Page 69 - Soil Degradation, Conservation and Remediation

2.2 Surface Sealing, Crusting, Hardsetting, and Compaction (Pc) 55

wheel causes 80 % of the potential compaction. Subsequent passes cause additional,
but progressively less, compaction. After four passes, the additional compaction
becomes very small.
Tillage can either create or help to alleviate soil compaction. Tillage operations
break up soil into smaller particles. Excessive tillage destroys the structure that
provides desirable pore space. Some tillage equipment, such as moldboard plows,
may aerate the soil and increase percolation at the surface while creating a com-
pacted layer just below tillage depth. Such a layer is called a “plowpan” or “hard
pan.” Disks can also produce a hard pan just below tillage depth while overtilling
the soil near the surface, especially where multiple passes are made.

2.2.3.2 Effects of Soil Compaction

The effects of soil compaction on soil properties and processes have been reviewed
by Soane et al. ( 1982 ), Lipiec and Stepniewski ( 1995 ), and Alakukku ( 1999 ). Soil
compaction has been found to affect almost all physical, chemical, and biological
properties and processes of soil to variable extents. Soil compaction modifi es the
pore size distribution of mineral soils, mainly by reducing the porosity and espe-
cially the macroporosity (diameter >30 μm; Eriksson 1982 ; Ehlers 1982 ). Besides

the volume and number of macropores, compaction also modifies the pore geome-
try, continuity, and morphology. Soil compaction has negative impacts on many soil
properties related to soil working, drainage, crop growth, and the environment.

Compaction due to fi eld traffic increases the bulk density (Arvidsson 1998 ), shear
strength and penetrometer resistance (Blackwell et al. 1986 ) of soils, limiting root
growth and increasing the draft requirement in tillage. Soil compaction reduces

infiltration (Pietola et al. 2005 ) and saturated hydraulic conductivity (Alakukku
et al. 2003 ). Soil compaction reduces CO 2 and O 2 exchange (Simojoki et al. 1991 ).
Drainage problems appear due to loss of permeability by compaction in the subsoil. Soil
compaction may lead to waterlogging. Poorly drained soil may also dry slowly,
reducing the number of days available for field operations. The reduction in drainage

rate attributed to soil compaction can be expected to increase the emissions of
greenhouse gases from soil (Ball et al. 1999 ), for instance, by increasing denitrifi ca-
tion. Compaction increases surface runoff and topsoil erosion (Fullen 1985 ). By

affecting soil properties and processes, soil compaction influences crop growth,
yield, and the use efficiency of water and fertilizers. Soil compaction reduces yield

(Hanssen 1996 ), crop water use efficiency (Radford et al. 2001 ), and nutrient uptake

(Alakukku 2000 ).
Typical responses of plants to soil compaction include low seed germination,
reduced seedling emergence, reduction of number and length of roots, restriction of
downward penetration of the main root axes, decrease in leaf thickness, increase in
shoot-to-root ratio, and decrease in crop grain yield (Fageria et al. 2006 ). The degree
of restriction of root growth in compact soil depends also on the species and the age
of the plants (Masle 2002 ). Inhibited plant growth is mostly attributed to reduced
rooting volume (Grzesiak et al. 2002 ; Masle 2002 ). The restrictive effect of soil