Soil formation processes

Correct answer: A. It receives more solar energy

More sunlight increases evaporation and may reduce moisture. For exams, link slope aspect with microclimate.

Correct answer: A. Weathering creates initial material for soil formation

Weathering converts rock into regolith on which soil processes act. In exams, see weathering as the first physical base.

Correct answer: A. Regolith need not have life and horizon development

Regolith is loose weathered material, while soil has biological activity and horizons. In exams, treat soil as a developed natural body.

Correct answer: B. Iron

Oxidation of iron can give soil red or yellow color. In exams, connect color change with chemical weathering.

Correct answer: A. By chemically altering minerals with water

Hydrolysis helps change silicate minerals into new soil minerals. For exams, treat it as a key chemical weathering process.

Correct answer: B. By dissolving minerals with carbon dioxide rich water

Carbon dioxide rich water forms weak acid and dissolves some minerals. In exams, link carbonation with limestone weathering.

Correct answer: A. Acid leaching

Under acidic conditions, some metallic and soluble materials become more mobile. For exams, connect low pH with leaching.

Correct answer: C. Argillic accumulation or illuviation

Clay particles washed from upper layers accumulate in the B horizon. In exams, understand clay accumulation as illuviation.

Correct answer: A. It affects water retention and aeration

The proportion of sand, silt and clay controls water and air availability. For exams, link texture with moisture and aeration.

Correct answer: A. It forms particle aggregates and pore spaces

Good structure improves root penetration, air movement and water flow. In exams, connect structure with porosity.

Correct answer: A. Water holding and nutrient availability

Direct answer: Option A is correct. Humus is the dark, partly decomposed organic material formed from dead leaves, roots, animals, and other remains. It improves soil structure by making soil loose, crumbly, and able to hold air and water. It also stores and slowly releases nutrients, so plants can use them over time. Therefore, the most relevant improvement is greater water-holding capacity and nutrient availability. Option A gives both benefits. Option B is wrong because humus cannot change the height of a slope; slope is a landform feature. Option C is wrong because humus does not determine the hardness of parent rock. Parent rock may weather, but humus is not the cause of its basic hardness. Option D is wrong because humus has no role in increasing the length of a river. The correct concept is soil fertility: more suitable humus generally means better moisture retention, structure, and nutrient supply. Memory cue: humus is the soil’s natural storehouse for moisture and plant nutrients.

Correct answer: A. They mix soil and increase aeration

Earthworms turn and mix soil and help decomposition of organic matter. In exams, consider them natural soil engineers.

Correct answer: A. Mixing and turning of soil by organisms

Organisms, roots and earthworms mix soil particles and affect the profile. For exams, remember bioturbation as biological mixing.

Correct answer: B. Waterlogging and reducing conditions

Poor drainage keeps water in soil and reduces oxygen. In exams, connect drainage with color and aeration conditions.

Correct answer: A. Vegetation is sparse and organic production is low

Low rainfall reduces vegetation, so fewer organic remains enter the soil. For exams, link deserts with low humus.

Correct answer: A. Dense root systems add more organic matter

Grass roots add organic matter deeper into the soil. In exams, link grasslands with a dark humus-rich horizon.

Correct answer: A. More leaching and acidity

Rainfall can wash away basic cations, increasing acidity. For exams, link low base saturation with leaching.

Correct answer: A. Surface area becomes larger

Smaller particles have larger surface area, so water and acids contact more material. In exams, link fine particles with faster reactions.

Correct answer: A. Thin soil on plateau edge and deep soil on valley floor

Material is removed from upper slopes and deposited in lower areas. In exams, compare upper and lower slope positions.

Correct answer: A. When deforestation increases erosion

Deforestation exposes the surface and developed soil may be washed away. In exams, see human impact as either conservation or degradation.

Correct answer: A. Leaching moves materials downward while capillary rise may bring solution upward

Leaching involves downward water movement, while evaporation in dry conditions can draw water upward. In exams, remember the direction difference.

Correct answer: B. Long-term soil forming processes

Horizons form over time through leaching, accumulation and biological activity. For exams, take clear profile as a sign of mature soil.

Correct answer: A. Soil may show properties different from local rock

Material brought by river, wind or glacier may differ from local bedrock. In exams, understand residual and transported soils.

Correct answer: A. Soil developed in place over its parent rock

Residual soil forms in place over its parent rock. For exams, link it with local parent material.

Correct answer: A. In cold dry climate and resistant parent rock

Coldness, dryness and resistant rock slow weathering and biological activity. For exams, look for a combination of unfavorable factors for slow rate.