Porosity (f)

 It is the ratio of the total volume of pore spaces (Vf) to the total volume of soils (Vt).

 Units percentage

f = Vf/Vt = Vw+Va/Vs+Vw+Va

Varies from 30 to 60%

Types of pores

2 types of pores

A) Macro pores – Larger than 0.06 mm dia

B) Micro pores – Smaller than 0.06 mm dia

Soil compaction

Compaction is the dynamic behavior of soil. 

The degree of compaction depends upon the nature of the soil, amount of energy applied, water content, and extent of manipulation of the soil.

Compaction is also associated with the rearrangement of the soil solid particles so that soil water and soil air are compressed within the pore space.

In many soils, a compacted layer is commonly found at the bottom of the zone of ploughing. This layer is termed as ‘ploughed sole’.

Age-old practices of tillage using heavy farm equipments create the problem of surface and sub-surface compaction in soils. These hard compact layers often restrict root penetration and growth and also reduce the water and nutrient uptake by crops.

Soil compaction changes soil moisture and thermal regimes and mechanical resistance in soils. However, in highly coarse-textured soils, loamy sand soils, compaction may be a boon to water retention and reduce percolation loss of water

Consistency of soil

•Soil consistency is the behavior of soil at various moisture contents (dry, moist, wet) to mechanical stresses or manipulations.

• It is commonly measured by feeling and manipulating the soil by hand or by pulling a tillage instrument through it.

•The consistency of soils is generally described at three soil moisture levels: wet, moist, and dry.

•Consistency of a soil sample changes with the amount of water present.

•For wet soils, it is expressed as both stickiness and plasticity, as defined below.

Atterberg Limits are basic measures of the nature of finegrained soil. Depending on the water content in the soil, it may appear in four states: solid, semi-solid, plastic, and liquid.

Liquid limit (LL): It is defined as the percentage moisture content at which a soil changes with decreasing wetness from liquid to plastic consistency or with increasing wetness from plastic to a liquid consistency. It is the water content at which a soil changes from plastic to liquid behavior.

Plastic limit (PL): it is the percentage of moisture content at which a soil changes with decreasing wetness from the plastic to the semi-solid consistency or with increasing wetness from the semi-solid to the plastic consistency. The plastic limit is the lower limit of the plastic state. A small increase in moisture above the plastic limit destroys the cohesion of the soil.

Mass water content/ Gravimetric water content (w)

• It is the mass of water (Mw) relative to the mass of oven-dry soil particles (Ms)

• W=Mw/Ms

• Expressed as a fraction

Volumetric water content

• It is the ratio of the total volume of water (Vw) occupied in pore spaces to the total volume of soil (Vt).

Degree of saturation

• It is the ratio of the volume of water present in the soil at a particular time (Vw) to the volume of pores (Vf).

Air-filled porosity (fa)

• It is the ratio of the volume of air space to the total volume of soil

• fa= Va/Vt

• Index for soil aeration status.

• Depends on texture, structure, OM(Organic Material)

Soil Permeability

• Soil permeability is the ease with which air, water, or plant roots penetrate or pass through soil.

• Soils with large pores that are connected are more permeable.

• Clayey soils can have greater total porosity than sand and still are less permeable than sands since the pores are small.

• Soil crusts

are soil surface layers that are distinct from the rest of the bulk\ soil, often hardened with a platy surface. Depending on the manner of formation, soil crusts can be biological or physical. Biological soil crusts are formed by communities of microorganisms that live on the soil surface whereas physical crusts are formed by the physical impact such as that of raindrops.

Problems caused by crusting

• Germinating seeds are unable to break the crust.

• Oxygen flow into and out of the soil is limited, reducing crop growth.

• Signs of soil crusting are poor crop emergence.

• Soil crusting is especially a problem in dry direct-seeded fields where the seed is covered by soil.

• Poor infiltration

• Increased runoff and erosion losses.