Application uniformity of
fertilizers and pesticides is critical, but it can be difficult
to determine when a fertilizer or chemical (fertigation/chemigation)
is applied through drip irrigation tape. Several factors such as
injector type, injection rate, and flow rate affect uniformity.
Several types of injectors are commonly used for fertigation of
horticultural crops. Venturi-principal injectors use differential
pressure to draw the fertilizer solution into the irrigation supply
line. A second type, i.e. a positive-displacement pump, forces the
fertilizer solution into the pressurized irrigation supply line.
A third type is the water-driven-piston proportional meter that
injects a predetermined amount of solution into the water that powers
the main piston.
Continuously diluting injector, primarily for experimental and demonstration
plots, has also been developed. Unlike the Venturi and pump systems,
this injector continuously adds fresh water to the fertilizer reservoir
while simultaneously drawing solution out into the irrigation supply
line. This design is a means of more easily applying multiple fertigation
treatments to small plots.
Much research has been conducted investigating drip irrigation system
design and water distribution uniformity. A report suggests that
fertilizer uniformity can be reatly influenced by injection method
and management during the injection process. Little work however,
has been conducted evaluating injector type and injection rate on
fertigation uniformity in a drip irrigation system.
Studies were conducted in a greenhouse and in the field. The objective
of the greenhouse experiment was to evaluate the effects of injector
type and fertilizer type on fertilizer distribution uniformity in
a drip irrigation system in a controlled environment. The objective
of the field experiment was to evaluate positive displacement pumps
in terms of injection rate and solution volume on fertilizer application
uniformity with this system.
Greenhouse Experiment
Injector type had a significant effect on fertilizer distribution
uniformity through drip irrigation tape. More uniform fertilizer
distribution was obtained with the venturi and proportional injectors.
Poor uniformity was obtained with the pump and continuous injectors.
In both situations, it appeared that the injectors were not correctly
sized to the water flow rates. In the case of the pump injector,
a bolus effect occurred because of the rapid injection with a slug
of fertilizer solution that remained intact in the irrigation stream.
With the continuous injector fertilizer appeared to be left in the
canister even after 2-h of injection time, which reduced distribution
of the fertilizer material.
A limitation of the continuous injector was the time required to
verify application uniformity relative to a specific delivery system.
Because all the fertilizers dissolved readily, fertilizer type did
not have an affect on uniformity distribution.
Field Experiment
Uniformity of fertilizer distribution in the field was greater with
a low injection rate that with a high injections rate. Injection
time with the low-flow pump took twice as long as the injection
time with the high-flow pump. It has been suggested that injecting
fertilizer over the longest possible time produces the greatest
field uniformity.
Although there was a trend toward more uniformity as solution volume
increased, differences in uniformity distribution among 5,10, and
20 gal. fertilizer solutions were not significant.
Conclusions
Injection rate had a significant effect on uniformity of fertilizer
distribution. Better fertilizer distribution in the greenhouse experiment
was obtained with the venturi and proportional injectors. In the
field, better distribution was obtained with the 1-gal/min pump.
Injection times were longer with these pumps than with the other
treatments in the greenhouse experiment, with the exception of the
continuous injector.
A problem with using long injection times is that the total irrigation
time (time to fill lines, inject, and then flush system) may be
longer than necessary to meet plant water needs. Injectors tested
in this experiment will give uniform fertilizer distribution in
the injector is properly sized with the water flow rate of the system.
In field situations, the problem encountered in the greenhouse with
injectors not being properly sized with the water flow rates of
the system is not a concern because flow rate in a field would be
10 to 50 times greater that in the greenhouse experiment.
