Agriculture Reference
In-Depth Information
are much larger ones for use in nurseries, market
gardens and in commercial glasshouses.
The type of tilth produced depends not only on the
soil conditions but also on the adjustment of forward
speed, rotor speed, blade design and layout, shield
angle and depth of working.
Creating the seedbed
Usually sandy soils are easily broken down to the
right size with cultivation equipment but there is a risk
of 'over cultivating' resulting in 'dusty' tilth. Heavier
soils are more difficult to cultivate. Traditionally, clay
soils are dug over (or ploughed) in the autumn and
exposed to wetting and drying and especially freezing
and thawing to produce a 'frost mould' (crumbs).
In this way, clods are easier to 'knock down' to a
crumb structure and level in the spring.
Timeliness
is essential when cultivating because not only does
it make cultivation easier when the soil is
friable
but
going on to the soil at the wrong time can damage the
good soil structures.
Once the seedbed is created, care is needed to
maintain it. Rain hitting the surface causes crumbs
to break up further. The particles released fill in
the gaps between the broken crumbs, reducing
water infiltration rates. As infiltration is reduced,
puddles form and the crumbs collapse even more
quickly. When the surface dries, a soil
cap
is formed
(Figure 12.10) which hinders gaseous exchange and
can trap germinating seeds.
In general, fine tilths should be avoided outdoors
until well into spring when conditions are becoming
more favourable and seedling emergence through any
developing cap is rapid. The surface can be protected
by using mulches (see p. 160) and a leaf canopy
reduces the problem.
Figure 12.10
Soil 'capping' is caused by the collapse
of the soil crumbs at the surface as a result of being
saturated with water and/or hit by raindrops
the need for so much cultivation is avoided by the
use of 'bed systems' or growing in containers (see
Chapter 15).
Bed systems
The compaction problems can be overcome by
cultivating in beds, which confines traffic to well-
defined paths between the growing areas. Effective
weed control methods, including modern herbicides,
have enabled the inversion of soil to be eliminated in
more situations than before.
On a garden scale, beds are constructed so that all
parts of the growing area can be reached from a
path; this eliminates the need to step on the growing
area. They can be laid out in many ways, but should
be no more than 1.2 m across. The paths should
be minimized while allowing access for all activities
through the growing season. Beds are usually raised
above the normal ground level to improve drainage
and ease of working. Much of this extra height results
from the addition of large quantities of suitable bulky
organic matter. Good topsoil is often taken from this
sacrificed path area to be used to raise the beds up
higher.
On larger areas, the width of the beds is adjusted to
the distance between the wheels of the vehicles used
so that the growing area is unaffected by the traffic
passing during the life of the plants. The equivalent
of this is done in farming and is easily seen from the
roadside in fields where cereals are being grown.
'No dig' methods
The traditional preparation of a seedbed, involving
as it does the inversion of the soil and creating
the appropriate tilth, is very demanding on energy,
labour and time. Furthermore, the cultivations
tend to interfere with the natural structure-forming
agents, such as earthworms (see p. 159), and when
undertaken at the wrong time, they create pans or
leave a bare, loose soil vulnerable to erosion (see soil
structure, p. 147). It also reduces the organic matter
levels at the top of the profile where it is most useful.
'No dig' methods have increasingly been adopted
in growing in order to eliminate these weaknesses
especially as weeds can be controlled with herbicides.
In gardens, on allotments and horticulture generally
