The reason for using any irrigation system is to supply adequate moisture for the crop. This requires the replacement of water lost through the processes of evaporation from the soil and transpiration from the leaves (evapotranspiration). Evapotranspiration can be nearly zero during cool, calm and humid weather, but can exceed 1/3" per day when it is hot, windy and humidity is low. This loss can be measured with evaporation pans (see General Irrigation section).
The need for irrigation can be determined by measuring soil moisture. Many growers do this by grabbing a handful of soil from the area of the root zone and making a judgment as to the need for irrigation or to determine if it is time to shut the system off. An experienced grower can do a good job of scheduling irrigation this way.
Instruments can be used to measure the moisture status of the soil. A tensiometer is a practical tool growers can use. A tensiometer is a plastic pipe with a ceramic cup at one end and a vacuum gauge at the other. The cup is wetted to saturation, and then the pipe is filled with water. The pipe is inserted into the ground at the root zone. A rule of thumb is to place the tensiometer about 6" from the drip tape at a depth of one-third the entire root zone. As the soil dries, moisture is pulled from the tensiometer through the ceramic cup. This creates a vacuum, or tension, which is measured on a meter at the top of the tensiometer. When the tension rises to a certain reading, irrigation should begin, and it should stop when the gauge drops to a certain level. Some growers place tensiometers at two levels; one at 6" deep and another at about 12" to 18" depending on the root zone of the crop. The upper unit indicates when soil moisture near the surface is being depleted (begin irrigating) and the lower one shows when the moisture has moved to the bottom of the root zone (stop irrigation). The appropriate gauge readings are based on soil type. These units work well in sandy to loam soils, but are not useful in clay soils. They can be purchased through several field equipment suppliers.
A gypsum block is less commonly used by growers. It measures the soil moisture by measuring electrical resistance. After the gypsum is saturated with water, it is embedded in the soil. Soil water moves in and out of the gypsum block as the soil is wetted or dried. As the wetness of the gypsum changes, the electrical resistance changes. The block needs to be replaced after all the gypsum dissolves.
An important point to remember with drip irrigation is that the water is applied near the plant and that there is very little available moisture outside of the root zone. The root zone of a specific crop is much closer to the plant under a drip system than with other types of irrigation or where there is significant rainfall during the growing season. Soil moisture should be measured in the root zone, not away from the plant. During periods of warm weather, the moisture in the root zone can be rapidly depleted.
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 to 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.