Humification leads to:

The correct answer is Option (3) → Accumulation of humus

Decomposition breaks dead organic matter (detritus) into simple inorganic substances. Its steps are fragmentation, leaching, catabolism, humification and mineralisation. Humification is the step that forms humus, a dark material that breaks down very slowly and stores nutrients in the soil.

Option (1) Soil erosion — Incorrect. Soil erosion is the removal of topsoil by wind or water. It is not a step of decomposition.

Option (2) Soil conservation — Incorrect. Soil conservation means human methods of protecting soil, not a natural step of decomposition.

Option (3) Accumulation of humus — Correct. Humification builds up humus, which acts as a reservoir of nutrients.

Option (4) Accumulation of salts — Incorrect. Inorganic nutrients are released by mineralisation, the next step. A build-up of salts in soil is salinity, not humification.

NCERT Extract:
Decomposition – “… Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism. It is important to note that all the above steps in decomposition operate simultaneously on the detritus (Figure 12.1). Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.”