Agriculture for Engineers 4 (3+1)

Lesson 14. Essential Plant Nutrients

14.2 The soil as a Nutrient Source for Plants

14.2.1 Mineral Nutrients in the Soil

Mineral nutrients occur in the soil in both dissolved and bound form. Only a small fraction (less than 0.2%) of the mineral nutrient supply is dissolved in soil water. Most of the remainder, i.e., almost 98% is either bound in organic detritus, humus and relatively insoluble inorganic compounds or incorporated in minerals. These constitute a nutrient reserve, which becomes available very slowly as a result of weathering and mineralization of humus. The remaining 2% is adsorbed on soil colloids. The soil solution, the soil colloids and the reserves of mineral substances in the soil are in a state of dynamic equilibrium, which ensures continued replenishment of supplies of nutrient elements.

14.2.2 Adsorption and Exchange of ions in the soil

Both clay minerals and humic colloids have a negative net charge so that they attract and adsorb primarily cations. There are also some positively charged sites where anions can accumulate. How tightly a cation is held depends on its charge and degree of hydration. In general, ions with high valences are attracted more strongly for example; Ca²⁺ is more strongly attracted than K⁺. Among ions with the same valence those with little hydration are retained more firmly than those that are strongly hydrated. The tendency for cations adsorption decreases in the order Al³⁺, Ca²⁺, Mg²⁺, NH₄⁺, K⁺ and Na⁺

The swarm of ions around particles of clay and humus works as an intermediary between the solid soil phase and the soil solution. If ions are added to or withdrawn from the soil solution, exchange takes place between solid and liquid phases. Adsorptive binding of nutrient ions offers a number of advantages nutrients liberated by weathering and the decomposition of humus are captured and protected from leaching the concentration of the soil solution is kept low and relatively constant; so that the plant roots and soil organisms are not exposed to extreme osmotic conditions; when required by the plant, however, the adsorbed nutrients are readily available.

Nutrient release and path for absorption

14.3 Essential and beneficial elements

14.3.1 The criteria of essentiality: In order to distinguish elements, which are essential from those which may be taken up by the plant but are not essential, Arnon (1954) has laid down the following criteria:

(1) The plant must be unable to grow normally or complete its life-cycle in the absence of the element;

(2) The element is specific and can not be replaced by another; and

The element plays a direct role in metabolism.

Arnon’s criteria of essentiality of elements for plant growth may be more correct. However, it appears to be too regid from practical point of view. Nicholas proposed the term “Functional or Metabolic nutrients”, whether or not its action is specific. Elements such as sodium, cobalt, vanadium etc are classified as essential when less restrictive definition of essentiality is used.

 

14.3.2 Classification of essential plant nutrients

 (i) On the basis of amount of nutrients present in plants, they can be classified in to three groups:

 Nutrients

	Average concentration in plant tissue	Relative numbers of atoms compared to Mo as one	Function in plant	Nutrient category
N	1.5%	1,000,000	Proteins, amino acids	Primary Macronutrients
P	0.2%	30,000	Nucleic acids, ATP
K	1.0%	400,000	Catalyst, ion transport
Ca	0.5%	200,000	Cell wall component	Secondary
Mg	0.2%	100,000	Part of chlorophyll
S	0.1%	30,000	Amino acids
Fe	100 mg/kg	2,000	Chlorophyll synthesis	Micronutrients
Cu	6 mg/kg	100	Component of enzymes
Mn	20 mg/kg	2000	Activates enzymes
Zn	20 mg/kg	300	Activates enzymes
B	20 mg/kg	2000	Cell wall component
Mo	0.1 mg/kg	1	Involve in N fixation
Cl	100 mg/kg	3,000	Photosynthesis reactions

 

(ii) According to mobility

    (a) In soil:

          1. Mobile: NO₃^-, SO₄^2-, BO₃^3-, Cl^- and Mn^2-

          2. Less mobile: NH₄^2-, K⁺, Ca²⁺, Mg²⁺ and Cu²⁺

3. Immobile: H₂PO₄^-, HPO₄^2- and Zn²⁺

    (b) In plant:

1. Highly mobile: N, P and K

2. Moderately mobile: Zn

3. Less mobile: S, Fe, Mn, Cu, Mo and Cl

4. Immobile: Ca and B

(iii) According to metal and non metal

1. Metal: K, Ca, Mg, Fe, Mn, Zn and Cu

2. Non metal: N, P, S, B, Mo and Cl

(iv) According to cation and anion

1. Cation: K, Ca, Mg, Fe, Mn, Zn and Cu

2. Anion: NO₃, H₃PO₄ and SO₄

14.4 Beneficial elements

Apart from vanadium, silicon, aluminum, iodine, selenium and gallium, which have been shown to be essential for particular species of plants, there are several other elements, like rubidium, strontium, nickel, chromium and arsenic, which at very low concentrations and often under specific conditions have been shown to stimulate the growth of certain plants or to have other beneficial effects. These elements, the essentiality of which for growth and metabolism has not been unequivocally established but which are shown to exert beneficial effects at very low concentrations are often referred to as 'beneficial elements', 

14.5 Forms of nutrients in soil

In soil, Nutrient present in different forms are as under:

S. No.	Nutrient	Forms
1.	Nitrogen	Organic N (97%) and Mineral N  NH4+, NO3-
2.	Phosphorus	Solution P, Calcium, Iron, Aluminium and Occluded P, Organic  P (25%-90%) and  Mineral P
3.	Potassium	Water soluble K, Exchangeable K, Fixed K and Mineral K (90-98%),
4.	Sulphur	Sulphate S, Non sulphate S, Adsorbed S, Organic S(95%)  and Total S,
5.	Micronutrients	Water soluble ion,  Exchangeable, Adsorbed, chelated or complexed ion, Cation held in secondary clay mineral and insoluble metal oxides and cation held in primary mineral

 

14.6 Mechanisms of nutrient transport to plants

          Two important theories, namely, soil solution theory and contact exchange theory explain nutrient availability to plants.

14.6.1 Soil solution theory :

(a) Mass flow: Movement of nutrient ions and salts along with moving water.

(b) Diffusion : Occurs when there is concentration gradient of nutrients between root surface and surrounding soil solution. Ions move from the region of high concentration to the region of low concentration.

14.6.2 Contact exchange theory:

The important of contact exchange in nutrient transport is less than with soil solution movement. A close contact between root surface and soil colloids allows a direct exchange of ions.

 14.7 Role of Macro and Micro-nutrients

 Essential elements perform several functions. They participate in various metabolic processes in the plant cells such as permeability of cell membrane, maintenance of osmotic concentration of cell sap, electrontransport systems, buffering action, enzymatic activity and act as major constituents of macromolecules and co-enzymes.

Nutrient (Element)	Role of Nutrients
1.Nitrogen (N)	(1)  Nitrogen is constituent of chlorophyll. (2)  N makes plant dark green. (3) It increases vegetative growth of protein content and cation   exchange capacity in plant roots. (4)  Encourages the formation of good quality foliage.
2.Phosphorus (P)	(1) It stimulates root growth and formation. (2) It helps in cell division and hastens maturity. (3) It makes plant more tolerant to drought, cold, insects and diseases (4) It increases P and Ca in plants and also increases nodule formation in pulses. (5) It increases tillers and ratio of grain to straw in crop.
3.Potassium (K)	(1) K helps in translocation also imparts, vigour and growth to plants. (2) It makes plant more tolerant to drought, cold, insects and diseases (3) It reduces lodging and increases the availability of N and P. (4) It increases the size of root and tuber.
4. Calcium (Ca)	(1) It promotes early root growth. (2) Ca is constituent of cell and increases stiffness in straw (stem). (3) It increases the calcium content in plants and also increases the nodulation of legumes. (4) It improves soil structure and keeps soil neutral.
5. Magnesium (Mg)	(1) It is a constituent of chlorophyll. (2) It increase photosynthesis and regulates uptake of nutrients. (3) It also promotes the formation of oils and fats.
6. Sulphur (S)	(1) It helps in chlorophyll formation. (2) It stimulates root growth, seed formation and nodule formation. (3) It encourages plant growth. (4) S is constituent of enzymes and proteins and increases oil content.
7. Iron (Fe)	(1) It helps in chlorophyll formation. (2) It acts as oxygen carrier and helps in protein synthesis.
8. Manganese (Mn)	(1) It acts as a catalyst in oxidation reduction reaction and as an activator of many enzymes. (2) It also helps in chlorophyll synthesis.
9. Zinc (Zn)	(1) It constituent of a number of enzymes. (2) It helps in formation of growth hormones and enhances heat and frost resistance. (3) It also acts as catalyst in chlorophyll formation.
10. Boron (B)	(1) It helps in uptake and efficient utilization of calcium. (2) It also helps in protein synthesis.
11. Copper (Cu)	(1) It helps in oxidation-reduction reaction. (2) It also plays constituent of certain protein.
12. Molybdenum       (Mo)	(1) It helps in fixation of atmospheric nitrogen by nodule bacteria in legume. (2) It also helps in protein synthesis.
13. Chlorine (Cl)	(1) It is essential for photosynthesis process and keeps osmotic pressure normal in cell sap. (2) It encourages growth in crop plants.