Fundamentals of Soil Science (1+1)

Soil Consistency

Soil Consistency

• Soil consistency is a term used to describe the resistance of soil to mechanical stress or manipulation at various moisture contents.
• According to Russell and Russell (1950), “Soil consistency designates the manifestations of the physical forces of cohesion and adhesion acting within the soil at various moisture contents including the behaviour towards gravity, pressure, thrust and pull, tendency to adhere to foreign bodies and the sensations which are evidenced (by the fingers of the observer) as feel”.

Soil consistence is described at three moisture levels namely ‘wet’, ‘moist’ and ‘dry’.

1. Wet Soils:

• In wet soils the consistency is denoted by terms stickiness and plasticity.
• Stickiness is grouped into four categories namely i) non sticky, ii) slightly sticky, iii) sticky and iv) very sticky.
• Plasticity of a soil is its capacity to be moulded (to change its shape depending on stress) and to retain the shape even when the stress is removed.
• Soils containing more than 15% clay exhibit plasticity – pliability and the capacity of being molded. There are four degrees in plasticity namely i) non plastic, ii) slightly plastic, iii) plastic and iv) very plastic.

2. Moist Soil:

• Moist soil with least coherence adheres very strongly and resists crushing between the thumb and forefinger.
• The different categories are i. Loose-non coherent, ii. Very friable - coherent, but very easily crushed, iii. Friable - easily crushed, iv. Firm - crushable with moderate pressure, v. Very firm - crushable only under strong pressure and vi. Extremely firm - completely resistant to crushing. (type and amount of clay and humus influence this consistency)

3. Dry Soil:

• 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.

Atterberg’s Limits of Soil Consistency

• Atterberg’s limits are used to measure the physical condition of soil at different water contents. These limits can be seen as the indices of workability of soil at various water contents. These depend on texture, organic matter content and amount of clay in the soil.
• It is generally described at three soil moisture levels such as dry, moist and wet and terms used to describe soil consistency 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 and plastic consistency is optimum condition for puddling.
• Soils are rated for consistency as a part of describing a soil profile and for estimating suitability for traffic and tillage.
• Knowledge of plastic limit and plasticity index is required to characterize the shear strength, in terms of the normal stress applied and the water content of the soil.
• From a practical point of view, the sticky point provides an estimate of the maximum water content at which normal soils will scour during 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. Soil is near saturation, it behaves like softened butter.
• Lower Plastic limit: Water content at which soil changes from a plastic to semi-rigid and friable state. Between upper and lower plastic limits, soil can be moulded into various shapes without breaking.
• 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. It 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.