Chapter 9: Soil
मृदा (मृदा परिच्छेदिका, संस्तर स्थितियाँ, बलुई, मृण्मय एवं दुमटी मिट्टी, जल अंतःस्रवण दर)
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- Soil Formation: Soil is generated by weathering of parental rocks by the kinetic and chemical actions of wind, rain, temperature shifts, and biological organisms over thousands of years.
- Soil Profile (मृदा परिच्छेदिका - Vertical Section):
- - A-Horizon (Topsoil / शीर्षमृदा): Dark in color, rich in dark organic matter (Humus), soft, porous, high water-retention capacity; habitat for earthworms, rodents, beetles, and plant roots.
- - B-Horizon (Subsoil / मध्य परत): Less humus, but richer in inorganic mineral salts; harder and more compact.
- - C-Horizon: Composed of cracked, weathered rock lumps with fissures.
- - Bedrock (आधार शैल): Non-porous massive solid rock below C-horizon; cannot be dug with a spade.
- Soil Textural Types:
- - Sandy Soil (बलुई मिट्टी): Large particles with wide spaces filled with air; water drains out very rapidly; light, dry, and aerated; poor water retention.
- - Clayey Soil (मृण्मय मिट्टी): Extremely tiny, fine particles tightly packed; almost no air spaces; retains water tenaciously; heavy and sticky when wet; ideal for crops requiring standing water like paddy (rice) and wheat.
- - Loamy Soil (दुमटी मिट्टी): Optimum mixture of sand, clay, and silt along with rich humus; possesses the ideal balance of aeration and water holding; BEST SOIL FOR GROWING VEGETABLES AND CROPS.
- Percolation Rate Calculation Formula (Guaranteed Numerical MCQ):
- $$\text{Percolation Rate (mL/min)} = \frac{\text{Amount of water (mL)}}{\text{Percolation time (min)}}$$
- - Sandy soil has the HIGHEST percolation rate (water drains fast).
- - Clayey soil has the LOWEST percolation rate and highest water retention capacity.
- Trap: Which soil has the highest water percolation rate? Correct: SANDY SOIL (बलुई मिट्टी), because of wide particle gaps.
- Trap: Which soil has the highest water retention capacity? Correct: CLAYEY SOIL (मृण्मय मिट्टी), because fine particles hold water tightly.
- Trap: In percolation numericals, students often invert the formula. Remember: Amount of water in mL divided by Time in minutes, NOT time divided by water.
NCERT Core Facts: Pedological Horizons & Percolation Equations
CTET Paper 2 science frequently tests soil profile horizons and numerical percolation calculations:
1. Soil Profile Horizons (शीर्ष से आधार तक)
[ A-Horizon (Topsoil) ] ── Rich in Humus; porous; anchors roots; dark color
│
[ B-Horizon (Subsoil) ] ── Rich in minerals; less organic matter; compact
│
[ C-Horizon ] ── Weathered rock chunks with cracks & crevices
│
[ Bedrock (आधार शैल) ] ── Solid unweathered rock; impervious base2. Comparative Soil Matrix
| Soil Type | Particle Size | Aeration Level | Water Drainage / Retention | Ideal Crops |
|---|---|---|---|---|
| Sandy (बलुई) | Large | High (O_2 rich) | Drains instantly; very low retention | Cactus, melons, peanuts |
| Clayey (मृण्मय) | Very Fine | Low (poor aeration) | Drains very slowly; highest retention | Paddy (Rice), Wheat, Gram |
| Loamy (दुमटी) | Balanced mix | Ideal | Retains optimum moisture with good aeration | Lentils, Mustard, Vegetables |
3. Master Numerical: Water Percolation Rate
A student poured 200 mL of water through a soil sample cylinder. It took 40 minutes for the water to percolate completely.
Test Your Mastery: Soil
Authentic CBSE exam questions tested directly from this NCERT chapter.
In a science laboratory experiment on soil, a student observes that 300 mL of water takes 60 minutes to percolate through a given soil sample. What is the rate of percolation of water for this soil?
मृदा पर एक विज्ञान प्रयोगशाला प्रयोग में, एक छात्र देखता है कि दिए गए मिट्टी के नमूने से 300 mL पानी को अंतःस्रवित होने में 60 मिनट का समय लगता है। इस मिट्टी के लिए जल अंतःस्रवण की दर क्या है?
Which of the following soil types possesses the HIGHEST water retention capacity and the LOWEST percolation rate?
निम्नलिखित में से किस प्रकार की मिट्टी में जल धारण क्षमता सबसे अधिक और जल अंतःस्रवण दर सबसे कम होती है?
Which horizon of the soil profile is richest in humus and supports most biological life?
The A-horizon (Topsoil), because decomposing plant and animal debris accumulate at the surface, creating rich dark organic humus.
Which soil type is best suited for growing garden crops and wheat?
Loamy soil, because its mixture of sand, clay, silt, and humus provides both adequate drainage and balanced water retention.