SOIL CONSISTENCY- MEASURE AND ATTERBERG LIMIT
Meaning
- Soil consistency refers to the resistance of soil to mechanical stress or manipulation at various moisture contents. In short, it describes the resistance of soils to deformation and rupture. Therefore, soil consistency is the strength with which soil materials are held together.
- Soil consistency is determined for wet, moist and dry soil samples. In case of wet soils, it is expressed as both stickiness and plasticity. Soil consistency may be estimated in the field using simple tests or may be measured more accurately in the laboratory.
Measure of soil consistency
Soil consistence is measured at three moisture levels namely ‘wet’, ‘moist’ and ‘dry’.
- Wet Soils
- Testing is done when the soil is saturated with water, like immediately after a good rainfall.
- In wet soils, the consistency is denoted by terms stickiness and plasticity.
- Stickiness is the ability of soil materials to adhere to other objects, and it may be slightly sticky, sticky, very sticky or non-sticky.
- Plasticity of a soil is its capacity to change its shape depending on stress and to retain the shape even when the stress is removed. There are also four degrees in plasticity namely, non-plastic, slightly plastic, plastic and very plastic.
- Moist Soil
- Testing is done when the soil is moist but not wet, for example, 24 hours after a good rainfall.
- Moist soil with least coherence adheres very strongly and resists crushing between the thumb and forefinger.
- The different categories are-
Loose-non coherent (single-grain structure)
Very friable – coherent, but very easily crushed (soil crushes easily under very gentle pressure but will stick together if pressed again).
Friable – easily crushed.
Firm – crushable with moderate pressure,
Very firm – crushable only under strong pressure and
Extremely firm – completely resistant to crushing or crushed only under very strong pressure.
- Dry Soil
- Testing is done when the soil has been air-dried.
- In dry soil, the degree of resistance is related to the attraction of particles for each other. The different categories are:
Loose – non coherent
Soft – breaks with slight pressure and becomes powder
Slightly hard – break under moderate pressure
Hard – breaks with difficulty with pressure
Very hard – very resistant to pressure
Extremely hard – extreme resistance and cannot be broken.
Determination of Soil Consistency using Atterberg limits
- It is clear that the consistency of a soil sample changes with the amount of water present.
- Atterberg’s limits are used to measure the physical condition of soil at different water contents. These depend on texture, organic matter content and amount of clay in the soil.
- An Atterberg Limit corresponds to the moisture content at which a soil sample changes from one consistency to another. Two of the Atterberg Limits are of particular interest for aquaculture, the liquid limit and the plastic limit, which are defined from three soil consistencies:
Liquid consistency- fluid or liquid mud.
Plastic consistency- kneading and moulding to shape are possible.
Semi-solid consistency- no more kneading is possible and the volume decreases (shrinkage) as the sample dries up. - Terms used to describe soilconsistency are hard or harsh for dry soil, soft or friable for moist soil and plastic and sticky for wet soil.
- Friable consistency is the optimum condition for tillage and other agricultural operations while, plastic consistency is optimum condition for puddling.
- The rating of soil consistency helps in describing a soil profile and also used for estimating suitability for traffic and tillage.
- Based on water content, limits of soil consistency are briefly described below:
Flocculation limit: Moisture content at which soil suspension is transformed from liquid state to a semi-liquid state with appreciable increase in viscosity.
Liquid limit (upper plastic limit): Moisture content at which soil-water system changes from viscous fluid to a plastic body.
Plastic limit: The percentage 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 will destroy the cohesion of the soil.
Shrinkage limit: Moisture content at which soil changes from semi-rigid to a rigid solid with no change in specific volume as drying proceeds further.
Sticky limit: Minimum moisture content at which soil paste will adhere to a steel spatula drawn over its surface.
Plasticity index: Difference in moisture contents between liquid limit and lower plastic limit. It indicates ‘clayeyness’ or potential plasticity of soil. Hence, depends upon clay content and nature of clay.
Friable (soft) consistency: The water content in this range permits easier crumbling of the soil. Friable consistency presents the optimum conditions for tillage and preparation of seed-bed. This is reached at moisture contents slightly less than lower plastic limit.
Harsh consistency: Upon dehydration soil becomes hard due to clay cementation and the consistency is called harsh. It requires more power to plough soil at this water content and soil becomes cloddy when ploughed.
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