Understanding water flow in a fertilizer injection system I recently set up a small EZ-Flo fertigation
system.
This is a tank full of fertilizer solution that you attach to a pipe that you are using for a sprinkler or
irrigation. As water flows down the pipe, small amounts of fertilizer are
"injected" into the water from the tank. This is a passive process entirely
driven by the pressure from the water source.
There is an adjustable valve in the pipe at the site at which it
attaches to the tank. There are branches in the pipe on either side of this
valve. These are an input to (before the valve) and output
from (after the valve) the tank. Both the input and the output are
thin, flexible hoses that connect to the tank, where they become dip tubes that
travel to the bottom of the tank.
When the system is working, as water flows down the pipe, some of it is
diverted down the input into the tank, where it mixes with fertilizer.
Simultaneously, fertilizer solution is siphoned up the output, where it joins
the pipe on the other side of the flow restrictor.
The above EZ-Flo link is a simple illustrative video of how the system
functions. Mine is basically like the one shown, with the exception that the
fertilizer tank is about 1 foot under the pipe.
I have observed that, if the pipe's valve is too open (i.e. not
restricting flow), then no fertilizer seems to be injected-- there doesn't seem
to be any outflow from the tank. It is hard to tell exactly what is happening, but the (transparent) outflow tube remains clear. When the valve is further closed, the outflow tube fills with the (colored) fertilizer solution, indicating that it is being siphoned up into the pipe. So it seems 1 of 3 things is happening when the valve is too open:

*

*The clear pipe water flows down to fill both the input and the output, rather than water flowing down the input and fertilizer up the output.

*Air remains in the output tube and the pipe water flows over it.

*Somehow a very diluted form of the fertilizer (so diluted it looks clear) is taken up the output. But I don't know how this would possibly occur, since the input water has to travel through the fertilizer solution.

Can someone explain what is happening?
 A: This how the EZ-Flo fertigation system works-
When you close the ballvalve a little(or more than a little), the pressure of the water inside the pipe (before the valve) increases. So the water inside it tries to escape through the input tube into the tank. When it reaches the tank it pushes the fertilizer inside the tank for more space. And the fertilizer is pushed out through the output tube into the water pipe where, it gets mixed with the water flowing.
Now, when you don't close the valve, the water doesn't have any reason to flow down the input tube and up the output tube i.e., it has got a straight path through the pipe. So it travels straight  without mixing with the fertilizer. One may think that the force of gravity might be enough for the water to travel down the input tube. But air occupies space. So the air inside the tube prevents the water from entering the tube. Moreover, even if the water somehow makes its way down the input tube, it will take some work for it to push the fertilizer up the output tube which can't be provided by the force of gravity.

"It seems that the pipe water flows down to fill both the input and the output, rather than water flowing down the input and fertilizer up the output."

No, it doesn't. The pipe water doesn't flow down to fill both the input and the output tubes. It's just your feeling. Maybe the water inside input tube and the fertilizer inside the output tube(if you've used them before) will still remain there. Or if you haven't used the EZ-Flo fertigation system before experimenting with the ballvalve, the air inside the tubes will remain there(which will look transparent).
