AgriLife Researchers at Uvalde examine nitrogen fertilization for olive tree establishment

UVALDE – Researchers at the Texas A&M AgriLife Research and Extension Center in Uvalde have completed initial investigations on nitrogen fertilization management for olive tree establishment in Texas.

VIDEO: Nitrogen Supplementation for Olive Tree Establishment in Texas, https://youtu.be/gUeuBAvkSOA

The two-phase study included both nursery/greenhouse and field experiment components.

Dr. Daniel Leskovar, center director and Texas A&M AgriLife Research vegetable physiologist, and Dr. Yahia Othman, research associate with AgriLife Research, served as the primary investigators for the study. Study supporters and collaborators included the Texas Department of Agriculture Specialty Crop Block Grant Program, Texas Olive Oil Council and Dr. Raul Cabrera of Rutgers University.

“This study is part of a large project conducted by AgriLife Extension colleagues,” Leskovar noted. “The the goal of this particular study was to enhance seedling vigor and transplant stress tolerance to improve olive stand establishment in southwestern Texas regions.”

The nursery pot and soil-based field experiments were designed to determine the impact of the nitrogen rates and sources on root growth and shoot development of young first-year olive trees, Leskovar explained.

Even though this experiment was designed for a minimum of two years, Leskovar said some preliminary recommendations relating to nitrogen management for olive tree establishment can be made from initial study results.

For the nursery experiment, a standard Arbequina olive cultivar was selected and planted in a peat moss, bark and sand substrate with nitrogen treatment levels of 0, 20 and 40 kilograms per hectare. Nitrogen sources were urea at 46 percent nitrogen; ammonium nitrate at 33 percent nitrogen; calcium nitrate at 15.5 percent and 19 percent calcium; and Osmocote at 16 percent nitrogen, plus 6 percent phosphate and 9 percent potash. Applications were made weekly for 20 weeks, except in the case of the Osmocote, which was given a one-time application during the first week.

Regular plant measurements were taken and include leaf area and number, shoot length, stem diameter, root components, gas exchange and nitrogen content in plant roots and shoots.

The field experiment established in the spring of 2015 consisted of spacing young Arbequina olive plants at 12 feet apart in rows and 10 feet between plants in each row. This spacing  provided a planting density of 363 plants per acre.

Plants were treated with nitrogen at levels of 0, 20 and 40 kilograms per hectare. Nitrogen treatment sources used were urea at 46 percent nitrogen and calcium nitrate at 15.5 percent nitrogen and 19 percent calcium. The total nitrogen application was split, with technicians providing 40 percent of the desired amount at the time of transplanting and 60 percent after 40 days.

Eight replications were done over a total of 56 plots. Measurements were taken of root and shoot length, plant height, stem diameter, number of branches, gas exchange and root and shoot nitrogen concentration.

The researchers’ initial conclusions are very interesting, Leskovar said.  

“From the nursery study they noted that even though nitrogen is required to establish young olive trees in pots, the rate is dependent on the source. In addition, for the nursery study we found fertilizers with ammonium sources, such as ammonium nitrate or urea at 40 kilograms per hectare, appear to work better than nitrate alone. Growth and physiology was not affected by Osmocote, except for more branching at that same rate.”

He said initial investigations of the field study showed stem diameter and branch number were reduced at the higher nitrogen rates of 40 and 60 kilograms per hectare.

“We also observed that leaf physiology was reduced with urea at higher than 20 kilograms per hectare, while no differences were observed between control, calcium nitrate and urea at the lower rate,” he explained.

“It looks like during the nursery period, slow release nitrogen applied once may be more practical and economical than spoon-feeding nitrogen to the young plants once a week,” he said. “Little to no nitrogen is required to establish young olive trees in the field and nitrogen may not be needed if pre-plant soil nitrogen levels are normal. Also, higher rates of nitrogen application during plant establishment may negatively affect growth.”

Leskovar said the next step will be to evaluate root and shoot growth in the second growing season in the nursery/greenhouse and also determine how nitrogen levels and source affect leaf and root nitrogen contents in the field.

“Another aspect of the study will be to measure root length density,”he said. “This is a more detailed and time-consuming task but very important to understanding how nitrogen sources such as ammonium or nitrate modulate root dynamics in the media or soil profile. Then we can determine if linkages exist in shoot growth and plant physiology over the early stages of the life cycle.”