If you have a moderate-to-large collection of plants, you might be caught in that “no man’s land” between mixing up watering cans of fertilizer and purchasing a metering pump. Siphon-type proportioners are a good way to simplify life, but do have their limitations. Let’s start by looking at how they function.
Siphon proportioners utilize a venturi device that narrows the flow of a water stream to increase its velocity, then expands again. That expansion reduces the pressure, creating suction that draws the fertilizer solution into the primary stream.
Most commercial fertilizer siphon have a 16:1 ratio built in. That is, for every 16 gallons of water that pass through it, one gallon of fertilizer is drawn in. Unfortunately, if the inlet water pressure or flow rate is too low, or if the back-pressure from a long, narrow hose after the device is too high, the ratio is changed, and in extreme circumstances, can stop it from drawing the fertilizer solution in at all.
There are several strategies that can be used to avoid that:
- Be certain your spigot is wide open to maximize the water pressure.
- Use as large a diameter and short a length of hose afterwards as is possible.
- Attach the device to a section of large-diameter hose, rather than the spigot itself, allowing you to shorten the hose on the delivery end.
No matter what you do, I strongly advise that you do a quick calibration exercise to determine the true mixing ratio. This is how I did it (having a helper is a “plus”.):
- Fill a milk jug or small bucket with a measured gallon or two of water into which you’ve introduced a strong dye, such as food coloring.
- With the water running at full flow through your siphoning device, drop the suction line into the dyed water and immediately begin filling a 5-gallon bucket from the delivery end of your watering wand.
- When it is full, switch to a second 5-gallon bucket, and dump the first. Alternate as needed.
- Count the number of times you have filled the bucket, stopping when your dyed water runs out.
At that point, you know the true injection ratio of your siphoning device under your specific conditions.
If your circumstances of water pressure and/or delivery hose prevent you from using such a device, you might consider using a hose-end sprayer, instead.
As far as determining the proper concentration for either method, we have provided a Fertilizer Concentrate Calculator for your convenience.