3 Important Physical Constraints of Soil

This article throws light upon the three important physical constraints of soil. The physical constraints are: 1. Soil Compaction 2. Soil Cracking 3. Soil Crusting.

Physical Constraint # 1. Soil Compaction:

In the compaction process, soil particles are packed together in a closer state of contact indicated by a change in bulk density, porosity etc.

Under a dynamic load sufficient to increase the density of soil, the following changes may take place:

(i) Compression of soil solids.

(ii) Compression of liquid and gases.

(iii) Changes in liquid and gas contents in the pore space (both macro and micro pore spaces).

(iv) Re-arrangement of soil solids.

A dry pulverized soil cannot be compacted to high density due to incompressible nature of soil particles and high internal friction. But as soil water content increases, the thickness of water film around the particles increases thereby reducing the cohesion between the particles and allowing them to slide over each other under the applied force/load.

The soil can now be compacted readily to a greater density. With an increase in the water content in the soil up to a liquid limit, the compaction is not possible as soil behaves like a fluid and loses its load bearing capacity.

Physical Constraint # 2. Soil Cracking:

It has high relationships with the phenomenon of the shrinkage which arises due to expansion and contraction of soil mass. It has been found that as shrinkage process progresses, the lines of cleavage develop at the points of least resistance normally corresponding to the plane of highest water content.

The cleavage of the soil body is indiscriminate and occurs horizontally, vertically, laterally etc. This results into formation of large irregularly shaped cracks.

Physical Constraint # 3. Soil Crusting:

Soil crusting is a phenomenon associated with deterioration of soil structure, where the natural soil aggregates break and disperse. If the dispersion is followed by rapid drying the soil solids rearrange into crust.

When rain drops strike the exposed soil surface the kinetic energy thus dissipated disintegrates the aggregates and the dispersive action of water leaves the surface few mm of the soil into a mono-grain/single grain state.

The dispersed particles are carried into the soil with the infiltrating water blocking the soil pores. At the start of the drying surface tension forces pull the soil particles together, tending to form a dense and strong soil layer known as soil crust. The thickness of the rain formed soil crust depends upon the size of the rain drops.

Larger drops having more energy destroy the original structure to a greater depth. These crusts are about 5 mm thick. In a cloddy soil, the material removed from the surface of the clod by rain drop impact erodes into inter clod areas resulting in thinner and weaker crust.

Surface crusting is a major structural feature of soils of arid and semi-arid regions, where crust formation is chiefly due to the aggregate destruction caused by impact of rain drops or irrigation. Crust can form on all soils textural types excepting sands.

But the problem of soil crusting is worst on silty clay loams of alluvial soils which are under terraces and levees due to their unstable soil structure. In soil crusting, a kind of structure develops in the upper few mms of the soil which differs markedly from the structure below sub-surface soil horizon.

This structure is characterized by bulk density, low non-capillary pore space, stratification and orientation of soil solids. When dry it is harder than the rest of the soil mass due to cementation of particles. The hydraulic conductivity of this layer is low encouraging run-off and soil loss. Soil crusting is the most important factor in interfering with seedling emergence and growth and largely determines the crop stand.