Weathering and Soil Formation

Humus

Step 1: Soil contains both inorganic matter (like sand, silt, clay) and organic matter. Step 2: The organic matter in soil is called humus. Step 3: Humus is dark brown or black in colour and is derived from the decomposition (breaking down) of dead plants and animals. Step 4: Humus is very important because it adds nutrients to the soil and improves its fertility, helping crops grow well. Step 5: Regolith is the subsoil (B-horizon), silica is a mineral, and kaolin is clay formed from feldspar — none of these are the organic matter in soil.

A-horizon

Step 1: A soil profile shows the different layers of soil from top to bottom. Step 2: The O-horizon is the very top layer, made of decayed organic matter (humus). It is missing in dry/arid areas. Step 3: The A-horizon, also called topsoil, lies just below the O-horizon. It is rich in both humus and minerals, and contains most of the nutrients needed for plant growth. Farmers value this layer greatly. Step 4: The B-horizon (subsoil) is poor in humus but has some minerals washed down from above. Step 5: The D-horizon is solid, unweathered bedrock — no plants can grow there. So, A-horizon is the

Rocks are broken down at the place where they exist, without being moved

Step 1: Weathering is the process of disintegration and decomposition of rocks. Step 2: The key feature of weathering is that it happens in situ — meaning 'in place'. The rocks break down right where they are, without being moved to another location. Step 3: This is why it is called a 'static' process — 'static' means not moving. Step 4: Denudation, on the other hand, is a dynamic process — it not only breaks down rocks but also transports the weathered material and deposits it elsewhere. Step 5: The other options describe transportation and deposition, which are features of denudation, not we

Mechanical weathering

Step 1: There are three types of weathering — mechanical, chemical, and biological. Step 2: Mechanical weathering (also called physical weathering) involves the physical breaking apart of rocks due to factors like temperature changes, frost, and wind. Step 3: Importantly, mechanical weathering only changes the SIZE of the rock — it breaks rocks into smaller pieces. It does NOT change the chemical composition of the minerals in the rock. Step 4: Chemical weathering, on the other hand, changes the chemical composition and mineral makeup of rocks through processes like oxidation, hydration, and c