Agriculture for Engineers 4 (3+1)

Lesson 6. Soil Physical Properties and Soil Particle Distribution -II

A. Consistency when soil is wet

When soil is at field capacity the consistency is described m terms of stickiness and plasticity.

Stickiness: The quality of adhesion to other objects is called stickiness. The degree of stickiness is identified by the following terms: non sticky, slightly sticky, sticky and very sticky.

Plasticity: Plasticity is the capacity of the soil to mould. When stress is applied, the shape changed and after removing the stress it keep the changed shape, Terms used to describe the degree of plasticity are: non plastic, slightly plastic, plastic and very plastic.

B. Consistency when soil is moist

For slightly wet condition of the soil (moisture content between air dry and field capacity) consistency is described in the following terms.

Type                       Characteristics

Loose                     Non coherent

Very friable            Coherent but easily crushed

Friable                    Easily crushed

Firm                       Crushed under moderate pressure

Very firm               Crushed only under strong pressure

Extremely Firm      Resist crushing between thumb and forefinger

C. Consistency when soil is dry

Under dry condition of the soil (air dry) consistency is characterized by rigidity and hardness.

Type                       Characteristics

Loose                   Non coherent

Soft                      Breaks under slight pressure between thumb and fore finger to a powdery mass

Slightly hard        Breaks under pressure

Hard                     Breaks with difficulty under pressure

Very hard            Very resistant to pressure can not be broken between thumb and fore finger

Extremely           Extreme resistance to pressure cannot be broken in the hand hard.

 D. Significance of soil consistence

Optimum soil consistence increases water holding capacity and plant food material. Thus, it increases the fertility of the soil.

E. Soil Crusting and Crust Formation

Soil crust is the thin compact layer of higher bulk density formed at the soil surface following dispersion of natural soil aggregates as a result of wetting or impact of raindrops or sprinkler irrigation and its subsequent rapid drying due to radiant energy of the sun.

When raindrops strike the exposed dry soil surface, there is disintegration and dispersion of the aggregates. The finer clay particles move down along with infiltrating water and clog the pores immediately beneath the surface thereby sealing the soil surface. Also, the dispersed soil may remain in suspension, coarse particles start to settle out but fine clay particles remain in suspension. As the water evaporates, clay settles on the top of coarse particles, forming a crust on drying. The soil particles tend to pull together due to surface layer with decreased porosity. Soil crusting is a major structural feature of soils of arid and semi-arid regions. Improved management of soil organic matter and use of certain amendments can help to prevent clay dispersion and crust formation.

 6.2 SOIL DENSITY

Density represents weight (mass) per unit volume of a substance.

Density= Mass / Volume

Soil density is expressed in two well accepted concepts (A) particle density and (B) bulk density.

A. Particle density: The weight per unit volume of the solid portion of soil is called particle density. Particle density is also known as true density. Generally, particle density of normal soils is 2.65 g/cc. This is high when heavy mineral is present in the soil viz. magnetite and limonite. Increase in the organic matter the soil decreases the particle density. Particle density varies with texture of soils (Table 6.1).

Table 6.1 : Particle density of different soil textural classes

Textural class	Particle density (g/cc)
Coarse sand	2.655
Fine sand	2.659
Silt	2.798
Clay	2.837

 B. Bulk density: The oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk density. The bulk density of a soil is always smaller, than its particle density. Bulk density normally decreases as mineral soils become finer in texture. The Bulk density of organic matter is about 0.5 g/cc. The bulk density varies indirectly with the total pore space present in the soil and gives a good estimate of porosity of soil. Bulk density is of greater important than particle density in understanding the physical behavior of soils. Soils with low bulk densities have favorable physical conditions (Table 6.2).

Table 6.2: Bulk density of different soil textural classes

Textural class	Bulk density (g/cc)	Pore space (%)
Sandy soil	1.6	40
Loam	1.4	47
Silt loam	1.3	50
Clay	1.1	58

 Using the bulk density of any soil the weight of that soil can be calculated unto the desired depth.

Example: If the bulk density of the cultivated field is 1.48 glee what will be the weight of hectare filed (soil) to the depth of 15 cm.

Volume of the soil (Field) of hectare filed (soil) to the = Length x Width x Depth

depth of 15 cm.                   = 100 x 100 x 0 .15

                                            = 1500 cubic meter

1 cubic meter = 1 x 100 x 1 x 100 x 1 x 100 = 1000000 cubic centimeter

1500 cubic meter= 1500 x 1000000 = 1, 50, 00, 00,000 cubic centimeter

Weight of 1 cubic centimeter = 1.48 g

So in this case weight 1, 50, 00, 00,000 cubic centimeters will be

= 1, 50, 00, 00, 000 x 1.48 (Volume of 15 cm Hectare furrow x Bulk density)

= 2,22,00,00,000 g

i.e. 2, 20, 20, 00,000/1,000 = 22, 20,000 kg/ha to the depth of 15 cm

 6.3 POROSITY OF SOIL

A. Pore space

The volume of soil mass that is not occupied by soil particles is known as pore space. This space is occupied either by air or water. It directly controls the amount of water and air in the soil and thus indirectly controls plant growth and crop production. There are two types of pore (i) Macro pore and (ii) Micro pore and both are important for crop growth.

(i) Macro pore or non capillary pore: Pores more than 0.06 mm in diameter are considered as macro pores. These pores allow readily movement of air and water and do not hold much water under normal condition. Sands and sandy soils have a large number of macro pores.

(ii) Micro pore or capillary pore: In contrast, in the micro pores, movement of air and water is restricted to some extent. Clays and clayey soils have a greater number of micro pores. Size of the individual pores, rather than total pore space in a soil is more significant in its plant growth relationship. Equal distributions of micro and macro pores are much desirable for better aeration, permeability, and drainage and water retention.

B. Soil porosity: Soil porosity is the percentage pores space. Porosity refers to that percentage of soil volume which is occupied by pore spaces. It can be calculated by the formula:

Since, % pore space + %solid space= 100 and

           % pore space = 100 - % solid space

Example: A soil having bulk density of 1.3 and particle density of 2. 65 have the    following percentage of pore space.

C. Factors affecting the soil porosity

(i) Soil texture: in sandy soils, pores are quite large, thus total pore space is less. In fine textures soils, there is possibility of more granulation and more total pore space because there are macro pores between individual particles and within granuals micro pores (Table 5.5).

Table 5.5: Relation between textural class and pore space

Textural class	Clay content	Av. Pore space (%)
Heavy clay	>55	51.09
Clay	40-55	48.45
Sandy clay loam	20-30	45.45
Sand	<10	42.54

 (ii) Soil structure: Good aggregated soil structure is having greater pore space than the soil having single grain structure or structure less. Soil with same size aggregate but one is single grain structure while other is aggregate structure, more space will be in later on because of additional pore space between the primary particles. Granular or crumbly type of structure has more porosity than plate like.

(iii) Arrangement of soil particle: When the spheres like particles are arranged in columnar form it gives the most open system of packing. Thus, the number of pore space will be less. When the particles are arranged in the pyramidal form it gives the closest system of packing. So in this system porosity would be more.

(iv) Organic matter: Increasing in the organic matter content in the soil, increase in the percentage of pore space.

(v) Micro-organisms: Micro-organisms like earthworm and insects increase the macro pores in the soil.

(vi) Depth of the soil: Pore space in the sub-soil is generally low compared to surface soil layers.

(vii) Cropping: Cropping tends to lower the total pore space in comparison to virgin or un-cropped soils. This reduction is associated with a decrease in the organic matter content. Continuous cropping often results in a reduction of large or macro pore spaces (Table 6.4).

Table 6.4 : Effect of continuous cropping on total pore space

Soil Treatment	Organic matter %	Pore space %
		Total	Macro	Micro
Virgin Soil	5.6	58.3	32.7	25.6
Cultivated Soil	2.9	50.2	16.0	34.2

 6.4 SOIL COLOUR

The colour of soil varies widely. It is an easily observable characteristic and is an important criterion in description and classification of soils. Colour of a soil is inherited from its parent rock material (termed as lithochromic) for example red soils developed from red sand stone. Often the soil colour is a result of soil forming process and is termed as acquired or pedochromic. The soil colour is best determined by the comparison with the Munsell colour.

The colour of the soil is a result of the light reflected from the soil. Soil colour rotation is divided into three parts:

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

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

Chroma: It represents the purity of the colour (strength of the colour)

The Munsell colour notations are systematic and letter designations of each of these three variables (Hue, Value and Chroma). For example, the numerical notation 2.5YR5/6 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.

A. Soil colour and composition

(a) Black and dark gray colour: The variations from black to dark gray colour of  soil are mainly due to organic matter.

(b) Brown colour: This is the most common soil colour and is due to. A mixture of the organic matter and iron oxides.

(c) Red yellow colour: Red colour is associated with unhydrated ferric oxides, whereas yellow colour indicates some degree of hydration.

(d) White colour: Silica and lime generally impart white colour.

(e) Bluish and greenish colour: Some of the bluish and greenish colours are due to the presence of ferrous compounds. This reducing condition occurs in ill drained soil.

(f) Mottling colour: Colour variation or mottling in soils indicates alternating oxidizing and reducing conditions due to a fluctuating water table.