of the wall. Calcium neutralises organic acids like oxalic acid forming calcium oxalate. It is also absorbed as salts.
Iron does not actually enter into the composition of chlorophyll, but minute quantities are essential for the formation of chlorophyll. Plants become chlorotic in absence of iron. Perhaps iron acts as a catalyst for the formation of complex chlorophyll structure.
Sodium, chlorine, and silicon are present in the ash. But they are not essential for the normal growth of the plants.