All the effects of soil forming processes, weathering, translocation
leaching and so on leave the soil with a particular 'structure'.
Soil structure refers to the shape, size and degree of development
of the aggregation, if any, of the primary soil particles into
naturally or artificially formed structural units and to their
spatial arrangement. In describing soil structure, we refer to
soil 'peds' and soil fragments or 'clods'.
Soil peds are natural, relatively permanent aggregates, separated
from each other by voids or natural surfaces of weakness. Peds
persist through cycles of wetting and drying. Soil Fragments and
Clods are artificial structural units, formed at or near the surface
by cultivation or frost action, and are not peds.
Degradation of soil structures occur by a number of manners including:
• Slaking - the breaking down of aggregates
by rain impact or under wet conditions; sorting soil particles
and washing them into depressions – such slaking can form
surface crusts or caps hindering shoot emergence in young crops.
The winter of 2002 in the UK saw many fields in the South East
becoming 'capped' by the intense pounding of rain on the soil surface
- which dramatically affected subsequent soil water runoff.
• Cementation - in certain cases, a subsoil
iron 'pan' or hard ferrous layer can form, effectively preventing
root penetration. Iron pans can be surprisingly hard, and if dug
out can be held up and may need to be 'snapped' to break them!
• Sodium leaching - as saline soil solutions
are diluted, sodium-saturated clays become unstable causing clay
deflocculation and structural collapse.
• Cultivation - the downward compression
from machinery when the soil is too 'plastic' can form a barrier
to rooting and drainage as massive or platy structures form. This
emphasises the need for the correct design of vehicles designed
to work the soil. The following picture highlights a highly compressed
soil, with a massive structure showing horizontal fracturing and
stressed roots. So called 'plough pans' can also form if a plough
of a set depth is used over and over again year after year. Where
this happens a hard layer of soil can build up just under the depth
of the plough. This hard layer can impede field drainage and adversely
affect crop yield.