winter lettuce production
“By holding the root temperature at
about 75°F, it is often possible to lower air
temperatures without sacrificing quality or yield.”
Temperature
Some growers like to warm their nutrient solution in
the winter, especially in NFT systems, in which water is
continuously circulating over the roots. By holding the
root temperature at about 75°F, it is often possible to lower
air temperatures without sacrificing quality or yield. Since
fuel costs for greenhouse temperature control can become
significant in the winter, heating the nutrient solution
can save production costs. Just make sure the nutrient
temperature does not exceed 78°F. The warmer the water,
the less dissolved oxygen the water can hold, setting up
anaerobic conditions that can promote root rot.
Nutrients
Since lettuce is a vegetative crop, a well-balanced grow formula is all that is necessary to produce a high-quality crop.
Grow formulas are proportionately higher in nitrogen than
bloom formulas, usually with an N-P-K ratio of about 3-1-5.
The nitrate form of nitrogen is usually preferred, with no
more than about 3-10% of the available nitrogen in the
ammonium form. Ammonium ions are rapidly taken up
and used by lettuce plants for fast vegetative growth, but
too much ammonium nitrogen results in “rank” growth,
producing soft, weak tissue with poor shelf life. Nitrate
nitrogen is used more slowly by the plant, producing stronger, healthier lettuce plants.
Amino acid blends may also be beneficial in a complete
nutrition program for lettuce. Amino acids are intermediate chelators, improving the availability of calcium and
other important cations to plants. When phosphoric acid
is added to hard water, the bicarbonates are burned off as
carbon dioxide and water, but the calcium ions react to form
calcium phosphate. Calcium phosphate is what bone is made
of, and it is 95% water insoluble, so calcium phosphate locks
up both calcium and phosphate, making them unavailable
to the plant. When amino acids are added, however, the calcium remains available. Primary amino acid chelators such
as glutamic acid and glycine attach to the calcium ions like a
claw, preventing them from reacting with the phosphorous.
At the same time, glutamic acid and glycine stimulate root
cells to open up calcium ion channels, taking up calcium
thousands to millions times faster than simple osmosis.
Inside the plant, the calcium is used to strengthen the
cell walls and help protect the lettuce from temperature
stress, pathogen attacks and common physiological
diseases such as tip burn.
Water and EC
Unlike tomatoes and other fruiting crops, which
can handle high levels of nutrient salts, lettuce has
a low salt tolerance, so using a clean, slightly soft
water source with relatively low levels of minerals
and low levels of sodium is beneficial. During the
summer months, the total EC of the nutrient solution should be maintained at 10 cf or below. During
the winter months, with lower light levels, the EC
should be raised a little to help prevent stretching, but
the grower should still try not to exceed a total cf of 12.
By starting with raw water with a 2-4 cf and adding only a
mild nutrient formula, a good balance can be maintained
between fast growth and top quality.
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Maximum Yield USA | January 2015