back to basics
Taken at face value, irrigation
appears to be a simple operation,
merely ‘watering plants’. But if
you get it wrong, crop growth
will be hampered and the quality
of the crop compromised.
Frequent watering will prevent
plants drying out between
waterings. The irrigation
cycle should, however, also
be long enough to ensure
that the medium drains out
adequately each time.
If you under-irrigate, the
plants will wilt and turn dark
green to blue-green. The stomata
will close to reduce moisture
loss through transpiration,
and this will increase the
plant’s internal temperature, as
cooling by evaporation cannot
take place. If it lasts for a long
time, yield will be reduced.
Over-irrigation will create
an oxygen deficit in the root
zone, which will damage the
roots. This will reduce water
and nutrient uptake and, in
extreme cases, cause wilting
and stunting. A crop produced
under these conditions does
not last or transport very well.
The following are important
when using a hydroponic system:- Growth medium
The roots of a crop must grow in
a well-drained, aerated medium
to enable proper root functioning.
If the medium is too fine or too
coarse, proper watering will
be difficult, leading to poor
plant growth and quality. Use
a well-drained medium with
high water-holding capacity
in an open-bag system. - Irrigation volume
As a general guideline, 10% to
20% of the volume of irrigated
water must drain freely out of
the growing medium after each
irrigation session. This will also
leach out excessive fertiliser
and salts that might otherwise
build up to a toxic level.
ABOVE:
Over-irrigation
can create an
oxygen deficiency
in the root zone,
which results in
poor water and
nutrient uptake.
US Department
of agricUltUreThe water-holding capacity of the growing medium
determines how often and how much to irrigate.
wilting
negatively
affects yield,
even if the
plants recover
c o m p l e t e ly
The irrigation levels and
cycles should be kept short
just after the seedlings have
been transplanted, as the water
drains quickly from the bags.
As the root system develops,
increase the length of each
irrigation and the interval between
successive irrigations to enable the
required 10% to 20% drainage.- Irrigate before water stress
Irrigation should start before
a plant shows early symptoms
of heat stress. These signs are
difficult for the human eye to
spot. A good rule of thumb
is to start an irrigation cycle
when the plants have used
30% to 50% of the available
water in the growing medium.
However, this will also depend
on the type of medium used.
Symptoms of wilting are a
sign that irrigation should
have taken place long ago.
Studies show that wilting
negatively affects yield, even
if the plants recover fully!
Various irrigation systems
are available. Next month, we’ll
look at micro-tube and drip
irrigation for bag cultures and
gravitational flow systems. - Source: Niederwieser, JG (ed).
- ‘Guide to Hydroponic Vegetable
Production’. Agricultural Research
Council, Roodeplaat Vegetable and
Ornamental Plant Institute.
▪
FWHydroponics
Irrigating your crop
44 farmer’sweekly 2 August 2019