July 2006 - Hydroponic Culture
of Lettuce II
Introduction:
Please refer
to the previous month’s
article “Hydroponic Culture of Lettuce I” for
details on varieties, starting the plants, nutrient
formulation and nutritional & environmental disorders,
pests and diseases. That article gives lots of specific
information for the growing of lettuce under greenhouse
hydroponic cultures. This article describes
in detail the NFT narrow channel growing system.
Production systems:
A. Agri-Systems
(narrow gutter) NFT:
This system differs from other NFT systems
in that it uses long, narrow, aluminum gutters
with a movable plastic tape cover (photo 1). The
engineering behind this system was to create an
automated system capable of increasing the percentage
of greenhouse floor area used directly for production.
The gutters are extruded aluminum measuring 3 inches
(7.6 cm) wide by 2 inches (5 cm) deep. Special
ridges or “cooling sinks” are molded
into the bottom of the extrusion to assist in cooling
the channels through conduction as cold air passes
underneath the channels. This design with heat
sink ridges permits the solution to remain cool over
more distance so that the channels may be up to 70
feet (21 meters). I do not recommend they exceed
that length as nutrient and oxygen gradients along
the gutters will cause nutrient imbalance and growth
disorders.
The gutters are supported by a galvanized pipe bench
frame at waist height (36 to 40 inches). The
frame has at least a 2% slope from the inlet end
to the catchment end of the channels. I
would recommend with these longer channels to slope
them from 3- to 5- percent. This greater inclination
will permit the nutrient solution to flow faster
below the root systems improving oxygenation and
stability of the nutrient solution.
The channels are cooled with the assistance of the
heat sinks on the bottom being exposed to cool air
forced under the benches. The air is
cooled with evaporative cooling pads at one end of
the greenhouse and forced underneath the benching
by large exhaust fans (photo 2). With polyethylene
side curtains mounted on the benching (photo 3),
and polyethylene convection tubes, cool air is directed
past the bottoms of the NFT channels cooling them
with the assistance of the heat-sink ridges on the
bottom of the gutters.
A nutrient tank cistern at
one end of the greenhouse stores 3000 to 4000 gallons
(10,000 – 15,000
liters) of solution per acre (0.4 hectare) of production
system. Generally, several smaller tanks of
1500 gallons (5500 liters), one for each section
of NFT channels is better so that growing sections
are independent of one another. The tanks may be
as small as 500 gallons (2000 liters) provided that
the greenhouse has an automatic electrical generator
backup system in case of power failure. With
a continuously re-circulating system more than half
of the total volume of solution remains in the NFT
gutters. If a pump stops the solution will
soon overflow the nutrient tank.
The solution is pumped through a 2-inch diameter
PVC inlet header at the higher end of the channels
and enters each channel via a 1⁄4-inch drip
line (photo 4). The nutrient solution is collected
at the lower end of the channels by a catchment gutter
and returned to the cistern by a main return pipe
(photos 5-6).
A flexible, moveable, tape cover slides on top of
two ridges of the inside face of the channel within
1⁄4-inch of the top edge of the channel as
shown in photo 1. The cover is made of 1/8-inch
thick plastic that is flexible enough to be rolled
up. This flexibility permits it to easily enter
the channel guides and be wound up on the other side
when harvesting by a simple motorized drum. One-inch
diameter holes are pre-punched in the moveable tape
at 6-inch (15-cm) centers for the lettuce plants. Channels
are spaced 6-inches (15-cm) apart giving the lettuce
a 6” by 6” spacing as shown in photo
3. Using this spacing with 70-ft channels and
a 6-ft aisle between sections, approximately 145,000
plants per acre may be grown on a 30- to 40-day cycle,
depending upon the available sunlight.
Seeds are sown in special plastic 154-celled “cream
cup” trays commonly used in restaurants (photo
7). Slits are cut in the bottom of the rows
of cups with a multi-bladed table saw to permit drainage
and the roots to emerge. Each cup (cell), about
1-inch deep by 3⁄4-inch in diameter, has a
small lip about 1/8-inch wide along the top edge. The
cell flats are filled with coarse vermiculite using
a flat filler machine.
Seeds are sown into each cell using a vacuum-seeding
wand or seeder as described in the earlier article
on lettuce culture (Vandana Seeder). The flats are
soaked with raw water after sowing and then kept
in a cooler for 1 to 2 days until germination occurs. Be
careful that the seedlings are removed from the cooler
as soon as their shoots start to emerge from the
seed, otherwise, they will rapidly extend and develop
into weak, spindly, seedlings, which will bolt (form
a shoot to flower) before the lettuce is fully mature. Such
bolted lettuce is not marketable. The seedlings
upon germinating are then transferred to an ebb-and-flow
benching system in a special seedling greenhouse
(photo 8). In the ebb-and-flow benches nutrient
solution is periodically pumped into the bed to flood
the trays sitting in the bed. The solution
then rapidly drains back to a cistern until the next
irrigation cycle. On the return the solution
is oxygenated as it falls back into the cistern. The
seedlings remain in the seedling house for 10 to
14 days until they are ready to transplant at the
2- to 3-true leaf stage (photo 9).
In preparation for transplanting each small cup with
its seedling is punched out of the trays and placed
in flats lying on their sides. A worker then
transplants these seedlings directly into the moveable
tape cover of the NFT gutter (photo10). A planting-harvesting
machine feeds the tape cover into the top of the
NFT channel while a worker places the seedlings into
the holes of the cover as it is moving into the channel. This
is done using two of these machines. One
at the end where the worker is transplanting has
a roll of the tape cover on a drum and another at
the far end of the gutters is pulling the tape into
the gutter. While the tape is moving, a worker is
placing the seedlings into the holes of the cover.
The lip of the cup sits on top of the moveable tape
with the rest of the cell positioned inside the gutter
so that the bottom is just above the bottom of the
gutter. This small distance between the
bottom of the cup and the gutter permits good oxygenation
and rapid flow of the solution past the suspended
roots in the gutter. The plant roots lying
in the bottom of the gutter permit the nutrient solution
to flow over them. Eventually, the roots will form
a complete root mat on the bottom of the gutter. For
this reason, it is important that the gutters have
adequate slope of 2% to 3% or greater so that the
solution will flow past the roots carrying oxygen
to the roots and not backing up behind the root mass.
A unique challenge exists in using this NFT system
in that the channels are spaced only 6 inches apart. This
close spacing, while adequate for lettuce, permits
the plants to growth tightly together as they mature. In
doing so, the moving of an individual row during
harvesting creates abrasion of the plants in the
moving gutter against the neighboring ones. This
can damage the lower leaves of the lettuce and break
them off the plant. To overcome this problem
and at the same time reduce light competition among
the rows of the plants, every third row is transplanted
until the entire section is completed (photo 11). Next,
every second row is transplanted and finally every
first row. In this way, each adjacent
row has plants of 10 days difference in age. Having
smaller plants next to older ones gives better light
to the crop and reduces the abrasion of one plant
against another during harvesting when the tape is
pulled from the channels.
During harvesting the moveable tape cover is attached
to the motorized drum of the planting-harvesting
machine and the tape is pulled along the NFT channel. As
the tape winds onto the machine, several workers
cut the lettuce heads from the tape and remove the
roots (photo 12). The harvested lettuce is
stored in a mobile cooler that transports the product
to the packing room where the lettuce is packaged
in heat-sealed polypropylene bags and placed into
cardboard boxes for cold storage until shipped (photo
13).
The moveable tape covers are sterilized between crops
in a vat of 10% bleach solution (photo 14). They
are rinsed and dried before re-using them in transplanting.
The NFT gutters are not sterilized between each crop,
but only every 3 to 4 months or as needed if a disease
problem arises. Pythium is the most
serious disease with lettuce. It gets into
the roots and kills them causing wilting of the plant
and stunting to finally death due to the inability
of the plant roots to take up sufficient water. It
can be controlled by keeping the nutrient solution
chilled to under 70 F and by using a Ridomil fungicide
in the solution of the cistern for the section being
cleaned prior to changing the nutrient solution.
A one-acre greenhouse can grow in excess of 140,000
plants. The greenhouse shown here is 20,000
square feet and grows 70,000 plants at any time. Based
upon a 32-day average cropping period, 2200 head
of lettuce can be harvested daily (photo 15).
This is a very efficient system for growing lettuce,
bok choy and some herbs as basil. The
crops must be a one-time harvest. Basil
is best cut as one harvest, so could be grown in
this way. The system, however,
is very expensive. Another disadvantage
is that you have no access to the sections of crops.
This can make any spot spraying difficult, but with
the use of fogging sprayers the grower does not need
access as the spray will carry sufficient distance
to fully cover the crop
