Soil as a resource: formation and the soil profile

Detail

Why soil is a resource

• Soil is the thin upper layer of the Earth's crust. Plants grow in it.
• It is called the most important natural resource because:
• plants take water and nutrients from it
• plants feed animals and people

• so almost all life on land depends on it.

• Soil is a living system. It holds:

• inorganic matter: rock particles and minerals

• organic matter: humus, plant roots, worms, insects and microbes.

• About 95% of the food we eat comes from soil [2].

• Soil forms very slowly:
• (NCERT: a few cm can take millions of years, Class 10; the process takes from a century to several millennia, Class 7)

• FAO says it can take up to 1,000 years to make just 2–3 cm of soil [2].

• Soil is treated as non-renewable (it cannot be replaced fast enough to matter):

• once it is lost, it does not come back within a human lifetime [2]

• 33% of the world's soils are already degraded, and over 90% could become degraded by 2050 (FAO estimate) [2].

• India's policy response: the Soil Health Card (SHC) scheme:

• launched on 19 February 2015 at Suratgarh (Rajasthan) [3]
• it tests 12 parameters [3]:
  • macronutrients (nutrients plants need in large amounts): N, P, K, S
  • micronutrients (nutrients needed in small amounts): Zn, Fe, Cu, Mn, B
  • pH, EC (electrical conductivity, a measure of salt in the soil) and OC (organic carbon)
• farmers get a new card every 2 years, with advice on fertiliser doses [3].

Factors of soil formation

Five factors decide what kind of soil forms. Pedology is the science that studies soils: how they form, what they are like, and how they are mapped [4].

Factor	What it does	Indian example
Parent rock (bedrock)	Gives the minerals, colour and texture	Deccan Trap basalt → black (regur) soil in Maharashtra and Gujarat; old crystalline igneous rocks → red soil in Tamil Nadu and Odisha
Climate (temperature, rainfall)	Controls how fast rock weathers and how much material water washes away	Heavy rain and high heat wash minerals out → laterite in Kerala, Karnataka and the Meghalaya plateau
Relief (slope and height)	Steep slopes lose soil, so it stays thin. Flat land collects deep soil	Thin, immature soil on Himalayan slopes; deep alluvium in the Ganga plains
Vegetation and other life	Adds humus. Roots and animals mix the soil	Forest floors in the Western Ghats are rich in humus
Time	The longer a soil has to develop, the clearer its layers	Old alluvium (bangar) vs new alluvium (khadar)

• Pedologists use the same five factors worldwide: parent material, topography, climate, organisms and time [4].

Agents of soil formation

• Temperature change:
• rock heats up by day and cools at night
• it expands and contracts again and again
• it cracks and breaks apart

• this is common in hot deserts such as the Thar.

• Running water: breaks rock, carries the pieces and drops them as alluvium (soil laid down by rivers), as in the Indo-Gangetic plain.

• Wind: wears rock down and carries fine dust. Wind-blown dust laid down in thick layers is called loess.
• Glaciers: grind rock into fine particles and leave them behind as moraines (heaps of rock and soil left by a glacier) in the Himalaya.
• Decomposers (bacteria, fungi and worms): break down dead plants and animals into humus.
• Chemical and organic change: water, oxygen and acids slowly change the minerals inside rocks. Iron rusts into iron oxide, for example.

Steps of soil formation (Class 7, Fig. 1.8)

1. Weathering: rock is broken down into small particles where it lies.
2. Organisms add humus. Humus (from Latin "earth") is dark, nutrient-rich organic matter. It forms from decayed plant and animal remains and holds moisture.
3. Layering: water moves material down through the soil. Over a long time, separate layers form.

The soil profile

• Soil profile (vertical section): a straight cut downward from the surface to the parent rock. It shows the soil's layers.
• Horizon: each layer of the profile. Horizons form because:
• living things add and mix organic matter from the top

• water moving downward carries material from one layer to another [4].

• Eluviation: water washes clay, iron and humus out of an upper layer.

• Illuviation: the washed material builds up in a lower layer.
• Solum (the "true soil"): the A, E and B horizons together. Most soil formation and soil life happens here [4].

Horizon	Name	What it is
O	Organic layer	Leaf litter and decaying organic matter on the surface
A	Topsoil	Dark and rich in humus, with the most biological activity (life in the soil). Eluviation starts here
E	Eluvial layer	Pale layer that has lost its clay and humus to the layer below. Clearest in leached forest soils [4]
B	Subsoil	Clay, iron and lime build up here (illuviation)
C	Weathered parent material	Broken rock, only a little changed
R	Bedrock	Solid rock that has not weathered

• Mature soils (old soils) have well-developed, clear horizons.

• Examples: laterite and old red soils of the Peninsula.

• Immature soils (young soils) have weak horizons or none at all:

• young alluvium (khadar): rivers add a new layer during almost every flood
• mountain soils: erosion on steep slopes removes soil before layers can form.

Physical and chemical properties

Texture (the size of the mineral particles):

Particle	Size class	Behaviour
Sand	Coarse	Drains fast and holds little water or nutrients
Silt	Between sand and clay	Smooth and fertile. Common in river alluvium
Clay	Finest	Holds water and nutrients, but becomes sticky when wet and cracks when dry
Loam	Mix of sand, silt and clay	The ideal soil for farming: it drains well and stays fertile

• Structure: how particles stick together into clumps called aggregates. They can be crumb-like (granular), block-like, plate-like or column-like.
• Good crumb structure lets air, water and roots move through the soil.

• Humus helps form this structure.

• Colour shows what the soil is made of:

• black: titaniferous magnetite (an iron–titanium mineral) and humus, as in regur soil
• red: iron oxides; red soil turns yellow when the iron oxide is hydrated (holds water)

• dark topsoil: high humus content.

• pH (a 0–14 scale; 7 is neutral, below 7 is acidic, above 7 is alkaline):

• acidic: laterite (heavy rain washes out bases such as calcium) and peat (rotting organic matter forms acids; found in Kerala and the Sundarbans)
• alkaline: usar soils (salty, alkaline soils called reh or kallar in Uttar Pradesh, Haryana and Punjab, caused by poor drainage and evaporation)
• pH is one of the 12 parameters on the Soil Health Card [3].

Prelims Hooks

• The five soil-forming factors are parent rock, climate, relief, organisms and time. Agents such as wind and glaciers are not in this list.
• Order of horizons from top to bottom: O → A → E → B → C → R. The solum is A + E + B [4].
• Eluviation is loss from the A/E horizon. Illuviation is build-up in the B horizon. Exam questions often swap the two.
• Humus comes from the Latin word for "earth". It is dark and nutrient-rich and holds moisture.
• Silt is between sand and clay in size. Loam is the ideal mix of all three.
• Black colour comes from titaniferous magnetite and humus. Red comes from iron oxides.
• Acidic: laterite and peat. Alkaline: usar (reh/kallar).
• Khadar and mountain soils are young soils with weak horizons. Laterite is a mature soil with clear horizons.
• Soil Health Card: launched 19 February 2015 at Suratgarh (Rajasthan). It checks 12 parameters, including pH, EC and OC, and is reissued every 2 years [3].
• FAO: up to 1,000 years for 2–3 cm of soil. 95% of food comes from soil [2].

Mains Points

• Soil is effectively non-renewable:
• it takes centuries to millennia to form
• erosion can remove it within decades
• one-third of the world's soils are already degraded [2]

• so conservation must focus on preventing loss, not on rebuilding soil (GS-III: environment and food security).

• Spatial pattern explained by formation factors:

• parent rock explains black soil on Deccan basalt and red soil on crystalline rocks
• climate explains laterite in hot, wet areas
• relief explains thin Himalayan soils and deep plains alluvium

• this links soil types to crop patterns (GS-I).

• The soil profile and conservation:

• fertility is concentrated in the A horizon
• sheet and gully erosion strip this layer first

• this lowers yield and raises the need for fertiliser (GS-III).

• Using soil properties in policy:

• the SHC scheme (2015) turns soil properties (pH, OC, EC, nutrients) into farm-level fertiliser advice [3]
• this reduces the overuse of urea and improves soil health over time.