No matter which form of fertilizer you choose it should be tested with your irrigation water before injection into your micro-irrigation system. In a clear glass jar dissolve the fertilizer.
Test: question in irrigation water at the concentration equivalent to the mixture that would be provided to the plants. Allow this mixture to set 12 - 18 hours in the dark. If the mixture is cloudy or forms a precipitant, it may clog your micro-irrigation system if injected. Remember that this test is specific to this water source.
Fertilizer injectors can be divided into pumps and pressure differential methods. Pumps must develop a pressure greater than that of the irrigation line at the injection point in order to introduce the fertilizer into the system. Small positive displacement pumps or centrifugal units are usually used. Positive displacement pumps are very good because they can be accurately metered. Both electrical and water powered positive displacement pumps are available. Piston and diaphragm pumps are available which do not require electricity to operate. They utilize a portion of the irrigation water flow as motive flow. As with a pressure differential method some degree of system pressure is required to operate the unit. Suction line injection; venturi devices, and proportional mixers all require a pressure differential to operate. While suction line injection is a common method, it is not a recommended injection method. There is no way to accurately meter the injection rate using a pump suction line injection setup. Suction line injection should not be used due to the danger of back-siphoning the fertilizer into the irrigation water source if the pump were to stop. In addition, the fertilizer solution is pulled through the pump and comes into contact with the pump seals, which could shorten their effective life. The most common, recommended pressure differential method is the venturi. Venturi injectors are very economical and require no electric power to operate. Venturi injectors suitable for commercial vegetable production are available from $50 - $500. As a rule of thumb about a 20% pressure drop is required to effectively operate a venturi injector. With any chemical injection system a sound backflow prevention device must be in place to prevent accidental contamination of the irrigation water source. Any chemicals, including fertilizer solutions, should be injected up-stream of the filter system. If a precipitant of some type forms, it will be caught in the filters, rather than the emitters.
Quality of water use with drip system and suggestion
Water sources for drip irrigation can include municipally treated water, well water, pond or reservoir water, and ditch, stream, or river water. Clean water is especially important to successful drip irrigation. The small orifices found in all drip emitters can be clogged easily by physical and chemical contaminants found in the water. Groundwater from wells is generally of good quality and should be used when possible. Groundwater may contain sand or chemical precipitates. Surface water can be used but often contains bacteria, algae, and other aquatic life. In any drip installation a screen filter or disc filter and chemical treatment of surface water is generally required. Usually, fast moving water contains higher levels of suspended particles, and reservoirs or ponds contain a relatively small amount of these particles. For small drip irrigation operations, the water source does not have to be excessively large. Most small systems require only 2 to 5 gallons per minute per system and can use a 3/4" to 1 " filter and a small fertilizer injector
Be aware of the effects of storage time and temperature on fertilizer solutions. Many solutions may salt out at lower temperatures. For example urea-ammonium-nitrate solutions will salt out at about 60 EF.
If injecting phosphorous and potassium, make sure you use soluble forms. White phosphoric acid and potassium nitrate are soluble forms of these nutrients.
Be aware that phosphate will react with calcium to form a precipitant (make sure you test before you inject!)
Always inject all fertilizers and chemicals before (upstream of) your filtration system.
Inject fertilizer toward the end of an irrigation cycle, but allow enough time for clear water to displace the fertilizer in the irrigation system. This ensures all the fertilizer reaches the plants and reduces fertilizer build-up on or in emitters.