SOIL PROPERTIES AND CLASSIFICATION

SOIL PROPERTIES The various properties of soil include-

1. Color

2. Texture

3. Humus

4. Soil structures

COLOUR

Colour is the foremost physical property that can be seen with naked eye and is useful in interpretation of valuable insight into the soil environment, thus it can be very important in assessment and classification of soil.

DIFFERENT SOIL COLOURS

The most influential colours in a well drained soil are white, red, brown and black.

White indicates the predominance of silica (quartz), or the presence of salts.

Red indicates the accumulation of iron oxides.

Brown and black indicate the level and type of organic matter. A colour triangle can be used to show the names and relationships between the influential colours. 

FACTORS DETERMINING SOIL COLOUR

Four main factors influence the colour of a soil:

1. Mineral matter derived from the constituents of the parent material

2. Organic matter

3. The nature and abundance of iron

4. Moisture content

INTERPRETATION USING SOIL COLOUR If we understand what determines soil

colour, then we can use colour to make some inferences about, history, chemistry and hydrology.

For example, an obvious change in colour between horizons is a real indication of changes in soil properties which are the result of biological activity, water movement and weathering.

DETERMINING SOIL COLOUR

Soil colour should be determined on moist surfaces of freshly broken (not sliced) soil samples.

Like any other soil property, colour must always be observed throughout soil profile, paying special attention to the differences between soil horizons. Colour characteristics such as mottle size, percentage and contrast should be observed and recorded.

A system that uses specially printed colour charts (Munsell Soil Colour Charts) gives an international standard. It divides colour into wavelength, lightness, and colour saturation.

Where a Munsell Chart is not available, simple names as listed in the triangle can be used.

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Munsell Color Charts

Hue = dominant spectral or “rainbow” color Red, Yellow, Blue, Green

Number increases and the color is more brilliant as grayness decreases

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Color Determination in the field

ALWAYS USE MOIST SOIL Munsell’s color charts values:-

1. Absolute black = 0

2. Absolute white = 10

3. Light soils hue value = 7 or more

4. Medium soils hue value = 5 – 6

5. Dark soils hue value = 4 or less

MOTTLES Spots of different colors in the soil Generally indicate that the soil has

periods of inadequate aeration each year

Usually rust colored Bluish, grayish, & greenish subsoils –

with or w/o mottles = indicate longer periods each year of waterlogged conditions & inadequate aeration.

HUMUS The term was coined in1790–1800; (< Latin:

earth, ground) Dark brown or black organic substance

made up of decayed plant or animal organic matter, that provides nutrients for plants and increases ability of soil to retain water

•In soil science, humus refers to any organic matter that has reached a point of stability, where it will break down no further and might, if conditions do not change, remain as it is for centuries. Humus significantly improves the structure of soil and contributes to moisture and nutrient retention.

•In agriculture, humus is sometimes also used to describe mature compost, or natural compost extracted from a forest or other spontaneous source for use to amend soil. It is also used to describe a topsoil horizon that contains organic matter

Humus also controls the colour of soil higher the humus content darker is the soil colour.

TEXTURE Refers to the size of particles.

Three types are….Sand (Large)Silt (Medium)Clay (Small)

TEXTURE = % of sand , silt and clay.

SOIL STRUCTURE The soil structures commonly

seen are-

1. Blocky

2. Platy

3. Massive

4. Prismatic

5. Granular

1. BLOCKY Particles cling

together in angular aggregates.

Typical of soils with high clay content.

Typical of B horizons.

beds are large about 5 to 50 millimeters.

2. PLATY

Large, thin beds.

Plate-like & arranged in overlapping horizontal layers.

3. MASSIVE

Soil has no visible structure.

Hard to break apart & appears in very large clods.

4. PRISMATIC Prismatic structure are

bounded by flat to rounded vertical faces. Units are longer vertically and top of the prisms are normally flat.

It is commonly found in B horizons.

5. GRANULAR Is the best for most

plants.

Particles cling together to form rounded aggregates.

It is commonly found in A horizons.

Beds are small usually between 1 to 10 millimeters.

SOIL CLASSIFICATION

Soil can be classified according to the grain size as-

1. Fine grained- Clay

2. Medium grained- Silt

3. Coarse grained- Sand

CLAY < 0.002 mm Flat plates or tiny

flakes Small clay particles

are colloids If suspended in

water will not settle

Wet clay is very sticky and is plastic or it can be molded readily into a shape or rod.

Easily formed into long ribbons

Pores spaces are very small and convoluted Movement of water

and air very slow

Water holding capacity Tremendous capacity

to adsorb water- not all available for plants.

Chemical adsorption is large

SILT < 0.05 mm to >

0.002 mm

Not visible without microscope

Quartz often dominant mineral in silt since other minerals have weathered away.

Does not feel gritty

Floury feel –smooth like silly putty

Smaller particles – retains more water for plants and have slower drainage than sand.

Easily washed away by flowing water – highly erosive.

Holds more plant nutrients than sand.

SAND Feels gritty Considered

non-cohesive – does not stick together in a mass unless it is very wet.

Sand has less nutrients for plants than smaller particles

Voids between sand particles promote free drainage and entry of air

Holds little water and prone to drought

On the basis of texture soil can be classified as :-1. Sandy soil2. Loamy soil3. Clayey soil

Sandy Soils

Coarse textureSandsLoamy sands

Loamy Soils- Coarse

Medium textureVery fine

sandy loam

LoamSilt loamSilt

Loamy Soils - Fine

Moderately fine textureSandy

clay loamClay loamSilty

clay loam

Clayey Soils

Fine textureSilty clayClaySandy clay

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