25 Soil- II (Essential, Macro and Micro Elements and Ion Exchange)

Prof. Ajay Taneja

Contents

1) Fertility of soil

2) Essential elements

3) Macronutrients and their functions

4) Micronutrients and their functions

5) Cation Exchange capacity

6) Anion exchange capacity

7) Exchangeable sodium percentage

8) Significance of Soil Ion Exchange

9) Soil pH

10) Adjustment of soil acidity

11) Nitrogen cycle

12) References

Fertility of Soil

Soil that produces abundant crops under the appropriate environmental conditions is coined the term “fertile”. Soil fertility reflects the soil- plant relationship with reference to plant nutrients available in soil. Understanding the principles of soil fertility is vital to efficient nutrient management, profitable crop protection and environmental protection.

By definition, soil fertility is defined as the innate capacity of soil to provide adequate amount of nutrients essential for plant growth during favorable conditions such as temperature, water and light, and the physical condition of soil while soil productivity is the economic concept that signifies the capability of soil to produce specified plants under well-defined and specified systems of management inputs and environmental conditions. It is a function of several factors rather than being a soil property alone.

Essential elements

For an element to be called as essential, it needs to meet the following criteria-

The plant cannot complete its life cycle (seed to new seed) without it. If the element is specific in its physiological function in plants

If the malady that develops in plant can be remedied only by that element.

The element’s function cannot be replaced by another element.

The element is directly involved in the plant’s growth and reproduction.

Overall, 18 elements have been identified as essential for the growth of all plants. They are further classified not on their relative importance but on basis of their relative amounts into: Macronutrients (C, H, O, N, P, K, Ca, Mg, S) and Micronutrients(B, Cu, Fe, Mn, Zn, Mo, Cl, Co, Ni)

Macronutrients

The elements occurring in ample levels in plant biomass and fluids are known as macronutrients. They are divided into three classes: Structural (C, H, O), Primary (N, P, K) and Secondary (Ca, Mg, S).

Table 1: Macro-elemental nutrient, their uptake form and their plant mobility

	Nutrient	Uptake form	Mobility in Plant	Mobility in Soil
1.	Carbon	CO2,H2CO3
2	Hydrogen	H+, OH-, H2O
3	Oxygen	O2
4	Nitrogen	NO3-, NH4+	Mobile	Mobile as NO3-, immobile as NH4+
5	Phosphorous	HPO42-, H2PO4-	Somewhat mobile	Immobile
6	Potassium	K+	Very mobile	Somewhat mobile
7	Calcium	Ca2+	Immobile	Somewhat mobile
8	Magnesium	Mg2+	Somewhat mobile	Immobile
9	Sulphur	SO4-	Mobile	Mobile

Functions of Macronutrients

(1) Carbon, Hydrogen, Oxygen- It is the foremost component of carbohydrates, proteins and fats (94% of dry weight of plants). They aid in oxidative breakdown of carbohydrates, proteins and fats during cellular respiration. They also provide energy for the growth and development of plants. They are also called structural nutrients.

(2) Nitrogen- It plays key role in, respiration and synthesis of proteins. It maintains fertility when bound to humus. It increases tillering and promotes vegetative growth of food crops. Deficiency leads to decrease in rate and degree of protein synthesis, development of chlorosis

 (3) Phosphorus- It makes up for structural component of membrane systems. Provides resistance to diseases in plants and also influences cell division, root growth, fruit and seed development. Deficiency leads to restriction in root and shoot growth, shedding of leaves and delaying in flowering

 (4) Potassium – It maintains water balance. It is an indispensable constituent of chlorophyll. It provides protection against fungal and bacterial diseases. Deficiency leads to development of chlorosis and drying back tips of shoots and reduced growth of shoots

 (5) Calcium- It controls soil acidity. It is involved in root elongation and cell division. It also acts as a buffer in plant system. It improves intake of plant nutrient by correcting the pH of soil. Deficiency leads to wrinkled leaves, shorter roots, decrease in growth of plant.

 (6) Magnesium- It is an important constituent of chlorophyll. It controls uptake of N and P from soil and also acts as a catalyst in activation of enzymes. Deficiency leads to premature defoliation

 (7) Sulphur- It promotes module formation, root growth and seed formation. It increases oil content in oil bearing plants. It is an essential nutrient for cereals, sugars. Deficiency leads to brittle leaves and stems.

Micronutrients

• These nutrients are required in lesser amounts as they are toxic at high levels
• They stimulate plant growth and actively participate in photosynthesis.
• They show catalytic behavior in plant processes.
• Their deficiency hampers plant growth, photosynthetic rate reducing crop yield.
• Deficiency may cause shedding of flowers, poor seed setting and attack of diseases.

Table 2: Micro-elemental nutrient, their uptake form and their plant mobility

Nutrient	Uptake form	Mobility in Plant	Mobility in Soil	Typical   Range   of Soil Content+
Iron	Fe2+, Fe3+	Immobile	Immobile	0.01-21%
Boron	H3BO3, BO3	Immobile	Very mobile	0.9-1000
Copper	Cu2+	Immobile	Immobile	2.5-60
Chlorine	Cl-	Mobile	Mobile	18-806
Manganese	Mn2+	Immobile	Mobile	<1-18300
Molybdenum	MoO4-	Immobile	Somewhat	0.07-5
Zinc	Zn2+	Immobile	Immobile	1.5-2000
Cobalt	Co2+	Immobile	Somewhat immobile	0.3-200
Nickel	Ni2+	Mobile	Somewhat mobile	0.1-1523

+Unit of measurement is mg/kg except where % is used

Functions of Micronutrients

(1) Iron- Aids in chlorophyll formation

(2) Boron- It regulates nutrients, production of sugar and carbohydrates and also is essential for fruit and seed.

(3) Copper – It is important for the proper reproductive growth. It abets root metabolism and helps in the consumption of proteins

(4) Manganese – It functions with enzyme systems involved in breakdown of carbohydrates and nitrogen metabolism. It regulates the intake and state of oxidation of various elements.

(5) Chloride- It aids plant carbohydrate metabolism and stimulates enzymatic activity

(6) Molybdenum- It helps in the use of nitrogen and in nitrogen fixation as it is constituent of nitrate reductase and nitrogenase enzyme

(7) Zinc- It contributes in mechanism for the transformation of carbohydrates and consumption of sugars and also constitutes a part of enzyme systems that regulate plant growth.

(8) Cobalt- It plays an indispensable role in Vitamin B12 production and thus assists in the process of nitrogen-fixation thus affecting plant growth and metabolism

(9) Nickel- It prevents urea accumulation to toxic levels and leaflet tip necrosis. It is essential for the mobilization of stored seed-nitrogen.

Cation Exchange Capacity (CEC)

It refers to the capacity of soil to exchange cations to enhance the availability of essential trace metals to plants. Mathematically it is defined as number of meq of monovalent cations that can be exchanged per 100g of soil. The value of CEC varies with soil conditions and pH.CEC increases with presence of negatively charged sites on minerals. Humus has high CEC (300-400 meq/100g) while that of organic matter soil (10-30 meq/100g)

Anion Exchange Capacity (AEC)

It is defined as the measurement of the positive charges in soils being affected by the amount of negative charges which a soil can absorb. The value of AEC is directly proportional to change in pH while in case of CEC, reverse trend is seen. The relative strength of adsorption of cations and anions can be illustrated by Lyotrophilic series as follows:

Exchangeable sodium percentage (ESP)

It reflects the alkalinity of soil. If ESP is greater than 15 then soil gets dispersed causing sodium toxicity. It also becomes impervious to water and hence unfavorable for plant growth. Moreover, the availability of plant nutrients is reduced and crop growth is affected. The exchangeable sodium reacts with carbon dioxide forming sodium carbonate which on accumulation renders soil alkaline and ultimately infertile.

                             Soil-Na + H2O               Soil-H + NaOH

2NaOH + CO2                Na2CO3 +H2O

Significance of Soil Ion Exchange

• Base saturation of soil is an indicator of the degree of weathering.
• CEC, supported by the data on clay content is useful for soil management purposes.
• CEC is an indicator of the nutrient storage capacity of soil.
• The information on exchangeable cations is used in soil classification.

Soil pH

It is a measure of soil acidity or alkalinity. pH is called a ‘master’ variable because:

• ü affects chemical, physical, and biological properties of soils
• ü Nutrient availability (optimum pH for most crops is 5.5 – 7)
• ü Metal toxicity and solubility e.g., Al toxicity at pH <5.5
• ü Microbial activity (especially important in the N cycle)pH= 6-8highest N contentpH= 6-7.5                  high P contentpH<6 (acidic soil) Formation of insoluble compounds of P with Fe and Al pH>6 (alkaline soil) Formation of insoluble compounds of P with Ca

Alteration of pH

pH can be reduced by

• Addition of iron or aluminium sulfate, S (forms sulphuric acid)
• Addition of urea, ammonium (nitrate, sulphate and phosphate) and mono potassium phosphate fertilizers
• Addition of pine needles, sawdust and acid peat.
• decomposition of organic matter in the form of plant litter, compost and manure

pH can be increased by

• Addition of lime (CaCO3 or MgCO3)
• Addition of wood and oyster shells
• Addition of calcium silicate (removes free hydrogen ions which in turn leads to neutralization of active acidity)

Adjustment of Soil Acidity

• If the soil becomes too acidic, it can be refurbished by liming
• If soil becomes too alkaline (in areas receiving scanty rainfall), it can be treated with aluminum or iron sulfate, releases acid on hydrolysis.

• Sulphur is added to soils is oxidized by bacterially mediated reactions to sulphuric acid

Nitrogen cycle

Molecular nitrogen from air is fixed by soil organisms. Nitrogen enters into the bodies of soil organism and organic substances (humus) in soil through the process of decomposition and is further recycled through plants and animals.

   Figure 1: Nitrogen cycle

Ammonification is a process in which nitrogen containing organic molecules present in soil is converted into ammonium compounds. The amount of accumulation of ammonium varies with organism, soil and environment.

Nitrification is a process in which ammonium or other reduced forms of nitrogen are oxidized to nitrite and nitrate with the help of nitrifying agents including nitrosomonas and nitrobacter. The nitrate formed is utilized by crops and soil organisms for nutrition purpose.

Denitrification is a process of conversion of nitrite and nitrate into molecular nitrogen. It is caused by Pseudomonas, Bacillus,Paracoccus. The anaerobic condition of soil is conducive to denitrification

Nitrogen cycle leads to formation of microbial cells and organic matter in soil and its intermediate by-products leads to formation of humus in soil.

References:

• Wild, A. Russell’s “Soil conditions and Plant growth”, (11thedn) Longman Scientific and Technical, Harlow, Essex, UK, 1998.
• Epstein, E. “Mineral Nutrition of Plants: Principles and Perspectives”, John Wiley & Sons, Inc. New York USA, 1972.
• Marschner, H. “Mineral Nutrition of higher plants”, Academic Press, London, U.K., 1995
•  Alexander, M. “Introduction to Soil Microbiology”, 2ndedn, John Wiley & Sons, New York, 1972 (Indian reprint, 1983, Wiley Eastern Limited, New Delhi).
• Biswas T. D. and Mukherjee S. K. “Textbook of Soil Science”, Tata Mc Graw- Hill Publishing Company Ltd, New Delhi, 1994.
• Chesworth, W. Ed., “Encyclopedia of Soil Science”, Springer, Dordrecht, The Netherlands, 2008.
• Manahan, S.E. “Environmental Chemistry”, CRC Press, Taylor & Francis, New York, 2010.
• http://www.marietta.edu/~biol/102/ecosystem.html
• http://soil.hostweb.org.uk/
• http://landresources.montana.edu/NM/Modules/mt44492.pdf