Soil formation processes

A stable old surface gives opportunity for horizons to develop. Exam tip: connect terrace age with soil maturity.

Base cations may move downward or out with water. Exam tip: connect it with soil chemistry and leaching.

In acidic conditions, material removal from upper layers may be stronger. Exam tip: connect acidic forest with washed horizon.

A hard pan can act as a physical barrier to water and root growth. Exam tip: write hardpan as a barrier.

In sodic conditions, particle dispersion can reduce drainage. Exam tip: connect sodium with poor structure.

Solonisation is a soil-forming process in which soluble salts accumulate in the soil, especially when evaporation is high and drainage is poor. The increasing salt content can make soil less suitable for plant growth. Thus, the process is connected with the soil problem of salinity, not with humus formation, freezing sand, or volcanic activity.

Option A is correct because it directly names the accumulation of salts. Humus is formed from decomposed organic matter, while the other two choices do not describe soil processes. Solonisation is often remembered by linking it with salinisation and the harmful build-up of soluble salts in soil.

Solodisation changes sodic condition toward leached and more acidic features. Exam tip: connect it with transformation of sodic soil.

Halomorphic soils have dominant saline or alkaline conditions. Exam tip: remember halo through salt.

Nitrifying organisms convert nitrogen into available form. Exam tip: connect nitrification with nutrient cycle.

Under anaerobic conditions, some organisms convert nitrate into gas. Exam tip: connect waterlogging with nitrogen loss.

Mineralisation converts organic matter into plant-usable nutrient forms. Exam tip: connect it with nutrient availability.

During growth, microbes can hold nutrients in their biomass. Exam tip: connect immobilisation with temporary locking.

At high carbon nitrogen ratio, organisms need extra nitrogen for decomposition. Exam tip: connect the ratio with decomposition rate.

Aggregates make soil structure stable and help roots. Exam tip: connect aggregates with structure.

Clay and humus are soil colloids. Their very large surface area and mainly negative charges attract and retain nutrient cations such as \(Ca^{2+}\), \(K^+\) and \(Mg^{2+}\). Thus, these nutrients are less easily leached away with water, increasing the soil’s nutrient-holding capacity. Large surface area alone is not enough; electrical charge is also needed to hold ions. Exam tip: link soil colloids and cation-exchange capacity with nutrient retention.

Colloids are tiny but effective in holding and exchanging ions. Exam tip: treat colloids as central to soil chemistry.

Direct answer: Option A is correct. Cation exchange capacity, or CEC, describes how well soil particles—especially clay and humus—hold positively charged nutrient ions such as calcium, magnesium, potassium, and ammonium. These nutrients can be exchanged with ions in the soil solution and taken up by plant roots. A high CEC means the soil can retain a larger store of nutrient cations and reduce their immediate loss through leaching. This often supports soil fertility, although fertility also depends on which nutrients are present and the soil’s pH. Option A is therefore correct. Option B is wrong because CEC does not create volcanoes. Option C is wrong because soil does not become only water; CEC concerns nutrient-ion retention. Option D is wrong because CEC cannot stop rainfall. The key idea is a nutrient-holding surface: more exchange sites generally mean greater ability to hold nutrient cations. Memory cue: high CEC means “more seats” for positively charged nutrients.

Negatively charged surfaces hold cations while some anions may move with water. Exam tip: remember the charge and leaching relation.

High buffering capacity protects soil from sudden chemical change. Exam tip: connect it with pH stability.

Lime can reduce acidity and improve nutrient availability. Exam tip: remember it as a correction for acidity.

Dryness reduces biological production and can leave salts near the surface. Exam tip: connect dry climate with salt accumulation.

Small surface differences affect water retention and material accumulation. Exam tip: connect microtopography with local soil differences.

Soil forms through interaction of many factors, not one cause. Exam tip: write interaction of factors in answers.