2.

Soil Color
Soil color is the color which can be observed through the naked eye. Soil color can provide important
information about its composition, fertility, and drainage.

Soil formation
1 2
Weathering Soil Genesis
Rock Parent Material Soil

• Rock
o Definition: The original bedrock or consolidated rock from which the soil will
eventually form.
o Types of Rocks: igneous (e.g., granite, basalt)
sedimentary (e.g., limestone, sandstone)
metamorphic (e.g., schist, marble).
• Rock Weathering:
o Physical Weathering: The mechanical breakdown of rocks into smaller particles
through processes such as freeze-thaw cycles, thermal expansion, and abrasion.
o Chemical Weathering: The alteration of rock minerals through chemical
reactions with water, oxygen, acids, and other chemicals (e.g., hydrolysis,
oxidation, carbonation).
o Biological Weathering: The contribution of living organisms (e.g., plant roots,
lichens) to the breakdown of rocks.
• Parent Material:
o Definition: The unconsolidated mineral or organic material from which the soil
develops, originating from the weathered rock.
o Types of Parent Material: Includes residual material (formed in place from the
underlying rock) and transported material (moved by wind, water, ice, or gravity).
o Influence on Soil Formation: The mineral composition and physical properties
of the parent material influence the texture, structure, and fertility of the resulting
soil.
• Soil Genesis:
o Addition of Organic Matter: Accumulation of decomposed plant and animal
material, forming humus which enriches the soil with nutrients.
o Formation of Soil Structure: Development of aggregates and soil horizons
through processes like leaching, eluviation, illuviation, and pedoturbation.
o Leaching and Eluviation: Removal of dissolved substances and fine particles
from the upper soil layers.
o Illuviation and Accumulation: Deposition of leached materials in the lower soil
layers, leading to the formation of distinct horizons (e.g., A, E, B horizons).
o Pedoturbation: Mixing of soil by biological activity and other physical processes,
enhancing soil development and horizon differentiation.
• Soil:
o Mature Soil Profile: The result of prolonged soil formation processes, featuring
well-developed horizons with distinct physical, chemical, and biological
characteristics.
1
 o Soil Horizons:
▪ O Horizon: Organic layer rich in decomposed material.
▪ A Horizon: Topsoil, dark and rich in organic matter.
▪ E Horizon: Eluviation layer, lighter in color due to leaching.
▪ B Horizon: Subsoil, zone of accumulation of leached materials.
▪ C Horizon: Weathered parent material with minimal organic content.
▪ R Horizon: Unweathered bedrock.

Soil color varies widely due to several factors, including its composition, mineral content,
organic matter, and environmental conditions. Here are the main reasons for the different
colors in soils, suitable for an A-level student:
1. Organic Matter
• Dark Brown to Black: Soils rich in organic matter, such as humus, typically appear
dark brown to black. This is because the decomposed plant and animal residues absorb
and retain moisture, giving the soil a darker color.
2. Mineral Content
• Iron Oxides:

2
 o Red: High concentrations of iron oxides, particularly hematite, give the soil a
red color. This is common in well-drained, aerobic conditions where iron is
oxidized.
o Yellow to Brown: Soils with goethite, another iron oxide, can appear yellowish
to brown. This can occur in both well-drained and poorly drained soils.
3. Soil Moisture
• Gray to Blue-Green: Poorly drained soils, or those with a high water table, often
exhibit gray to blue-green colors due to the reduction of iron in anaerobic conditions.
These soils might contain ferrous iron compounds, which are greenish or bluish.
4. Parent Material
• White to Light Gray: Soils derived from parent materials rich in quartz, gypsum, or
other light-colored minerals tend to be white or light gray. These materials contribute
little color to the soil.
• Dark Basaltic Rocks: Soils formed from basaltic parent materials can be dark due to
the minerals like augite and olivine.
5. Clay Content
• Varied Colors: Different types of clays can impart various colors to the soil. For
instance, kaolinite clays can make soil appear light-colored, while smectite and other
clay types might influence different hues based on their mineral composition.
6. Environmental Conditions
• Leaching: In areas with heavy rainfall, leaching can wash away minerals and organic
matter, leading to lighter-colored soils. Conversely, soils in arid regions might retain
more salts and minerals, influencing their color.
Examples and Observations:
• Tropical Soils: Often red or yellow due to high iron oxide content and intense
weathering.
• Temperate Forest Soils: Generally brown due to moderate organic matter and mineral
content.
• Desert Soils: Typically pale due to limited organic matter and high salt accumulation.

In Sri Lanka, the variation in soil color is influenced by the island's diverse climate,
geology, and topography. Here are some examples of different soil colors found in Sri
Lanka and the reasons behind these colors:

1. Red-Yellow Podzolic Soils

• Color: Red to yellow

• Location: Central highlands and wet zone
• Reason: These soils are rich in iron oxides, which give them a red or yellow color. The
humid conditions promote intense weathering and leaching, concentrating iron oxides
in the soil.

3
2. Reddish Brown Earths

• Color: Reddish-brown
• Location: Intermediate zone, especially in the dry regions like Kurunegala and
Anuradhapura
• Reason: The reddish-brown color is due to the presence of iron oxides. These soils are
less intensely weathered than the red-yellow podzolic soils and have moderate organic
matter content.

3. Alluvial Soils

• Color: Variable, often gray to brown

• Location: River valleys and floodplains, such as along the Mahaweli River and Kelani
River
• Reason: Alluvial soils are formed from sediments deposited by rivers. Their color can
vary widely depending on the source material, organic matter, and the extent of
waterlogging.

4. Grumusols (Black Soil)

• Color: Dark gray to black

• Location: North-central and north-western plains, particularly in regions like Jaffna
and Mannar
• Reason: These soils are high in clay and organic matter, which gives them a dark color.
They are also known for their high fertility and good moisture retention.

5. Latosols

• Color: Yellow to red

• Location: Upland areas and regions with high rainfall
• Reason: Latosols are highly weathered soils with significant iron and aluminum oxides.
The color varies from yellow to red depending on the specific iron compounds present.

6. Solodized-Solonetz and Solonchaks (Saline and Alkaline Soils)

• Color: Light gray to white

• Location: Coastal areas and regions with poor drainage, such as in Puttalam and
Hambantota districts
• Reason: The light color is due to the high concentration of salts and the presence of
sodium, which can lead to the bleaching of the soil.

Specific Examples:

• Kandy and Nuwara Eliya Districts: The red-yellow podzolic soils here are typical of
the wet zone's tea-growing areas. The iron oxide content gives these soils their
distinctive red and yellow hues.
• Jaffna Peninsula: The black soils, known locally as 'regosols,' are rich in organic
matter and minerals, making them very fertile for agriculture, especially for crops like
onions and tobacco.

4
 • Mahaweli River Basin: Alluvial soils in this region support extensive rice cultivation.
The color can range from gray to brown, depending on the sediment composition and
organic matter content.

Importance of soil color for crop cultivation

Soil color significantly impacts agriculture as it provides valuable clues about soil properties
that influence crop growth. Here are the key ways in which soil color affects agriculture:

1. Organic Matter Content

• Dark Brown to Black Soils:

o Implications: These soils are generally rich in organic matter, which improves
soil structure, water retention, and nutrient availability. They are often highly
fertile and ideal for growing a wide variety of crops.
o Examples: In Sri Lanka, black soils in regions like the Jaffna Peninsula are
highly fertile and support crops such as onions and tobacco.

2. Mineral Content and Fertility

• Red and Yellow Soils:

o Implications: These colors often indicate the presence of iron oxides. While
these soils can be well-drained and suitable for certain crops, they might be low
in essential nutrients like nitrogen and phosphorus. Regular fertilization may be
required to maintain fertility.
o Examples: Red-yellow podzolic soils in the central highlands of Sri Lanka are
suitable for tea cultivation but may require amendments to boost nutrient levels.

3. Drainage and Aeration

• Gray to Blue-Green Soils:

o Implications: These colors typically indicate poor drainage and anaerobic
conditions, which can lead to issues like root rot and poor crop growth. Such
soils may require drainage improvements for successful agriculture.
o Examples: Poorly drained alluvial soils along the Mahaweli River in Sri Lanka
may show gray hues and require careful water management for rice cultivation.

4. Soil Temperature

• Dark Soils:
o Implications: Dark soils absorb more heat from the sun, which can help warm
the soil early in the growing season, promoting faster germination and early
crop growth. This can be beneficial in cooler climates or seasons.
o Examples: Dark soils in the highland areas of Sri Lanka can help with early
planting of vegetables and other crops that benefit from warmer soil
temperatures.

5. Soil pH and Salinity

• Light Gray to White Soils:

5
 o Implications: These colors can indicate high salinity or alkalinity, which can
be detrimental to many crops. Special management practices, such as leaching
salts or applying gypsum, might be necessary to improve soil conditions.
o Examples: Coastal regions in Sri Lanka, like Puttalam, with light-colored soils
may face challenges with salinity, requiring careful selection of salt-tolerant
crops or soil amendments.

6. Soil Depth and Erosion

• Surface Soil Erosion:

o Implications: Changes in soil color can indicate erosion. For example, if dark
topsoil is eroded, lighter subsoil colors may become visible. This indicates a
loss of fertile topsoil and necessitates erosion control measures.
o Examples: In hilly regions of Sri Lanka, erosion of fertile red and brown soils
can expose less fertile subsoils, impacting crop yields and requiring
conservation practices.

Practical Applications for Farmers:

• Soil Testing: Farmers can use soil color as an initial indicator of soil health and follow
up with soil testing to determine nutrient levels, pH, and other properties.
• Crop Selection: Understanding soil color and its implications helps in selecting
appropriate crops. For example, light-colored saline soils might be planted with salt-
tolerant crops.
• Soil Amendments: Based on soil color and subsequent tests, farmers can apply organic
matter, fertilizers, lime, or gypsum to improve soil conditions.

Munsell color chart

The Munsell Soil Color Chart is a standardized tool used by soil scientists to describe and
communicate soil color accurately and consistently. It is an essential resource for soil
classification, agricultural planning, and environmental assessment. Here's a detailed
explanation of how the Munsell Soil Color Chart works and how to use it:

Structure of the Munsell Soil Color Chart

The Munsell Soil Color Chart organizes colors based on three attributes:

1. Hue: The type of color (e.g., red, yellow, green).

2. Value: The lightness or darkness of the color, ranging from 0 (pure black) to 10 (pure
white).
3. Chroma: The intensity or saturation of the color, ranging from 0 (neutral gray) to a
maximum intensity for that hue.

Components of the Chart

• Hue Pages: Each page in the chart represents a different hue (e.g., 10YR, 5YR,
2.5Y).
• Color Chips: Each page contains a grid of color chips arranged by value (rows) and
chroma (columns).

6
Reading the Chart

A typical color code looks like this: 10YR 5/3

• 10YR: Hue (Yellow-Red)

• 5: Value (Lightness)
• 3: Chroma (Intensity)

Steps to Use the Munsell Soil Color Chart

1. Collect a Soil Sample:

o Take a representative sample from the area and depth you are studying.
o Ensure the sample is moist but not muddy, as soil color can change when wet
or dry.
2. Prepare the Sample:
o Break up the soil clumps and spread the sample thinly on a white surface or
hold a small amount in your hand.
o If the soil is dry, moisten it slightly with water.
3. Use Natural Light:
o Perform the color comparison in natural light or under a standardized light
source to avoid color distortion.
4. Compare with the Chart:
o Hold the soil next to the color chips on the appropriate hue page.
o Find the closest match in terms of hue, value, and chroma.
o Record the color code that best matches your soil sample.

Interpreting Soil Colors with Munsell Codes

• Dark Brown to Black (e.g., 10YR 2/1): Indicates high organic matter content,
typically fertile soil.
• Red (e.g., 5YR 4/6): Indicates the presence of iron oxides, well-drained conditions.
• Yellow (e.g., 10YR 6/8): Also indicates iron oxides but in less oxidized states.
• Gray to Blue-Green (e.g., 2.5Y 6/1): Suggests poor drainage, reduced iron
conditions.
• White to Light Gray (e.g., 10YR 8/1): Could indicate high salinity, leached soils, or
presence of quartz or gypsum.

Practical Applications

• Agriculture: Farmers use soil color information to gauge soil fertility and determine
suitable crops.
• Soil Surveys: Soil scientists use Munsell codes to classify and map different soil
types.
• Environmental Monitoring: Researchers track soil color changes to study erosion,
contamination, and other environmental factors.

Example of Use in Sri Lanka

In Sri Lanka, for instance:

7
 • Red-Yellow Podzolic Soils (10YR 5/8): Found in central highlands, suitable for tea
cultivation due to good drainage but might need nutrient amendments.
• Reddish Brown Earths (5YR 4/6): Found in dry zones like Kurunegala, indicating
well-drained but possibly low in organic matter.
• Alluvial Soils (10YR 4/4): Found along river valleys, indicating fertile soils suitable
for paddy cultivation.

By using the Munsell Soil Color Chart, professionals can make informed decisions about land
use, soil management, and agricultural practices, ensuring sustainable and productive use of
soil resources.