When growers in Las Cruces, N.M., began installing drip irrigation systems in the early 1990s, they were harvesting 16 tons of green chili peppers per acre. Today, those growers are averaging 33 tons per acre.

Green Chile Pepper
APPLYING NUTRIENTS through drip irrigation systems has helped improve chili pepper yields.

Drip irrigation isn’t the only reason chili yields have doubled. Plant breeders have developed higher-yielding varieties, and harvesting techniques have improved. But being able to put water, nitrogen, phosphorus and potassium where plant roots can best use them has also helped.

“In 1993, when we saw the first drip systems, the average green chili pepper yield was 16 tons per acre,” says William Cox, owner of CoxCo Ag Services in Las Cruces. “With peppers selling for $295 per ton, revenues averaged $4,400 per acre. In 2005, peppers were still selling for $295 per ton, but revenues had climbed to more than $9,000 per acre due to the higher yields.”

Prior to installing drip irrigation, growers often left 30 percent of their acres out of production because of lack of water for the furrow systems most of them used, said Cox, a private consultant to the growers. With drip systems they can plant all their acres.

Not all who have adopted fertigation systems have been able to double their yields. But most experience significant yield and quality benefits from the technology.

“Fertigation offers a boost in water and nutrient efficiency by simultaneously placing moisture and nutrients directly in the root zone where they are most needed, frequently resulting in the need for less fertilizer and water,” says IPNI agronomist Rob Mikkelsen.

Chili peppers growers in the Las Cruces area have shown that to be the case. Before drip irrigation, they were putting out 90 gallons of a nitrogen solution in three sidedress applications. They typically apply 40 gallons through the drip system.

“Drip irrigation allows us to keep the moisture levels in the field at optimum all the time,” says Cox. “Growers weren’t always able to get around to all their fields often enough with furrow irrigation, which is one of the reasons they left some acres out each year. We’ve also been able to cut our nitrogen usage in half.”

The reason growers can reduce the nitrogen – and phosphorus, potassium and other nutrients – is that fertigation systems typically allow them to precision-apply the amounts the plants need when they need them.

The increased efficiency of fertilizer applications results from the small, controlled amount of nutrients that are applied throughout the season in contrast to the large amounts that may be put out at planting, according to researchers with the University of Florida.

Fertigation systems also appear to help move more nutrients, such as potassium, deeper into the root zone. In their research, scientists with California Polytechnic State University in San Luis Obispo have found that many soils cannot supply sufficient K fast enough during periods of heavy plant uptake.

“There can be advantages to fertigating potassium on the K release rate-limiting soils even though soil levels appear high,” says Charles M. Burt, an agricultural engineer at Cal Poly. (During pollination, corn plans can require up to 15 pounds of K per acre, for example.)

“Researchers found that fertigated potassium at a high concentration (190 ppm) moved two feet downward and spread three feet laterally in a Sycamore clay loam under drip emitters near Colusa, Calif. On a Wyman clay loam near Gridley, the potassium moved one foot downward and two feet laterally from drip emitters.”

Burt says this deep movement of potassium is only possible with fertigating through drip irrigation. “Fertigating potassium with sprinkler irrigation usually results in the potassium moving only two to four inches because it is spread over more surface area.”

He and his coauthors of a book on fertigation say one way to get potassium fertilizers to move deeper into the soil is to saturate all of the cation exchange sites with potassium. “The danger with this strategy is that the potassium equilibrium will be upset with super saturation. The K injection rate may be upset, resulting in an undesirable soil fertility reaction.”

Fertigation systems can be expensive but the increased efficiency of fertilizer application and increased yields may help offset the added investment. University of Florida studies have shown yield increases of more than 100 percent for tomatoes and other vegetable crops with fertigation through drip systems even in the wetter climate of the Southeast.

Growers interested in fertigation should begin with a suitable irrigation system and an adequate supply of water, according to Mikkelsen, western regional director for the International Plant Nutrition Institute who is based in Davis, Calif.

Fertigating with flood or basin-type irrigation does not require high quality water. However, growers using drip irrigation systems must take care to never plug the small emitters. While many fertigation techniques can be effective, they require different levels of management, Mikkelsen notes.

“Addition of fertilizers to irrigation water needs to be done carefully to avoid precipitation with the water or other added chemicals when mixed together,” he says. “For example, high levels of calcium in the water can react with some fertilizers to coat pipes, sprinklers and emitters with mineral deposits.”

On some soils, nitrogen and potassium are the primary nutrients that will be injected through drip irrigation systems. Ammonium nitrate, calcium nitrate and other nitrogen solutions can be used to supply the nitrogen through drip irrigation systems.

All potassium fertilizers are water soluble, but sulfate of potash is popular for fertigation because it is suitable for vegetable crops such as avocados, berries and peppers that are sensitive to high levels of chloride. SOP or potassium sulfate also is considered to be an excellent source of sulfur.

Growers should be careful in applying potassium sulfate in water with high levels of calcium. SOP may form a mealy substance if it is run together with the gypsum in high calcium waters, according to Burt.

Another key to success in drip irrigation is system uniformity. “Nutrients applied through the irrigation system cannot be delivered to the crop in the right amount unless the water delivery is uniform across the field,” says Mikkelsen.

“A mere 10 percent change above or below the optimal water supply can impact the yield of many crops. Over-irrigation can reduce plant growth by causing poor soil aeration, increasing root diseases and leaching nutrients from the root zone. A lack of water increases plant stress, limits photosynthesis and stunts cell development.”

Growers should also take care to inject the fertilizer so that it is distributed through the entire field. “The injected nutrients do not immediately start coming out of the irrigation system,” says Mikkelsen. “It takes time for the water and fertilizer to travel through the irrigation system plumbing. Do not inject the fertilizer and then quickly shut down the irrigation system."