Page 60 - Soil Degradation, Conservation and Remediation

46 2 Physical Deterioration of Soil

take place due to faulty soil management. It has been observed that the following
processes may account largely for the physical deterioration of soil.

1. Coalescence : Slow deposit of fine soil particles between individual aggregates,
welding them together into a massive structure. Soils that have been tilled in a
very dry state are particularly prone to coalescence.
2. Slaking : Structural collapse when dry soil aggregates are wetted rapidly by rain
or irrigation. Soils with low organic carbon (<2 %) are subject to rapid slaking.
3. Dispersion : Disintegration of the slaking products into individual particles due
to high forces of repulsion between clay particles. These high forces are promoted
by high concentrations of exchangeable sodium.
4. Consolidation : Reduction of soil volume by destruction of pore space, mainly
large pores. Soils with low organic carbon (<2 %) are particularly prone to com-
paction, but soils high in organic matter exhibit elastic resilience that makes
them rebound.
5. Aggregate Pulverization : This happens when soils that are too dry are tilled,

which destroys aggregates into fine powder. On the other hand, tilling when soil
is too wet destroys aggregates by smearing, because soil strength is at a mini-
mum. Tillage should ideally happen when soil moisture is at a point called
“lower plastic limit.”
The physical deterioration is one of the major types of soil degradation. It has the
following subtypes: (1) surface sealing, surface crusting, hardsetting, and compac-
tion (Pc); (2) waterlogging (Pw); (3) lowering of water table (Pa); and (4) subsid-
ence of organic soils (Ps). Desertification is also a physical degradation process

which has become a major concern in the arid and semiarid regions.

2.2 Surface Sealing, Crusting, Hardsetting,
and Compaction (Pc)

2.2.1 Surface Sealing

The term soil sealing is used to describe a change in the nature of the soil (usually
surface soil) leading to impermeability. Soil particles are dispersed by the collapse
of soil aggregates due to physical pressures, such as raindrop impact, or due to a
chemical agent such as excess exchangeable sodium. Structure deterioration can
also take place for improper tillage operations. For example, peds are ground to fi ne
powders when soils are tilled in the dry condition. Fine particles enter into the mac-
ropores and reduce infiltration and percolation. On the other hand, tilling in the wet

condition puddles the soil. Unstable peds may slack when wetted. Thus, the surface
soil becomes impervious to water and plant roots. Sealed soils are lost to many uses
such as agriculture and forestry while the ecological soil functions are severely
impaired or even prevented. In addition, surrounding soils may be infl uenced by