Unit 1 - Soil Fertility and Plant Nutrition | MSc Horticulture & Agronomy

Soil Fertility and Plant Nutrition

HORSS-201 & AGRMI-202

UNIT I

Soil Fertility Concept

Soil fertility refers to a soil’s capacity to sustain plant growth by providing essential nutrients, water, oxygen, and a favourable physical, chemical, and biological environment.

Key characteristics of fertile soil include:

• Nutrient supply: Adequate levels of 17 essential elements (see below) in plant-available forms.
• Absence of toxins: Low concentrations of harmful substances (e.g., excess heavy metals).
• Structure: Good aggregation, porosity, and organic matter (humus) for moisture retention and root penetration.
• pH balance: Optimal range of 5.5–7.5 for nutrient availability and microbial activity.
• Maintains adequate organic matter and biological activity.
• It has good moisture retention and proper drainage.

Soil Fertility vs. Soil Productivity

• Soil Fertility: The nutrient-supplying capacity of the soil.
• Soil Productivity: The ability of the soil to produce crops under a defined set of management practices.

Note: All productive soils are fertile, but all fertile soils may not be productive.

Types of Soil Fertility

1. Natural (Inherent) Fertility: Fertility present without human intervention.
2. Acquired Fertility: Fertility developed through external inputs like fertilizers, manures, irrigation, etc.
3. Apparent Fertility: Temporary fertility based on favorable conditions like good weather or irrigation.

Fertility Evaluation

• Done through soil testing, plant tissue analysis, and crop response studies.
• Parameters like available N, P, K, pH, EC, organic carbon, and micronutrients are analysed.

Factors Affecting Soil Fertility

Physical Factors

1. Soil texture:
   
1. Loam (balanced clay, sand, silt) is ideal for nutrient retention and root growth.
2. Clay soils retain nutrients but may restrict aeration; sandy soils drain quickly.
2. Organic matter (humus): Enhances nutrient retention (especially nitrogen and phosphorus), soil structure, and microbial activity.
   
3. Moisture and aeration: Adequate moisture ensures nutrient solubility, while oxygen supports root respiration and aerobic microbes.
   
4. Soil depth: Deeper soils allow extensive root development and water storage.
   
5. Structure: Good structure promotes aeration and root penetration.
   
6. Porosity and Compaction: Affects oxygen supply and water movement.
   
7. Temperature: Influences microbial activity and nutrient availability.

Chemical Factors

1. Soil pH: Affects nutrient solubility (e.g., phosphorus becomes unavailable in acidic soils).
2. Cation exchange capacity (CEC): Determines the soil’s ability to hold positively charged ions (e.g., K⁺, Ca²⁺, Mg²⁺).
3. Mineral composition: Parent material influences nutrient availability (e.g., potassium in feldspar).

Biological Factors

1. Soil biota: Microbes (bacteria, fungi) decompose organic matter, fix nitrogen, and solubilize minerals.
2. Pathogen balance: Beneficial microbes suppress plant diseases.

Management Factors

1. Crop rotation: Prevents nutrient depletion (e.g., legumes fix nitrogen).
2. Tillage practices: Over-tillage reduces organic matter and disrupts soil structure.

Essential and Beneficial Elements

Essential Elements

Essential elements are nutrients indispensable for plant growth, development, and reproduction. The deficiency of any essential element cannot be substituted by another element, and it must be directly involved in plant metabolism.

Criteria of Essentiality (Arnon and Stout, 1939)

An element is considered essential if:

1. A plant cannot complete its life cycle without it.
2. The element is part of a vital plant constituent or metabolite.
3. Its function cannot be replaced by another element.

Classification of Essential Elements

A. Based on Quantity Required:

1. Macronutrients (Required in large quantities)
1. Primary Macronutrients: Nitrogen (N), Phosphorus (P), Potassium (K)
2. Secondary Macronutrients: Calcium (Ca), Magnesium (Mg), Sulfur (S)
2. Micronutrients (Required in small quantities)
1. Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni)

B. Based on Function:

1. Structural elements: C, H, O (make up 96% of plant dry matter; obtained from air and water)
2. Energy-related elements: N, P, S (involved in proteins, nucleic acids, ATP)
3. Charge balance and osmotic potential: K, Ca, Mg
4. Enzyme activators or cofactors: Fe, Zn, Mn, Cu, Mo, B, Cl, Ni

17 elements are critical for plant life cycles:

Element	Key Functions	Deficiency Symptoms
Nitrogen (N)	Protein Synthesis, chlorophyll, and growth	Yellowing of older leaves (chlorosis)
Phosphorus (P)	Energy transfer (ATP), root growth	Purplish leaves, poor root development
Potassium (K)	Enzyme activation, Osmotic regulation, disease resistance	Marginal chlorosis and scorching
Calcium (Ca)	Cell wall stability, membrane integrity	Tip burn, poor root growth
Magnesium (Mg)	Central atom in chlorophyll, enzyme activation	Interveinal chlorosis in older leaves
Sulfur (S)	Proteins, vitamins	General yellowing of young leaves
Micronutrients
Iron (Fe)	Chlorophyll formation, electron transport	Interveinal chlorosis of young leaves
Zinc (Zn)	Auxin synthesis, enzyme activation	Small leaves, rosetting
Boron (B)	Cell wall formation, sugar transport	Brittle leaves, poor fruit/seed set
Copper (Cu)	Photosynthesis, lignin synthesis	stunted growth, yellowing, leaf curling
Molybdenum (Mo)	Nitrogen fixation, nitrate reduction	yellowing of leaves, stunted growth, and deformed leaves
Chlorine (Cl)	Osmotic balance, photosynthesis	wilting of leaves, especially at the margins
Manganese (Mn)	Photosynthesis, enzyme activator	Interveinal chlorosis + necrotic spots

Beneficial Elements

These are not essential for all plants but beneficial or essential for some species or under specific conditions. They may:

• Improve growth or yield
• Replace essential elements in some metabolic roles
• Be essential for certain lower plants or specialized functions

Examples:

• Sodium (Na): Beneficial for C4 and CAM plants (e.g., sugar beet) and Enhances osmotic regulation in some crops (e.g., spinach).
  
• Silicon (Si): Enhances drought/pest/disease resistance, mechanical strength (not essential but very beneficial for grasses and rice)
  
• Cobalt (Co): Required by nitrogen-fixing bacteria in legumes
  
• Selenium (Se): Beneficial for some plants, toxic in excess
  
• Aluminum (Al): Beneficial for tea plant root growth in acidic soils (toxic in most others)

Table: Essential Elements and Their Typical Concentration

Element	Symbol	Typical Range (ppm in soil)
Nitrogen	N	0.1–0.5% (organic)
Phosphorus	P	5–50 ppm
Potassium	K	100–500 ppm
Calcium	Ca	1000–10,000 ppm
Magnesium	Mg	100–1000 ppm
Sulphur	S	10–200 ppm
Iron	Fe	2–100 ppm
Zinc	Zn	0.5–5 ppm
Copper	Cu	0.2–2 ppm
Manganese	Mn	5–50 ppm
Boron	B	0.1–2 ppm
Molybdenum	Mo	0.1–5 ppm
Chlorine	Cl	100–3000 ppm
Nickel	Ni	<1 ppm

Key Interactions

• Law of the Minimum: Plant growth is limited by the scarcest nutrient.
• Nutrient immobilization vs. mineralization: Microbial activity determines nutrient availability.
• pH-dependent solubility: Example: Iron becomes inaccessible in alkaline soils, causing chlorosis.

---

Soil Fertility and Plant Nutrition-  Click here for Notes of all units

Want a PDF of all the notes? Send a DM to me: https://www.instagram.com/puspendrps