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Posted by
Maendeleo Vijijini
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My grandmother used to tell me that I am not supposed to plant the same crop on the same land consecutively.
And
when I asked her why not? Her reply would be it was not right. Well,
that is where her understanding ended, but all she was telling me was to
rotate crops to fight pests for more yields. Now, let’s explore the
benefits, rules and the science behind crop rotation.
The
first rule is that crops of the same family should never be planted
following each other because they share the same pests and diseases.
At the time of growing the first crop, there will be pest and diseases that will build up. Therefore, by the time you plant the second crop of the same family, the diseases and pest will be ready to attack it.
At the time of growing the first crop, there will be pest and diseases that will build up. Therefore, by the time you plant the second crop of the same family, the diseases and pest will be ready to attack it.
Losses
from such an attack can be up to 100 per cent. For example in an area
where you have planted tomatoes, you cannot rotate with a crop in the
Solanaceae family such as the nightshade, potatoes, eggplant, peppers
(capsicum) or tobacco. Some weeds such as datura are also in this
family.
Weeds in most cases are not beneficial to the
crop since they act as alternate hosts of both pests and diseases. This
is apart from competing with crops for nutrients, space, sunlight and
water.
The second rule is that in the sequence, there
needs to be a legume. Leguminous crops include the common bean, lentils,
garden peas, soya beans, peanut/groundnuts, chickpeas, pigeon peas,
cowpea and green grams and pastures such as alfafa, clover and lupin,
among others.
The advantage of having a legume in the
sequence is that it fixes the atmospheric nitrogen into the soil through
a process known as biological nitrogen fixation with the help of
micro-organisms found in their roots known as rhizobia. This means the
atmospheric nitrogen, which is in its molecular form N2 is converted to
ammonia (NH3).
The ammonia is then converted into
ammonium (NH4+) which is the form that is utilised by the plant. After
the legumes are harvested, the amino acids in the crop are realised back
into the soil where they are converted into nitrate (NO3-), which is a
form that is utilised by plants. You can rotate a cereal such as maize
with soya bean or potato with common bean.
Furthermore,
legumes are also called green manure. This is because of their ability
to biologically fix nitrogen into the soil. For you to get the maximum
out of green manure, you need to incorporate the legume into the soil.
Green manure also has the advantage of improving the soil structure and
the water-holding capacity of the soil.
The third rule
of crop rotation is to rotate crops with different rooting depth. The
deep-rooted crop should be rotated with the shallow-rooted. For example,
sunflower (deep-rooted) should rotate with wheat (shallow-rooted).
The benefits of crop rotation
Crop
rotation keeps the farmer in production throughout the year. Although
the planting seasons are determined by the amount of rainfall, a farmer
should take advantage of this using a rotational sequence
During the long rains, you can plant the high rainfall crops in this sequence:
Wheat >> Beans >> Tomatoes
During the dry season with the intermediate rains:
On
the farm where the wheat (cereal) was, plant chickpea (legume), where
beans were plant finger millet (cereal) and where tomatoes (solanaceae)
were, plant pigeon pea (legume). All these crops are drought-tolerant,
thus, there is no need to skip the season.
Chickpea >> finger millet >> pigeon pea
During the short rains, plant:
Maize (the variety will depend on where you are) soya beans and butternuts.
Crop
rotation further helps to reduce the amount of fertiliser used,
especially if you incorporate legumes, thus cutting the cost of
production and increasing the profit margins.
Though
not proven, there is an increase in yields in a rotation sequence, which
some agronomists term as the rotational effect. The growing of the
different crops on the same piece of land in the same season also
cushions the farmer from losses.
Just like
intercropping, crop rotation sequence can cushion the farmer against
adverse weather condition especially if dry land crops are incorporated.
Crop rotation helps in the management of some pests
such as root-knot nematode and diseases that have established in the
soil over time.
Soil erosion still remains one of the
greatest threats to land degradation. Crop rotation is a strategy that
can be used to curb this scourge.
Different crops such
as beans and other wide leaf plants can be used to reduce the raindrop
impact, crops such as grasses can be used to reduce sediment detachment,
transport, surface runoff and eventually soil loss.
Crop
rotation encourages the utilisation of various crop species, it assists
in the build-up of soil organic matter, improves soil structure and the
chemical and biological soil environment.
Soil
organic matter has additional advantage of improving water infiltration
and retention, increased drought-tolerance and decreased soil erosion.
The
main disadvantage of crop rotation is that the types of the crops
chosen depends on the farmer’s preference, which is influenced by the
environment especially water availability and temperatures.
The writer is based at the Department of Crops, Horticulture and Soils, Egerton University.
SOURCE: NMG
SOURCE: NMG
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