Introduction to Soil Science

1. Soil is the natural medium for plant growth.
2. Pedology studies soil formation, classification, and mapping.
3. Edaphology studies soil’s influence on plants and organisms.
4. Pedogenesis is the process of soil formation.
5. The five soil-forming factors are parent material, climate, organisms, relief, and time.
6. Soil profile shows vertical layers called horizons (O, A, B, C, R).
7. The smallest sampling unit of soil is a pedon.
8. A group of similar pedons forms a polypedon.

Soil Formation and Weathering

1. Weathering is the breakdown of rocks into smaller particles.
2. Physical, chemical, and biological processes contribute to weathering.
3. Parent material determines initial soil properties.
4. Residual soils form in place; transported soils are moved by agents like wind or water.
5. Alluvial, colluvial, and aeolian soils are examples of transported soils.
6. Eluviation = removal of materials; Illuviation = accumulation of materials.

Soil Physical Properties

1. Texture = proportion of sand, silt, and clay.
2. Structure = arrangement of soil particles into aggregates.
3. Types of soil structure: granular, blocky, prismatic, platy.
4. Bulk density indicates compaction; ideal range = 1.1–1.6 g/cm³.
5. Porosity determines total pore space.
6. Permeability refers to ease of water movement.
7. Soil color indicates organic matter, oxidation, and drainage.
8. Munsell color system classifies color by hue, value, and chroma.
9. Soil consistency shows resistance to deformation.
10. Tilth refers to the physical condition suitable for plant growth.

Soil Water

1. Soil water exists as gravitational, capillary, and hygroscopic water.
2. Field capacity = water retained after drainage.
3. Wilting point = plants cannot extract water.
4. Available water = field capacity – wilting point.
5. Soil moisture tension measures energy status of soil water.
6. Infiltration = water entry into soil; Percolation = downward movement.
7. Soil water potential includes matric, osmotic, and gravitational components.

Soil Air and Temperature

1. Soil air has more CO₂ and less O₂ than atmosphere.
2. Aeration affects microbial activity and root growth.
3. Anaerobic conditions cause denitrification and methane emission.
4. Soil temperature influences seed germination and enzyme activity.
5. Heat transfer in soil occurs by conduction, convection, and radiation.

Soil Chemical Properties

1. Soil pH shows acidity or alkalinity.
2. Ideal pH range for crops = 6.0–7.5.
3. Cation Exchange Capacity (CEC) indicates nutrient-holding ability.
4. Anion exchange occurs in acidic soils with positive charges.
5. Base saturation = % of exchange sites occupied by basic cations.
6. Buffering capacity resists pH change.
7. Soil colloids hold nutrients and water.

Soil Biological Properties

1. Soil microorganisms decompose organic matter.
2. Rhizobium fixes nitrogen in legumes.
3. Azotobacter and Azospirillum fix N in non-legumes.
4. Cyanobacteria (BGA) fix nitrogen in paddy soils.
5. Mycorrhizae improve nutrient and water uptake.
6. Actinomycetes decompose cellulose and lignin.
7. Soil enzymes catalyze nutrient transformations.
8. Soil respiration measures microbial and root activity.

Soil Organic Matter

1. Humus is stable decomposed organic matter.
2. Organic matter improves structure and CEC.
3. Mineralization converts organic nutrients to inorganic.
4. Immobilization converts nutrients into organic form.
5. Humification forms stable humus.
6. Soil organic carbon (SOC) is key to soil health and carbon sequestration.

Soil Classification

1. Soil taxonomy groups soils by measurable properties.
2. There are 12 USDA soil orders (Entisols, Inceptisols, Vertisols, etc.).
3. Entisols – young soils; Inceptisols – weak horizon; Vertisols – shrink-swell clays.
4. Aridisols – arid; Mollisols – fertile grassland soils; Alfisols – moderately leached.
5. Ultisols – acidic, low base saturation; Oxisols – highly weathered tropical soils.
6. Andisols – volcanic ash; Histosols – organic; Gelisols – permafrost.