Soil Properties
Physical properties (texture, structure, bulk density, porosity), chemical properties (pH, CEC, organic matter, soil reaction), soil water constants (field capacity, PWP), soil colour and horizon development.
Frequently Asked Questions
What is the soil texture triangle and why does it matter for exams?
The soil texture triangle classifies soils into 12 textural classes based on relative percentages of sand, silt, and clay. Loam is the central class and most ideal for agriculture. Exam focus: clay has the highest water-holding capacity and CEC; sand has the highest permeability and lowest water retention. Particle size: sand 0.05–2 mm, silt 0.002–0.05 mm, clay < 0.002 mm.
What is CEC and what are the typical values for different soil materials?
CEC (Cation Exchange Capacity) is the total capacity of soil to hold exchangeable cations — measured in cmol(+)/kg. Typical values: sand 1–5, loam 10–15, clay 20–50, organic matter 200–400 cmol/kg. Montmorillonite clay has the highest CEC (80–150 cmol/kg); kaolinite the lowest (3–15 cmol/kg). This is why black soils (Vertisols) have higher fertility than red soils.
What is the difference between field capacity and permanent wilting point?
Field capacity (FC) is the moisture content retained after excess water drains away — held at −0.33 bar (−33 kPa) tension. Permanent wilting point (PWP) is the moisture level plants cannot extract — held at −15 bar (−1500 kPa). Available water = FC − PWP. Loamy soils store the most available water; sandy soils the least.
What are kaolinite and montmorillonite and how do they differ?
Kaolinite is a 1:1 clay mineral (one silica + one alumina sheet) — non-expanding, low CEC (3–15 cmol/kg), dominant in highly weathered tropical red soils. Montmorillonite is a 2:1 clay mineral (two silica sheets sandwiching one alumina sheet) — expanding lattice, very high CEC (80–150 cmol/kg), dominant in black cotton soils (Vertisols). The expanding nature of montmorillonite causes cracking in dry conditions.
What is bulk density and what does a high value indicate?
Bulk density (BD) = dry soil mass ÷ total soil volume (including pores), expressed in g/cm³. Ideal BD for crop growth: 1.1–1.3 g/cm³ for loam; compacted soils > 1.6 g/cm³ restrict root growth. Particle density of mineral soil is always ~2.65 g/cm³. BD increases with compaction and decreases with organic matter addition.
Which exams test soil properties most heavily?
Soil properties carry high weightage in IBPS AFO (3–5 questions on soil science per exam), NABARD Grade A (Agriculture paper), ICAR JRF Soil Science discipline (20+ questions), Pre-PG entrances at IARI, BHU, ANGRAU, and TNAU, and state Agriculture Officer PSC exams. Most tested: soil texture triangle, clay minerals, CEC values, pH ranges, field capacity vs PWP, and USDA soil taxonomy.