Fundamentals of Soil Science (1+1)

Soil Colour

Soil Colour

• The colour of the soil is a result of the light reflected from the soil. Soil colour is an easily observable soil property and gives an immediate indication of the soil condition.
• Soil colour is inherited from its parent material and that is referred to as litho-chromic, e.g. red soils developed from red sandstone.
• Besides soil colour also develops during soil formation through different soil forming processes and that is referred to as acquired or pedo-chromic colour e.g. red soils developed from granite gneiss or schist.

Significance

• Soil colour gives us a clue regarding the drainage condition of the soil, amount of organic matter present and type of parent material from which soil has developed.
• In general, the dark coloured soils are considered fertile and rich in organic matter. The mottlings, when present in lower horizons of a profile indicates poor drainage conditions. Iron compounds, in various states of oxidation and reduction are major colouring agents of subsoil horizons.
• The colour of soil containing iron oxides varies from red and rust brown to yellow depending on the degree of hydration.
• Reduced iron normally displays a green-blue tinge. Soils in anaerobic conditions, such as those in poorly drained depressions, will normally have dull, grey B-horizons. Alternatively, aerobic soils have bright reddish-brownish colours.

Factors affecting soil colour
Various factors influencing the soil colour are as follows:

Parent material / Composition:

• Soils containing higher amount of iron compounds generally impart red, brown and yellow tinge colour. The red colour of the soils is due to unhydrated iron oxide. It indicates that the soils are old and more weathered.
• The yellow colour in the soil is due to iron oxide (limonite) and more moist conditions. Due to presence of large amounts of silica and lime or both in the soil the colour of the soil appears like white or light coloured.
• In case of black cotton soils, the black colour is due to titaniferrous magnetite. Such soils may have less organic matter but still they are black in colour. Thus the parent material has direct bearing on the colour of the soil formed from it.

Soil moisture:

• A well drained soil will have normal colours as compared to a poorly drained soil. The poorly drained soil will show the sign of graying due to anaerobic or reduced conditions.
• When soils are waterlogged for a longer period, the permanent reduced condition will develop. The presence of ferrous compounds resulting from the reducing condition in waterlogged soils impart bluish and greenish colour.
• During monsoon period due to heavy rain the reduction of soil occurs and during dry period the oxidation of soil also takes place. Due to this alternate oxidation and reduction (due to alternate wetting and drying), some coloured patches develop. These patches are known as mottles. This mottled colour is due to residual products of this process especially iron and manganese compounds.
• The background soil surface is known as matrix. The matrix and mottle colour are recorded separately.

Organic matter:

• Soils containing high amount of organic matter show the colour variation from black to dark brown. In forest soils and grassland soils, more organic matter is added to the soil every year. This makes the soil darker in colour.
• In hot and dry regions the organic matter is readily decomposed and lost. Hence, these soils are lighter in colour. Thus, organic matter is important clolour imparting constituent in soils.

Determination of soil colour:

• The soil colours are determined by using Munsell colour chart. In this chart, different colour chips are systematically arranged by three variables namely Hue, Value and Chroma.
• Hue - it indicates the dominant spectral colour (red, yellow, blue and green).
• Value - it indicates lightness or darkness of a colour (the amount of reflected light).
• Chroma - it represents the purity of the colour (strength of the colour).
• The hue is written on the top right hand corner of the chart.
• The values are written on the left hand side and the colour becomes lighter as we move upward.
• The chroma is written at the bottom and increases towards right hand side.
• The combination of these variables is used to describe the colour of a soil as “Munsell notation”. The Munsell colour notations are systematic, numerical with letter designations of each of these three variables (Hue, value and chroma).
• English names of the colours are written on the left page. They have more uniformity for internation use.
• For example, the numerical notation 2.5 YR 5/6 suggests a hue of 2.5 YR, value of 5 and chroma of 6. The equivalent or parallel soil colour name for this Munsell notation is 'red'.