Agriculture Reference
In-Depth Information
the footprints left behind are evidence of compaction.
As a soil dries out, it becomes ideal for cultivating:
load-bearing and friable
. If they dry out further, the
lighter soils can fragment too easily when raked and
produce dusty tilth, whereas heavier soils can become
too hard to break down without a great deal of effort.
Dusty
Ideal
Cloddy
Sand
Silt
Friable
is the consistency of the soil when it is
easily cultivated, i.e. readily forms crumbs.
Clay
Figure 12.8
Soil crumbs - the 'ideal' arrangement
of small and large pores for establishing plants is
illustrated alongside a 'dusty' tilth with too few large
pores and a 'cloddy' tilth that has too many large pores
The delay to work on the land can be reduced by the
use of boards to walk on: the pressure on the ground
is spread and compaction is minimized. Equipment
with very wide tyres (or tracks) does the same thing
for mechanized work on a larger scale.
Good crumb structure can be destroyed by cultivating
at the wrong time, but also by working the soil too
much, that is, 'over cultivation'. Soil crumbs are
vulnerable to collapse when wetted especially if they
are low in organic matter. Puddles on seedbeds lead
to crumbs collapsing and the particles released fill
the gaps between remaining crumbs; on drying these
can form 'soil caps', which is a crusty layer in the soil
that reduces gaseous exchange and can hold back
seedlings.
The crumbs at the surface collapse if they stand in
puddles and are easily broken by large droplets of
water from rainfall or irrigation, particularly when there
is no protection from plants or mulching. Care should
be taken when applying water (irrigation).
the crumbs can be large enough to allow water to
be pulled out by gravity. This means that after being
fully wetted and allowed to drain, there will be mainly
water in the crumbs and mainly air between the
crumbs; ideal for plant growth and for the beneficial
organisms living in the soil.
Tilth
is the crumb structure of the seedbed.
The fineness of a seedbed should be related to the size
of seeds, so it usually consists of crumbs between 0.5
and 5 mm in diameter. Cloddy surfaces lead to poor
germination of all but the largest seeds and make weed
control difficult. If made too fine, then there will be too
few soil pores that hold both air and water. Fine crumbs
tend to form a 'soil cap' when wetted (see p. 150).
Soil conditioners
such as manure and compost (see
organic matter, p. 158) help the soil to form a good
crumb structure. When fresh they can 'open up'
the soil (i.e. improve aeration and drainage) and the
humus created from it improves crumb formation in
very sandy soil and in heavy clays. Lime is added to
remedy soil acidity, which ensures that the beneficial
soil organisms are encouraged, but it also contributes
'calcium' that encourages clay to form crumbs. When
clay needs to be improved without raising pH, gypsum
(calcium sulphate) can be added to the soil.
Cultivations
In temperate areas, the conventional preparation of
land for planting starts with thorough disturbance of
the top 15-30 cm of soil (primary cultivation), usually
by digging on small plots and ploughing on larger
areas. Mainly rakes or harrows are used to prepare the
final surface (secondary cultivation).
Digging
Digging inverts the soil and this arduous activity is
undertaken to loosen the soil and bury trash. The
land is broken up into clods and an increased area is
exposed to weathering. As the soil is inverted, weeds,
plant residues and bulky manures are incorporated.
The depth of routine digging should be related to
the depth of topsoil because bringing up the subsoil
reduces fertility in the vital top layers, seriously
affecting germination of seeds and the establishment
of plants. Single digging, no more than the depth of
the spade, is usually undertaken on an annual basis
Timeliness
The right time for soil cultivation depends on the
weather, but more specifically the
soil consistency
(sometimes referred to as the 'workability' of the soil).
It influences the timing and effect of cultivations on the
soil, the
cultivation window
. Many soils are too sticky
when wet; soil clings to the equipment and walking on
the ground at this stage damages the soil surface and
