4. Organic vegetable production

weeds by exuding a chemical,
a phenomenon known as
allelopathy. Allelopathic
influences can be both an
advantage (if they affect weeds)
or a disadvantage (if they affect
the crop).
• Precede soil-depleting crops with
soil-replenishing crops. A rotation
should generally consist of soildepleting
and soil-replenishing
crops. Legumes can provide
nitrogen for subsequent crops.
Other crops, such as those with
deep tap roots, have the ability
to exploit a greater area of the
soil nutritive reserve. Turning
in refuse from these crops will
help recycle deep nutrients for
use by shallow-rooted crops.
Crops with fibrous root systems
are also important for nutrient
exploration. Some crops are
chosen on the basis of their
ability to add to soil organic
matter. With green manure
crops, the aim should be to have
a range of species that fulfil all of
these requirements.
The composition and timing of
incorporating a green manure
crop can be designed either to
supply essential nutrients (if
ploughed under when young or,
for legumes, at early flowering)
or to boost organic matter and
improve soil structure (if allowed
to produce maximum crop
biomass before being ploughed
in). Table 4.1 shows two possible
green manure mixes, their cost,
and their biomass contribution.
Crops having higher nitrogen
Table 4.1 Two possible green manure mixes: cost and
biomass contribution
Field pea mix
50kg/ha field pea (Dundale)
20kg/ha oats (echidna)
1.4kg/ha fodder mustard (Winfried)
60kg/ha faba beans (mixed)
Biomass yield/ha: 48.450 tonnes
Sowing rate: 131.4kg/ha (total mix)
Cost of mix: $159.0/ha
Vetch mix
requirements should follow
green manure crops. These
include crops such as lettuce
and sweet corn, which have a
shallow, fibrous root system.
These would then be followed in
the rotation by flowering crops
such as broccoli and cauliflower,
which have lower nutritional
requirements, and then by a
deeper rooted fruiting vegetable
such as pumpkin. The final
crop in the rotation would be a
root vegetable: these have the
greatest ability to exploit the soil
profile for remaining nutrients.
This could then be followed by
two green manure crops—one
to increase soil organic matter
(for example, an oat-based crop)
and the other legume based—to
precede the start of the next
vegetable rotation. Alternatively,
a single green manure crop and
compost could be applied.
Incorporation of a green manure
crop in the rotation does not
preclude supplying additional
nutrients before or during a
vegetable crop’s growth. Soil
and leaf analysis will help to
determine these requirements.
Section 4.4 discusses crops’
nutritional requirements in
greater detail.
• Use the root physiology of crops
to help improve soil structure.
Some crop species—such as
sunflowers—have extensive and
penetrating root systems. These
can also be incorporated in the
rotation, reducing the need for
primary tillage.
30kg/ha vetch (Popany)
20kg/ha oats (echidna)
1.4kg/ha fodder mustard (Winfried)
30kg/ha faba beans (mixed)
Biomass yield/ha: 39.967 tonnes
Sowing rate: 81.4kg/ha (total mix)
Cost of mix: $94.3/ha
4.2.3 Spatial design
considerations
Row and inter-row spacing, the
number of crop rows per bed, and
interplanting with other species
to act as insectaries or trap crops
are some of the spatial design
considerations when planning a
cropping phase.
Weed management
Weeds grow best where there is
minimal competition—for example,
where there are gaps in a crop
stand. Because weeds are better
competitors, they will occupy these
sites rapidly. Several practices can
reduce the potential for weeds to
invade a site:
• decreasing the inter-row
spacing—that is, increasing the
crop sowing rate
• decreasing the distance between
rows or beds
• increasing the number of crop
rows on a bed
• growing a competitive crop or a
crop that is readily cultivated—
for example, pumpkin and
potatoes.
The aim is to close the crop canopy
as quickly as possible. In the case
of crops that never establish a
competitive canopy—such as
onions and, to a lesser extent,
carrots—other strategies are
needed. Of course, any strategy
that changes crop or row spacing
must be compatible with the
available machinery and equipment.
Experimentation might be needed
in order to determine the optimum
spacing for each crop. Increasing
the sowing rate can affect the
total yield and the size of the
product—for example, the head size
of cauliflowers and the bulb size of
onions.
Pest management
The crop spacing and its
relationship with other crops
22