Page 11 - Science Class 08

Pests can be kept away from crops by se ng done by using machines called harvesters. As
up scare-crows, by n-bea ng and by using
insec cides and pes cides. But these chemicals
are used (sprayed) when the insects and pests
are at the weakest stage of their life cycle.
Grown-up pests become resistant to these
chemicals.
But these chemicals must be sprayed in proper
amounts only. Over-spraying of these chemicals
can certainly harm the plants and pollute the
environment as well. Remember that spray of
these chemicals causes them to enter the food-
chain, to s ck to fruits and vegetables. So, they
must be properly washed before use.
When swarms of locusts and grasshoppers
a ack crops, manual spraying of these
chemicals cannot be done. Small aircra s are
used to spray these chemicals over vast areas.
Some common insec cides are BHC (commonly
known as gammaxane), malathion and disyston.
Crops are also damaged by diseases caused by
fungi, bacteria and viruses. These diseases are
transmi ed through seeds, air, soil or through
insects. Rust and smut are important diseases
of wheat which are caused by fungi. Blight of
potatoes is also a fungal disease. The diseases
caused by fungi can be protected by using
fungicides.
Harves ng, Threshing and Winnowing
We have read about how soil is prepared, how
seeds are sown and how we protect them.
Now, it is the me to study about harves ng,
threshing and winnowing.
When a crop has completed its life-cycle from
sowing of seeds to matura on and yielding of
seeds again, it is called a ripe crop. And then, it
is reaped or picked. This reaping or picking of a
crop is called harves ng.
Harves ng is manually done by using sickles.
But crops like wheat and paddy can also be
Science-8
for vegetables, fruits, flowers, tea, etc. hand
picking method is used.
As for threshing, it was done in the past by
hand or with the help of animals using a device
made of co on-s cks. This device was drawn
by oxen over the harvested crop — wheat, etc.
ll the chaff and grains were separated. But
these days, machines, called thresher are used.
A thresher is powered by electricity or diesel.
A combine is an advanced machine which
harvests as well as threshes the crops at the
same me. But the use of combine is harmful
in two ways. One, that it wastes a good amount
of chaff that makes good fodder for the ca le.
Secondly, it leaves much more stubble in the
fields that has to be burnt which causes a lot of
air pollu on.
A mixture of grain and chaff is obtained a er
threshing a wheat-harvest. They are separated
by the conven onal method of winnowing. It is
done using a flat baskets called a winnowing fan.
The mixture of chaff and grain is let fall from
the winnowing fan raising it over the head on
the days with a moderate wind. The chaff,
being lighter, is blown to a distance before it
falls down to the ground. But the grain, being
heavier, falls just below the front edge of the
winnowing fan.
Thus, two heaps are obtained — one of grain
and other of chaff.
Storage
The grain is filled in gunny bags while the chaff
is stored in a cabin made of dry corn stalks. It is
called a koop (well) in Hindi.
Grain is generally sold to governmental and
private agencies that store it in godowns. In
homes, it is stored in big barrels or bins or mud
structures called kothis. But large-scale storing
is done in silos or granaries.
11