Soil Colour: Factors, Determination and Its Implication

After reading this article you will learn about:- 1. Factors Affecting Soil Colour 2. Determination of Soil Colour 3. Implication.

Factors Affecting Soil Colour:

There are various factors or soil constituents that influence the soil colour which are as follows:

(i) Organic Matter:

Soils containing high amount of organic matter show the colour variation from black to dark brown.

(ii) Iron Compounds:

Soils containing higher amount of iron compounds generally impart red, brown and yellow tinge colour.

(iii) Silica, Lime and Other Salts:

Sometimes soils contain either large amounts of silica and lime or both.

Due to presence of such materials in the soil the colour of the soil appears like white or light coloured.

(iv) Mixture of Organic Matter and Iron Oxides:

Very often soil contains a certain amount of organic matter and iron oxides. As a result of their existence in soil, the most common soil colour is found and known as brown.

(v) Alternate Wetting and Drying Condition:

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 development of such alternating oxidation and reduction condition, the colour of soil in different horizons of the soil profile is variegated or mottled. This mottled colour is due to residual products of this process especially iron and manganese compounds.

(vi) Oxidation-Reduction 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.

Therefore, it may be concluded that the soil colour indirectly indicative of many other important soil properties. Besides soil colour directly modify the soil temperature e.g. dark coloured soils absorb more heat than light coloured soils.

Determination of Soil Colour:

The soil colours are best determined by the comparison with the Munsell colour. This colour chart is commonly used for this purpose. The colour of the soil is a result of the light reflected from the soil.

Soil colour rotation is divided into three parts:

(i) Hue:

It denotes the dominant spectral colour (red, yellow, blue and green).

(ii) Value:

It denotes the lightness or darkness of a colour (the amount of reflected light).

(iii) Chroma:

It represents the purity of the colour (strength of the colour). The Munsell colour notations are systematic numerical and letter designations of each of these three variables (hue, value and chroma). 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’.

Implication of Soil Colour:

(i) Colour is one of the most useful and important characteristics for identification and classification of soils.

(ii) Colour often help to distinguish the different horizons of a soil profile, for example A₁ horizon is darker and B₂ horizon is brighter than adjacent horizons.

(iii) Sometimes colour helps in identifying diagnostic horizons used in soil classification such as a molicepipedon has a colour so dark that it’s both value and chroma are 3 or less.

(iv) Soil colour is an indicator of the soil moisture regimes under which a soil was developed. When a dry soil becomes moist, soil colours become darker by 1/2 to 3 steps in value, may change from – 1/2 to +2 steps in chroma and there is seldom change of hue.

Some of the largest difference in value between dry and moist colours occurs in grey or greyish brown horizons having moderate to moderately low contents of organic matter.

(v) As oxidation and reduction changes the colour of iron containing minerals, the hydraulic regime or drainage status of a soil can be predicted from soil colour. Bright (high chroma) colours such as red colour throughout the profile are symptoms of well-drained soil through which water drains out easily and in which plenty of oxygen is available.

Prolonged water logging condition reduces iron oxide coatings and changes high chroma (red or brown) colours to low chroma (grey, bluish or grey-green) colours i.e. gray colour.

Presence of gray (low chroma colour) either alone or mixed in molted form with various shades of gray, brown and yellow is used in identifying imperfectly and poorly drained soils or is indicative of water logged conditions during at-least a major part of the plant growing season.