Soil Fertility and Plant Nutrition

1. Soil fertility = ability to supply nutrients.
2. Essential nutrients = 17 (macros + micros).
3. Macronutrients: N, P, K, Ca, Mg, S.
4. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
5. Liebig’s Law of Minimum — yield limited by most deficient nutrient.
6. Law of Diminishing Returns — yield increase declines with each input.
7. Deficiency symptoms depend on nutrient mobility.
8. Soil testing identifies nutrient status.
9. Fertilizer use efficiency improved by split doses and balanced application.

Problem Soils and Reclamation

1. Acid soils – pH < 6.5, corrected with lime.
2. Saline soils – EC > 4 dS/m, pH < 8.5.
3. Sodic soils – ESP > 15, pH > 8.5.
4. Gypsum used for sodic soil reclamation.
5. Leaching removes excess salts using good-quality water.
6. Drainage prevents waterlogging and salinization.

Soil Physics

1. Soil physics studies physical properties and processes of soil.
2. Soil texture affects porosity, water movement, and nutrient retention.
3. Fine-textured soils hold more water than sandy soils.
4. Bulk density increases with compaction and decreases with organic matter.
5. Porosity = (1 – bulk density / particle density) × 100.
6. Average particle density of soil = 2.65 g/cm³.
7. Infiltration rate = rate at which water enters the soil surface.
8. Percolation = downward movement of water through soil.
9. Hydraulic conductivity measures water transmission through soil pores.
10. Capillary rise is greater in clay soils due to smaller pores.
11. Field capacity is reached after gravitational water drains away.
12. Permanent wilting point = when plants cannot extract water.
13. Available water = field capacity – wilting point.
14. Soil moisture constants are important for irrigation scheduling.
15. Soil compaction decreases infiltration and root penetration.
16. Soil aggregates improve structure and aeration.
17. Crusting reduces infiltration and seedling emergence.
18. Soil tilth improves with organic matter and proper tillage.
19. Soil temperature controls biological activity and germination.
20. Thermal conductivity increases with moisture content.

Soil Chemistry

1. Soil chemistry deals with composition and reactions in soil.
2. Soil solution is the liquid phase containing dissolved nutrients.
3. Nutrient ions move by mass flow, diffusion, and root interception.
4. Adsorption = attachment of ions on soil colloid surfaces.
5. Desorption = release of ions back to the solution.
6. Cation Exchange Capacity (CEC) measures soil’s nutrient-holding ability.
7. CEC is higher in clay and organic matter-rich soils.
8. Anion exchange occurs in acidic soils with positive charge on colloids.
9. Base saturation = % of CEC occupied by basic cations.
10. pH < 7 = acidic; pH > 7 = alkaline; neutral = pH 7.
11. Soil buffering resists change in pH.
12. Lime neutralizes acidity by releasing Ca²⁺ and carbonate ions.
13. Gypsum corrects sodicity by replacing Na⁺ with Ca²⁺.
14. Redox potential indicates oxidation-reduction status.
15. Reduced soils form under waterlogged or anaerobic conditions.
16. Oxidized soils are well-drained and aerated.
17. Soil salinity measured by electrical conductivity (EC).
18. Sodicity measured by exchangeable sodium percentage (ESP).
19. Sodium adsorption ratio (SAR) = Na⁺ / √(Ca²⁺ + Mg²⁺)/2.
20. Fertility and chemical balance depend on exchangeable ions.

Soil Biology and Microorganisms

1. Soil biology studies living organisms in soil.
2. Bacteria dominate soil microflora and drive nutrient cycling.
3. Fungi decompose organic matter and form mycorrhizal associations.
4. Actinomycetes decompose resistant materials like cellulose.
5. Algae contribute to soil fertility by fixing nitrogen.
6. Protozoa feed on bacteria and regulate microbial populations.
7. Earthworms improve aeration and soil aggregation.
8. Nitrosomonas converts NH₄⁺ to NO₂⁻ (first step of nitrification).
9. Nitrobacter converts NO₂⁻ to NO₃⁻ (second step of nitrification).
10. Denitrification converts nitrate into gaseous nitrogen forms.
11. Rhizobium fixes atmospheric nitrogen in legumes.
12. Azotobacter and Azospirillum are free-living N-fixers.
13. Blue-green algae (Cyanobacteria) fix nitrogen in paddy fields.
14. Mycorrhizae increase phosphorus and water uptake.
15. Soil enzymes like urease, phosphatase, and dehydrogenase catalyze reactions.
16. Soil respiration indicates microbial and root activity.
17. Biological nitrogen fixation (BNF) reduces fertilizer dependence.
18. Organic carbon fuels microbial metabolism.
19. Soil biodiversity ensures ecosystem stability.
20. Decomposition releases nutrients for plant uptake.