In this article we will discuss about:- 1. Meaning of Mineral Nutrition 2. Criteria of Essentiality 3. Mobility of Nutrients 4. Classification of Essential Nutrients 5. Hidden Hunger 6. Deficiency Symptoms.
Meaning of Mineral Nutrition:
The term mineral nutrient is generally used to refer to an inorganic ion obtained from the soil and required for plant growth. The process of absorption, translocation and assimilation of nutrients by the plants is known as mineral nutrition.
The elements C, H, and O are not minerals. The rest of the elements are absorbed from the soil and these are called mineral elements since they are derived from minerals. These mineral elements are mainly absorbed in ionic form and to some extent in non-ionic form.
Criteria of Essentiality:
Plant body contains about 30 elements and in some cases as many as 60 elements Arnon and Stout (1939) proposed criteria of essentiality which was refined by Arnon (1954).
(a) The deficiency of the element makes it impossible for the plants to complete the vegetative or reproductive stages of its life cycle.
(b) The deficiency is specific to element in question and as such can be prevented or corrected only by supplying that particular nutrient element to the plant.
(c) The elements must have a direct influence on the plant and must be directly involved in nutrition and metabolism of the plant.
Recent investigations show that point (b) cannot be accepted absolutely as Mo may be substituted by vanadium (V); Similarly CI by Br; K by Rb (Rubidium); Ca by strontium (Sr).
Na is known to increase the yield of several crops like beets, turnip, carrot etc. and hence it is found to be essential for the farmers. At Arnon’s time Only 16 elements were considered as essential.
Nicholas (1961) from Long Ashton Institute (USA) proposed the term ‘Functional Nutrient’ for any mineral element that functions in plant metabolism whether or not its action is specific. With this criteria, Na, Co, V and Si are also included in functional nutrients in addition to 16 essential elements.
The 16 essential elements are-
Out of these 16 essential elements C, H, O, N, P, K, Ca, Mg, S are called Macro- elements or Macro- nutrients and the remaining elements (or nutrients) are called Micro- nutrients.
N, P, K macronutrients are called Primary nutrients and Ca, Mg, S are called Secondary nutrients.
Functional Nutrients = 16 essential elements + (Co, Na, V, Si)
Thus Functional nutrients are 20 in number for the plants. Presently Ni (Nickel) is considered as essential hence total essential nutrients are 17.
Mobility of Nutrients:
Mobility in Soil:
(a) Mobile nutrients – Such elements are highly soluble and are not adsorbed on clay complex e.g. –
NO-3, SO42-, BO33- , Mn2+, CI
(b) Less Mobile – Such elements are also soluble but adsorbed on clay complex and thus their mobility is reduced e.g. NH4+, K+, Ca2+, Mg2+, Cu2+
(c) Immobile – It is highly reactive and get fixed in the soil e.g. H2PO4–, HPO2- , Zn2+
(a) Highly Mobile – N, P & K
(b) Moderately mobile – Zn
(c) Less mobile – S, Fe, Cu, Mn, Mo & Cl
(d) Immobile – Ca & B.
Classification of Essential Nutrients:
Basis – Quantity of nutrients present in plants.
1. Basic Nutrients:
C, H, O constitute about 96% of the total dry matter of plants. Among them C & O constitute 45% each.
2. Macro/Major Nutrients:
Such elements are required in larger amount i.e. more than 1 ppm. NPK are Primary/Major Nutrients. Ca, Mg, S are Secondary nutrients because they are indirectly applied to the soils when NPK fertilizers which contain these nutrients are used.
3. Micronutrients:
It is required in small quantities (0.01-1000 ppm) and generally less than 1 ppm (Parts per million).Micronutrients are also called trace elements, Oligoelements, or Spurne elements. These are very efficient, and minute quantities produce optimum effects. On the other hand even a slight deficiency or excess is harmful to the plants.
Micronutrients are all the essential nutrients except NPK, Ca Mg S. e.g.- Mn, Cu, Mo, B, CI, Co, Fe and Zn. Fe and Zn behave as macronutrients but for historical reasons, included in micronutrients.
4. Ultra Micronutrients:
Concentration required 1 ppb (Parts per billion). Nicholas (1963) put forward this concept giving the example of Mo and Co but it is faulty classification and is not acceptable today because the quantity of nutrients absorbed depends upon the types of plant.
Beneficial Nutrients or Potential Micronutrients:
Such elements at very low concentrations and often under specific conditions have been shown to stimulate the growth of certain plants or to have Sure rate other beneficial effects e.g. Selenium (Se), Al, Rb, Sr, Ni, Chromium (Cr) and Arsenic (As).
Hidden Hunger:
The term Hidden Hunger is used to describe a plant that shows no obvious symptoms yet the nutrient content is sufficient to give the top profitable yield. Fertilization with the sure rate rather than bare economic optimum for on average yield helps to detain the top profitable yield. Sure rate is a rate a little above critical limit to be sure that there is no reduction in yield.
The Hidden Hunger is an expression in soil science to designate a condition in plants brought about through the lack of certain essential elements in the soil and is not easily detected by outward appearance.
Deficiency Symptoms on Leaves:
The deficiency symptoms can be distinguished on the basis of the followings:
(a) Region of occurrence.
(b) Presence or absence of dead spots and
(c) Chlorosis of entire leaf or interveinal chlorosis.
Deficiency Symptoms on Old Leaves:
N – Uniform chlorosis of leaves including veins. The leaves become stiff and erect especially in cereals. Cereals show characteristic ‘V’ shaped yellowing at the tip of lower leaves.
P – Small, erect, usually dark green leaves with a greenish red, greenish brown or purplish tinge. The rear side develops bronzy appearance ripening of crop.
Mg – Yellowing (chlorosis) in between the veins and veins remain green. Leaf is not erect, necrosis in extreme case only in the margins; upward curling of leaves along the leaf margin, most prevalent towards the end of growing crop season. Cereals fairly resistant but oat is more susceptible.
K – Yellowing starts from tips or margins of leaves extending to the centre of leaf base; Necrosis very soon, sharp difference between green and yellow and necrotic portions; curling of leaves towards undersurface. Tip burn, margin scorching, Brown spots near margin rolling of laminae; Barley most susceptible, Potato shows abnormal dark green colour of foliage followed by browning, cotton bolls are either small, knotty or poorly opened.
Mo – Translucent spots of irregular shape in between the veins of leaves; these spots are light green, yellow or brown; chlorotic mottling between veins of middle leaves. The affected spots are impregnated with resinous gum which exudes from rear side of the leaf from the reddish brown spots; Brassica sp. most susceptible, yellow spot of citrus, whiptail of cauliflower (rat diseases).
Interveinal Chlorosis:
Fe – The principal veins remain conspicuously green and other portions of the leaf turn yellow tending towards whiteness; chlorosis starts from base of leaf.
Mn – The principal veins as well as the smaller veins are green. Interveinal portion is yellowish not tending towards whiteness; Complete Interveinal chlorosis; chequered appearance to leaf; oat and Soybean most affected; The minimum level in healthy oat at flowering is 14 ppm; leaf top remains green and base dies; Grey speck of oat; Pahala blight of sugarcane,
(b) Veins not Remaining Green:
S – Leaves small and veins are paler than interveinal portion, No dead spots; plants not loose the lower leaves as in N- deficiency. Vegetable leaves develop yellowish green colour and become thick and firm.
Cu – Leaf yellowish tending towards whiteness; in extreme deficiency chlorosis of veins occurs and leaf loses lustre. Chlorosis spreads alongside the margins towards the base of leaves; development of several auxiliary buds when shoot apex dies; wheat very susceptible; multiple bud formation in leaf axils, resetting and excess gumming.
Ca – Bud leaf becomes chlorotic white with the base remaining green. About one-third chlorotic portion of the tip hooks downward and becomes brittle, root systems poorly developed or lack fibre.
B – Yellowish or chlorosis which starts from base to tip. Tip becomes very much elongated into a whip like structure and becomes brownish or blackish brown – leaves become thick and margin upwardly rolled; Hollow stem of cauliflower, Browning of cauliflower curd, Top sickness of tobacco and hard fruit of citrus.
Deficiency Symptoms on Both Old and New Leaves:
Zn – Lamina becomes chlorotic and veins remain green; dead spots over leaf including veins, tips and margins; Structural aberrances in root tips; dwarfing of vegetative growth: New leaves emerge white in colour (White bud of maize); Khaira disease of rice; In cereals deficiency appears in 2-4 leaves from top during vegetative stage. Plants bushy due to reduced internodal elongation (malformation).
N – Lodging and abortion of flowers; susceptible to pests and diseases.
P – Deficiency of Fe and Zn.
Fe – Tiny brown spots on lower leaves of rice starting from tips.
Mn – Stunted plant and tillering is often limited in lowland rice.
B – Chlorosis of tips of older leaves especially along the margins.