| Attn: Ed. I'm passing this message along to you. It's from a commercial lettuce grower in Brazil and deals with growing methods and high temperatures. It's from Raul Vergueiro Martins and was posted recently on the HSA list site. You may find his comments useful. Here in Brazil we deal every day with high temperatures, and our experience,
specially in
our North East regions, where day temperatures reach 42 Centigrade under
shadow.
Our experience here, contradicts all the normal rules dictated and used in
the majority of
countries.
So I would advise you to consider what we make here. 1 - The lenght of the channels must never be larger than 12 meters, with a
minimum of 10
meters. The channels must be open at both ends, to permit a good air
circulation inside
them
2 - The slope of the channels must be at least 5%. The best lettuces I
know here, are
produced in channels with a slope of 24% at a hill side.
3 - Nutrient solution flow must be between 1,8 and 2 liters/minute.
4 - High Oxigen levels are a must, and a Venturi must be mouned in the
solution tank,
working with a separated pump (Never use the same pump used for the solution
circulation,
as this can reduce the pump efficiency). The Venturi discharge in the tank
must be at a
minimum distance of 30 cm of the solution catching point, and at a maximum
of 50 cm. It
is said that there is no recovering of Oxigen in the channels, but this is
not real.
With high solution speed inside them, and with plants in the holes, we can
recover not
only the Oxigent lost to atmosphere, but also the one taken by the plants.
The greatest
loss takes place when the solution enters the channels.
The high solution speed, highly oxigenated, also keeps the roots clean from
solid
residues.
5 - To reduce solution temperature and to keep it under the lower
temperatures possible,
not only the tank, but also all the pipes, must be buried.
To reach the best low temperature, you must make a hole in the soil, and as
you make it
more and more deep, measure the inside temperature, untill it it reaches a
stable level
whatever i the deepness of the hole. When you reach the stable
temperature, measure the
hole depth. That value
will be the depth you will need to use, to buri the pipes.
Here, 1,20 to 1,50 meters are common depths. It is interesting to note
that this
temperature
will be linear during Winter and during Summer.
Under these conditions, the solution will raise from the soil at the
entrance of the
channels, and at the discharge side, it will immediatly be carried to the
soil, thus
keeping it cool.
Making this, with short channels, from one extreme to the other of them,
your temperature,
in high Summer, will raise no more than one Centigrade degree during
daylight.
It will cool again during night, if you keep the solution circulating 24
hours a day.
During Summer, it is advisable to keep the solution circulating 24
hours/day.
In my web site, you can see some photos of an instalation at Rio Largo -
Maceió - Alagoas
from
Mr. Ricardo Mello, where we work with a constant temperature at the plant's
root of 30
Centigrade degrees.
At Rio Largo, the inside temperatures of the greenhouse are around 37 to 39
Centigrade
degrees. Look at the plants, and see the result.
These temperatures in the roots, contradict all we normally know about
temperatures needed
for the plant's roots.
6 - Nutrient solution concentration, during Summer time, must never be
higher than 0,8 mS,
and during Winter time, you can reach the maximum of 1,2 mS.
7 - You dont't need to use a greenhouse, but you will need to use it if your
rains are too
heavy. If you need to use a greenhouse, it should be an "Umbrella" type,
with an opening
on the roof in "all the greenhouse lengh" for hot air exaust.
The greenhouse side columns must have a minimun height of 4,0 meters, and
its sides should
be completelly open. You can use only white side shading cloth to control
insects and
excessive winds.
May be you need to use black or aluminized shading cloth on the roof.
8 - Don't use water nebulizers under the greenhouse.
10 - For you to evaluate the our plants transpiration, in our North East
regions, if we
work with a solution tank with 20000 liters, filled with 16000 liters of
solution, during
Summer, we replace around 6000 liters of water all days (4000 liters during
Winter).
Such a tank feeds 3500 square meters of final growing benches with a plant
density of 12
plants (lettuce) per square meter.
11 - Keep a good distance between the centers of the plants, for a good air
circulation.
For Veronica variety (a hybrid resistent to hot wether), a minimum distance
between plant
centers should be 30 cm. Veronica lettuce heads, here, normally reach 30
cm diameter,
weighting from 350 to 420 grams per head, after a growing cicle (considering
pre-growing
and final growing) from 35 to 45 days. |
