Fertigation Outperforms Granular Applications of Nitrogen in Study

Five years of production data in a fertilizer study in blueberries shows fertigation applications of nitrogen far outperform granular applications.

“We found that year after year we got consistently higher yields when we fertigated than when we used granular nitrogen fertilizers,” said David Bryla, a research horticulturist with the USDA Agricultural Research Service’s Horticultural Crops Production and Genetic Improvement Research Unit in Corvallis.

“We definitely recommend using fertigation as opposed to granular fertilizers,” he said, “particularly during establishment.”

Speaking at the Oregon Blueberry Conference in February, USDA ARS research horticulturist David Bryla said research is showing that fertigation of nitrogen outperforms granular application.

In a presentation at the Oregon Blueberry Conference February 7 in Salem, Bryla said yields were between 12 and 40 percent higher under fertigation than under granular fertilizer applications. He added that there are several nitrogen fertilizers available that can be used for fertigation, including ammonium sulfate solutions, which are most commonly used, or urea solutions, which tend to be the least expensive.

Also, he said, ammonium thiosulfate works well, particularly if a grower is looking to lower soil pH.

And Bryla said it is important to place drip lines close to plants when fertigating, particularly when plants are young, because at that point they don’t have an expansive root system.

“Another thing about ammonium nitrogen is it is not very mobile in the soil,” he said. “So, where it comes out of the drip emitter is pretty much where it stays. So, if you have those drip lines too far from the plants, the plants will actually be nitrogen stressed the first year or two.”

Bryla also advised growers to fertigate at least once every two weeks.

Nitrogen rates can be increased slightly early in plant development when fertigating, he added, but as plants grow, they will need less nitrogen.

“You’ll notice that as the plants continue to grow, you actually need less nitrogen by fertigation than you do with granular fertilizers, because it is more efficient because you are applying it directly to the roots,” he said.

The five-year study included looks at several nutrients in addition to nitrogen, including potassium, which Bryla identified as the second most abundant nutrient in a blueberry plant.

“Potassium is essential for growth, as well as promoting root development,” he said. “It can also decrease problems from disease and cold temperatures.”

Potassium deficiency is not common in Oregon or in blueberries, Bryla said, but it can occur under heavy crop loads. Symptoms of potassium deficiency include scorching on leaf edges, leaf curling and sometimes necrotic spots on leaves.

“But be careful because it is also easy to overapply potassium,” Bryla said. “So, often what we see, rather than potassium deficiency in blueberries, is excess potassium, potassium toxicity.”

The research also looked at phosphorus, another key nutrient in blueberry production. Phosphorus deficiency symptoms include the appearance of a very dark green, purple tinge on leaves.

“Usually, phosphorus deficiency will occur at the beginning of the season because the roots aren’t very active at that point, so they aren’t taking up enough phosphorus,” Bryla said. “But usually over time, as the season progresses, these symptoms will disappear.”

Boron, a micronutrient, also was looked at in the study. Boron is very important for flower development and fruit set, Bryla said, and tends to be limited in Oregon blueberry fields. The most common symptom of boron deficiency is a lack of fruit set.

“Again, with boron, you have to be careful,” Bryla said. “It’s a micronutrient, so it is really easy to over apply it.” Overapplication can lead to dieback, he said. “So, you get a lot of problems if you apply too much boron, particularly in young plantings.”

Bryla said fertility guidelines recommend applying between 0 to 100 pounds an acre of potassium, depending on soil or leaf potassium levels. With phosphorus, guidelines call for applying between 0 and 60 pounds per acre, depending on leaf analysis. And when leaf boron levels are less than 30 parts per million, boron should be applied.

Recommendations call for applying potassium by fertigation, if needed. “But make sure that you know your leaf levels are telling you that you need potassium,” Bryla said, “because if you apply at a high rate, you can start causing problems, such as calcium and magnesium deficiency.”

The recommended threshold for when a potassium application is needed is under 0.2 percent, Bryla said. “If you are applying it when the leaf levels are higher, then more than likely, it is not providing any benefit.

“And certainly, once you start getting above 0.3 or 0.4 percent, you don’t want to be applying potassium to your plants,” he said.

Blueberry plants have a very low phosphorus requirement, Bryla said. “Leaf levels higher than 0.07 percent are sufficient,” he said. “So, 0.08 percent and above is fine for blueberries. As we saw in our study, additional phosphorus wasn’t really beneficial, so when we applied it, it had no effect. So, unless your levels are 0.07 or less, then you probably don’t need to be thinking about putting phosphorus on your plants.”