Southeastern U.S. Vegetable Crop Handbook - 2021 - 34

so physically clog emitters. In treating water containing iron, chlorine
will oxidize the iron dissolved in water, causing the iron to
precipitate so that it can be filtered and removed from the system.
Chlorine treatment should take place upstream of filters in
order to remove the precipitated iron and microorganisms from
the system. Chlorine is available as a gas, liquid, or solid. Chlorine
gas is extremely dangerous and caution should be exercised
if this method of treatment is chosen. Solid chlorine is available
as granules or tablets containing 65% to 70% calcium hypochlorite
but might react with other elements in irrigation water to form
precipitates which could clog emitters. Liquid chlorine is available
in many forms, including household bleach (sodium hypochlorite),
and is the easiest and often safest form to use for injection. Stock
solutions can be bought that have concentrations of 5.25%,
10%, or 15% available chlorine. Use chlorine only if the
product is labeled for use in irrigation systems.
Since chlorination is most effective at pH 6.5 to 7.5, some
commercial chlorination equipment also injects buffers to maintain
optimum pH for effective kill of microorganisms. This type
of equipment is more expensive, but more effective than simply
injecting sodium hypochlorite solution.
The required rate of chlorine injection is dependent on the
amount of microorganisms present in the water source, the amount
of iron in the irrigation water, and the method of treatment being
used. To remove iron from irrigation water, start by injecting 1
ppm of chlorine for each 1 ppm of iron present in the water. For
iron removal, chlorine should be injected continuously. Adequate
mixing of the water with chlorine is essential. For this reason,
be certain to mount the chlorine injector a distance upstream
from filters. An elbow between the injector and the filter will ensure
adequate mixing.
For treatment of algae and bacteria, a chlorine injection rate
that results in the presence of 1 to 2 ppm of " free " chlorine at the
end of the furthest lateral will assure that the proper amount of
chlorine is being injected. Free, or residual, chlorine can be tested
using an inexpensive DPD (diethyl-phenylene-diamine) test kit.
A swimming pool test kit can be used, but only if it measures free
chlorine. Many pool test kits only measure total chlorine.
If a chlorine test kit is unavailable, one of the following
schemes is suggested as a starting point:
For iron treatment:
* Inject liquid sodium hypochlorite continuously at a rate of 1
ppm for each 1 ppm of iron in irrigation water. In most cases,
3 to 5 ppm is sufficient.
For bacteria and algae treatment:
* Inject liquid sodium hypochlorite continuously at a rate of 5
to 10 ppm where the biological load is high.
* Inject 10 to 20 ppm during the last 30 minutes of each irrigation
cycle where the biological load is medium.
* Inject 50 ppm during the last 30 minutes of irrigation cycles
two times each month when biological load is low.
* Superchlorinate (inject at a rate of 200 to 500 ppm) once per
month for the length of time required to fill the entire system
with this solution and shut down the system. After 24 hours,
open the laterals and flush the lines.
The injection rates for stock solutions that contain 5.25%,
10% and 15% can be calculated from the following equations:
FOR 5.25% STOCK SOLUTION:
Injection rate of chlorine, gph = [(Desired available chlorination
level, ppm ) x (Irrigation flow rate, gpm)] divided by 875.
FOR A 10% STOCK SOLUTION:
Injection rate of chlorine, gph = [(Desired available chlorination
level, ppm ) x (Irrigation flow rate, gpm)] divided by
1,667.
FOR A 15% STOCK SOLUTION:
Injection rate of chlorine, gph = [(Desired available chlorination
level, ppm ) x (Irrigation flow rate, gpm)] divided by
2,500.
It is important to note that chlorine will cause water pH to rise.
This is critical because chlorine is most effective in acidic water. If
your water pH is above 7.5 before injection, it must be acidified for
chlorine injection to be effective.
IMPORTANT NOTES.
- Approved backflow control valves, low pressure drains, and
interlocks must be used in the injection system to prevent
contamination of the water source.
- Chlorine concentrations above 30 ppm may kill plants.
Fertilization. Before considering a fertilization program for
mulched-drip irrigated crops, be sure to have the soil pH checked.
If a liming material is needed to increase the soil pH, the material
should be applied and incorporated into the soil as far ahead of
mulching as practical. For most vege tables, adjust the soil pH to
around 6.5. When using drip irrigation in combination with mulch,
apply the recommended amount of preplant fertilizer and incorporate
it 5 to 6 inches into the soil before laying the mulch. If equipment
is available, apply the preplant fertilizer to the soil area that
will be covered by the mulch. This is more efficient than a broadcast
application to the entire field.
The most efficient method of fertilizing an established
mulched crop is through a drip irrigation system, which is installed
during the mulching operation. Due to the very small holes or orifices
in the drip tape, high quality liquid fertilizers, or water-soluble
fertilizers must be used. Since phosphorous is a stable non-mobile
soil nutrient and can cause clogging of the drip tape emitters,
it is best to apply 100% of the crop's phosphorous needs pre-plant.
Additionally, apply 20 to 40% of the crops's nitrogen and potassium
needs pre-plant. The remainder of the crop's nutrient needs can
be applied through the drip system with a high quality liquid fertilizer
such as 8-0-8, 7-0-7, or 10-0-10. Generally, it is not necessary
to add micronutrients through the drip system. Micronutrients
can be best and most economically applied pre-plant or as foliar
application if needed.
The amount of nutrients to apply through the drip system depends
upon the plant's growth stage. In general, smaller amounts
of nutrients are needed early in the plant's growth with peak demand
occurring during fruit maturation. The frequency of nutrient
application is most influenced by the soil's nutrient holding
capabilities. Clay soils with a high nutrient holding capacity could
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2021 Vegetable Crop Handbook for Southeastern United States
Southeastern U.S. Vegetable Crop Handbook - 2021

Table of Contents for the Digital Edition of Southeastern U.S. Vegetable Crop Handbook - 2021
