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Fertigation of Vegetable Crops

Posted: July 5, 2012

Total fertility requirements in plasticulture are not different than those in conventional open-soil culture. With a drip irrigation system, however, application can be much more precise and timed with crop development.


Bill Lamont (wlamont@psu.edu), Penn State Extension Vegetable Specialist

Soluble fertilizers can be added to the drip irrigation water to provide uniform crop fertilization. A simple "hozon brass siphon mixer" venturi injector draws soluble fertilizer from a bucket or jug into the line at a preset ratio (usually 1:16 or 1 gallon for every 16 gallons of water flowing through the line).

However, the hozon injection system, is suitable only for one-third to one-half acre plantings or less. Other venturi units are available in sizes up to two inches in diameter. More expensive injectors with greater capacity and accuracy use an electric or hydraulic "pump" to inject fertilizer solutions from a stock tank into the line. A hydraulic device, called a Dosatron, placed in the mainline can be set at various dilution rates and operates with water flowing directly through it. Use only high quality, soluble fertilizers that completely dissolve. All fertilizer injections should be made ahead of either the main filters on the line or the secondary filters if placed closer to the field, so that any contaminants are filtered out.

Fertigation is used most commonly to supply nitrogen and potassium, because they are highly soluble and move easily through soils to roots. Phosphate and micronutrients are best applied prior to planting and not injected through the irrigation system. Other chemigation applications may include pest control measures, but check label restrictions on use in chemigation applications. If any fertilizer or chemicals are applied through the system, a check valve or proper back-flow prevention devices are required to ensure that no contamination of the water source is possible. 

Preplant Fertilizer
Take a soil test to know what level of fertility is in your soil. Use a starter fertilizer, a small amount of fertilizer, either liquid or dry, that is applied in the bed in drip irrigated crops. This fertilizer would contain all of the phosphorus (P) and micronutrients and up to 20-30 percent of the nitrogen (N) and potassium (K). On soils testing very low in P and K, the starter can be broadcast or banded in the bed. If only small amounts of P and micronutrients are required, then it would probably be better to band these materials two to four inches below the bed surface and to the side of the plant row, but not between the drip tube and the row. In most cropping situations, approximately 20 to 30 pounds per acre of N and K would be sufficient in the starter fertilizer mixture. In situations where the soil test index for P is high or very high, then no P would be added to the soil.

Note on phosphorous and micronutrients. In general, simultaneous application of P and micronutrients is not recommended in drip irrigation systems. This is because of precipitation events that can happen between the fertilizers or between the P and the calcium or magnesium in the well water. If application of P is required during the season (such as during cold periods), it should be injected as phosphoric acid alone, in separate applications. Acidification of the irrigation water to pH 4.0 to 5.0 might be needed to keep the P in solution during this fertilizer application. Acidification can be achieved by using phosphoric, sulfuric, hydrochloric, or other acids to reduce the pH of the water. Concentrated acids always must be added to water, never the reverse.

Similar problems also occur for micronutrient injection. The key is to avoid precipitation. If micronutrients must be injected, then soluble forms, less subject to precipitation, such as chelates, should be used. Like P, micronutrients should be injected alone.
Injected Fertilizer     
Rates. In most situations, injected fertilizers will consist only of N and K. The amount of N to use is determined basically by the N requirement of the particular crop. This amount of N is recommended for each crop for each season. The current recommendations for open-soil culture can serve as "starting points" for developing local plasticulture recommendations.  

The amount of K to be injected is based on the soil-test predicted requirement of K for the crop minus the portion of this requirement that is applied in the bed as a starter. For example, if the soil tested medium in K, perhaps only 100 pounds per acre of fertilizer would be required for the season. If 20 percent of this K, i.e., 20 pounds, were applied in the bed as starter fertilizer, then 80 pounds would be injected through the season.

Sources. Several sources of N and K can be used for drip irrigation injection, but all sources must be highly water soluble to be effective. Nitrogen sources include ammonium nitrate, calcium nitrate, various N solutions, and urea. Potassium can come from potassium nitrate, potassium chloride, or potassium sulfate.

Frequencies. It is most convenient to think of rate of injection in terms of pounds of a particular nutrient per acre per day or week. For example, the recommended schedule of N injection for a particular crop might be to start out early in the season with l pound N per acre per day and finally inject 2.5 pounds per acre per day when the crop is at its peak growth rate. The general rule is that the amount of N and K injected per day or week starts out low and peaks with the crop demand for the nutrients. It is tied to the stage of crop growth or development. 

Nutrients can be injected into the system in various frequencies. Basically, the frequency of injection, whether once a day or once every two days or even once a week, depends on system design constraints, on soil type, and on grower preference. Research has shown that the frequency, even up to once per week, is not as important as achieving a correct rate of application of nutrients to the crop during a specified period of time. With computer control of drip irrigation systems, some growers find it easy to inject more frequently, such as once every day. This may have a slight advantage logistically. For example, injecting fertilizer on a more frequent basis would reduce the chances that nutrients were leached from the beds during a heavy rain storm or excessive irrigation compared to injecting larger amounts on a less frequent basis. If the chances for leaching losses are extremely low for any particular field, then injection once per week would be satisfactory. In any case, it is extremely important that the nutrients applied in any irrigation event are not subject to leaching either during that same irrigation event or by subsequent irrigation events. This is why knowledge of the crop root zone is important for optimum fertilizer management. It is critical to monitor the application of water and to realize that fertilizer application is linked closely to water application. To be a good fertigator, a grower first needs to be a good irrigator.

When injecting fertilizer in noncontinuous (bulk) fashion, such as once per day or once per week, it is important to keep in mind a few pointers about the operational sequences for the injection events. The drip irrigation systems always should be brought up to operating pressure prior to injecting any fertilizer or chemical. After the system has been pressurized fully, the fertilizer can be injected. Following the completion of the fertilizer injection, the drip irrigation system should be operated for a period of time to ensure flushing of the nutrients out of the tubes and into the soil. This period might be the next irrigation cycle of the day, if that water will not contain fertilizer. With these operation constraints in mind, it becomes very important to design the drip irrigation system so that fertilizer injection can be achieved in a reasonable amount of time without running the risk of overwatering the crop to get the fertilizer applied. This means that injection pumps, pipe sizes, and injection rates must be adjusted properly to apply the nutrients in the desired amount of time, so that the system can still be flushed without applying excess water during the injection and subsequent cycles.

In some systems, fertilizer is injected continuously (concentration injection) so that all irrigation water applied contains nutrients. This system is acceptable as long as no irrigation cycle is excessive, causing nutrients to be leached below the root zone.

It should be apparent from the above discussions that water application and fertilizer application are linked inextricably.