Fertilizer Calculations: Proportioners

Liquid feed fertilizer programs used in greenhouse crop production allows a grower to manipulate plant nutrition conveniently. This is done by using different fertilizers containing equal ratios of nutrients and adjustment of specific fertilizer elements within a fertilization program. These ratios are calculated using the formulas discussed in the previous issue.

To apply soluble fertilizer to a crop in its final dilution, fertilizer injectors are used to introduce the fertilizer into the irrigation water. There are two major categories of injectors: proportional fertilizer injectors and positive displacement fertilizer pumps (non-proportioning types).

Proportional fertilizer injectors

There are three categories of proportional injectors based upon their principle of operation. They include: 1) pressure differential injectors, 2) water motor injectors, and 3) water meter injectors.

Differential pressure injection may be accomplished by a pressure differential such that the pressure of the point of injection is less than at the intake of concentrated fertilizer. Concentrated fertilizer is then pulled into the irrigation water. This is accomplished by connecting a hose to the vacuum side of the irrigation pump and placing the other end in the concentrated fertilizer solution. An adjustable valve in the hose or a series of valves are used of control the volume of concentrated fertilizer solution withdrawn. This is done to vary the fertilizer concentration without modifying the concentrate. The injected fertilizer concentration can inadvertently be altered by changes in pump speed and line pressure due to leaks, clogged nozzles, and faulty valves- If air is allowed to enter the stock fertilizer suction line, the system will probably have to be reprimed.

Venturi type proportioners are similar in principle to the system described above. As water flows through a constriction in the proportioner, the water speed increases and pressure decreases. A tub attached at the constriction facilitates movement of fertilizer from the concentrated solution into the irrigation water. The Siphon Mixer uses the venturi principle and has a dilution ratio of about 1:15 with inlet water pressure of 30 psi. Changes in inlet water pressure, flow rate, or any factor creating back pressure on the output side of the proportioner such as a constrictive nozzle or a kink in the hose, will alter the dilution ratio. Generally, 50 feet or less of hose is used although some line pressure drop is experienced regardless of hose length. The Siphon Mixer has a dilution capacity of one gallon of concentrate in five minutes.


Displacing concentrated fertilizer dilution into the irrigation water is another example of pressure differential injection. There is a bag inside a metal tank. As water fills the tank, the bag (containing fertilizer concentrate) collapses forcing fertilizer concentrate into the irrigation water. This type of injector contains few moving parts; however, inlet water pressure and flow rate does alter injection accuracy. Flow control valves are used to compensate for variations in inlet flow rate.

Water motor controlled injectors use water flow to operate a piston or diaphragm that injects or forces fertilizer into the irrigation water by positive displacement. As water flows through the injector, the water moves a cam to turn and push a piston back and forth. Consequently, oscillation of the piston varies with water flow.

Water meter controlled injectors use a water meter mechanism to determine the flow rate and water powered diaphragm pumps to inject the fertilizer.

Positive displacement fertilizer pumps displace a fixed amount of concentrated fertilizer each time fertilizer is forced into the irrigation water. A piston or piston and diaphragm is used to displace the fertilizer. Injectors of this type are usually driven electrically and should have a common electrical circuit with the irrigation water pump so the injector will stop when the irrigation pump is stopped.

When positive displacement injectors are use, a blend-tank may be needed in the water line immediately following the point of injection to ensure adequate mixing of water and fertilizer. This is especially true if the fertilizer passes thorough pipe lengths insufficient to adequately mix the fertilizer.

Injection Calculations

After selecting an injection system for proportioning concentrated fertilizer into irrigation water the amount of fertilizer to be dissolved into the concentrate must be calculated. Regardless of the type of proportioner or ratio, the first step of determining the amount of fertilizer is to determine the volume of irrigation water to be applied. To do this, note the proportioner rate given by the manufacturer. These values are given as a ratio, for instance 1:100. What this means is that for each 100 units of water, 1 unit of concentrate is mixed for a total volume of 100 units. Therefore, 50 units of concentrate will yield 5,000 units of fertilizer and irrigation water. Note that "units" can be changed to gallons, liters, quarts, or any other volume to suit your needs.



After the total volume has been calculated, all that is left to determine is the amount of fertilizer to mix a concentrated solution. Take the value 8.09 oz. ammonium nitrate / 100 gal. of water which yields 200 ppm N calculated in Part 1, and simply multiply by 5,000. This results in the value 404.5 oz. or 25.3 pounds of ammonium nitrate that must be dissolved in 50 gallons of water using a 1:100 proportioner.

When mixing concentrated fertilizer salts in water, remember to use hot water. As salt dissolves in water, an endothermic reaction occurs. This means that heat is removed and the water chills. This is the same reaction used to chill ice cream in a hand crank freezer. Therefore, if hot water is not used, the fertilizer concentrate may chill to a temperature too low for adequate blending.