Fertigation is the injection of soluble fertilizer into irrigation water. Nitrogen and potassium are available in liquid or soluble solid form and can be applied through a drip system. Phosphorus, if needed, is usually broadcast at the beginning of the season.
By using a fertilizer injector, trickle irrigation can be used effectively to apply N and sometimes K during the growing season. The need for supplemental N can be determined using the PSNT as it is with other application methods. Samples for the PSNT should be taken from under the plastic, if used. Use a soil sampler to punch a small hole in the plastic and remove a core of soil. Be sure to avoid cutting the irrigation tape when sampling under plastic.
With conventional topdressing or sidedressing, it is common to apply all the N in one or two applications. With trickle irrigation, it is convenient to apply small amounts of N weekly or even daily, which is desirable from a N management standpoint. For example, if you want to apply about 50 lb N per acre, you can inject a little over seven lb N per acre per week for seven weeks, or about one lb per day if you prefer. Small weekly applications provide for more efficient crop use of N than one or two larger applications. Daily application offers little advantage over weekly application, but may be necessary if the injector cannot inject a week's worth of N during the appropriate irrigation run time. To prevent leaching, the irrigation system should not run longer than necessary to effectively wet the root zone of the crop. If there is not enough time to inject all the fertilizer needed for the week in one injection, then smaller, daily injections are preferable. Before injecting fertilizer, the entire system should be filled with water at full operating pressure. When all the fertilizer has been injected, the system should run long enough to flush all fertilizer from the lines. If fertilizer is left in the lines, clogging may occur due to chemical precipitates or growth of bacterial slimes.
There is a potential for certain fertilizer materials to react with chemicals in irrigation water. If the water pH is below 7.0, there is little potential for problems, but at pH 8.0 and above, the risk is high. At levels above 40-50 ppm, calcium and magnesium are likely to react with phosphorus, if present in the fertilizer, causing precipitation of phosphates. If fertilizer containing calcium is added to water with concentrations of bicarbonates above 2 meq/liter, calcium carbonate may precipitate. Sulfates in fertilizers can react with calcium in the water resulting in the precipitation of gypsum. These precipitates can clog emitters.
Phosphorus- and sulfate-containing fertilizers, if needed, should be applied before planting because we are not concerned about these leaching. Nitrogen is the element that is most appropriate for injection into trickle irrigation water. Calcium nitrate has the potential to cause clogging if the water pH and bicarbonate levels are high as noted above. If calcium nitrate causes clogging, potassium nitrate or urea can be used as an alternative N source.
Water testing labs can analyze water for pH, calcium, magnesium and bicarbonates. You can also perform a simple test: Mix fertilizer into a container of irrigation water at the same concentration it will be after injection into the trickle system. Cover the mixture to exclude dust and let it sit for at least the length of time it will be in the system before it reaches the soil. If the water becomes cloudy or a precipitate collects on the bottom of the container, you can expect this to happen in the irrigation system with the likelihood of clogging. If it is necessary to lower the water pH, acid can be injected into the irrigation water. This requires special handling precautions and special injection equipment. Be sure to carefully follow directions to avoid personal injury or damage to crops or equipment.