Soil formation processes

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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