Hass avocado trees absorb nutrients according to seasonal growth patterns, and matching fertilizer applications to those patterns can both maximize yields and reduce potential for groundwater pollution.

That’s what Carol Lovatt, professor of plant physiology at the University of California, Riverside, concluded after a multi-year study, beginning in 2001, of how cropping loads relate to nutrient uptake.

The work included digging up 60, 20-year-old, avocado trees at different times of year and dissecting, grinding, and chemically analyzing all parts of them. Half of the trees were defruited beforehand to establish on- and off-year bearing patterns.

Lovatt reported her findings at the recent 13th annual Fertilizer Research and Education Project (FREP) conference in Salinas.

The research, conducted in a commercial orchard of Somis Pacific in Moorpark, Calif., was funded by FREP and was aimed at finding best management practices for fertilizing the crop. Those practices are intended to provide optimum returns for growers as well as to protect the environment.

California avocado growers are eager for new practices to increase yields and maintain large fruit sizes in the face of competition from fruit from Mexico and Chile.

Nutrient needs

“An understanding of seasonal tree nutrient requirements is critical in developing best management fertilizer practices,” Lovatt said.

“Nutrient applications should be coincident with these periods of high tree demand. Careful analysis of tree growth patterns (particularly of fruits and leaves) can indicate when tree nutrient demand is high, and, thus, when nutrients should be applied to maximize tree nutrient uptake and reduce environmental pollution.

“Nutrient leaching losses should be minimized by coordinating fertilizer applications with periods of high plant demand.”

Rates and timing for nitrogen, phosphorus, and potassium fertilization of avocadoes, Lovatt said, have not been adequately defined. The practices commonly followed are largely borrowed from citrus, and only a few experiments have been routinely done on nutrient uptake and allocation.

“However,” she reported, “determining nutrient uptake in mature trees is considerably more difficult, requiring repeated tree excavations at important phenological periods over the season. Thus, few best management practices have been developed for perennial tree crops.”

Until recently, she said, avocado growers customarily applied 60 to 200 pounds of nitrogen per acre annually. Then regulatory agencies, concerned about potential contamination of groundwater, advised them to divide treatments into small monthly doses.

But, she added, these monthly applications don’t take into account the peak nitrogen needs of the trees, particularly during April with the set and early development of fruit and the vegetative flush.

Test applications

Lovatt and her cooperators applied ammonium sulfate (with 10 percent enriched 15N) in August, November, and June. Trees were excavated three months after application and analyzed for the percent of 15N recovered from their parts.

The researchers learned that alternate bearing had little effect on changes in leaf nitrogen concentrations. “This is surprising,” Lovatt said, “since avocado trees accumulated significant quantities in their fruit and this demand was not reflected in lower leaf N concentrations.

“In other alternate bearing species, such as pistachios, leaf N concentrations are frequently lower in on- versus off-year trees. This indicated that avocado trees are highly buffered against large N demands by the fruit.”

Analysis of above-ground tree nitrogen and potassium concentrations showed that tree N contents increased by almost 50 percent in both on- and off-year trees between the spring and fall. On-year trees contained almost 60 percent more potassium than off-year trees at fruit maturity in June and July.

Nutrient levels

An estimate of total tree nitrogen and potassium was found in the difference in tree nutrient content at bloom in February and at harvest the following July.

The study revealed that over the alternate bearing cycle, on-year trees took up 125 pounds of nitrogen and 171 pounds of potassium per acre, and fruits comprised 40 percent and 59 percent of the total nitrogen and potassium uptake, respectively.

In contrast, only 95 pounds of nitrogen and 48 pounds of potassium per acre were taken up in off-year trees. Almost 80 percent of the potassium in the tree was located in the leaves during the off year.

After tracing the seasonal uptake of nitrogen applied in August and November of 2002 in the trees, Lovatt said the 15N “accumulated equally between leaves, green twigs, and canopy branches of the off-year trees. In contrast, the roots accumulated the most 15N in the on-year trees. The lack of 15N translocation out of roots may reflect the lower nitrogen requirements for on-year trees at this time.”

Contradict reports

She added that these results appear to contradict earlier reports that nitrogen uptake, translocation, and allocation are a function of sink demand.

“Not so,” she said. “On-year trees in November would have fewer new vegetative shoots to support than off-year trees. And since they were going into an off-year bloom in the spring of 2003, they would also have fewer reproductive sinks.”

She also noted that 15N recovery rates were markedly lower when applied in November compared to August. The cold and wet weather likely contributed to these lower recovery rates in two ways. First, high rainfall events likely increased nitrogen leaching, and second, cold weather decreased tree growth and therefore reduced tree nitrogen demand.

Sharing project leader duties with Lovatt on the project was Richard Rosecrance, assistant professor of biology at California State University, Chico. Cooperators were Don Reeder and Scott Savard of Somis Pacific and Ben Faber, Ventura County farm advisor.

Lovatt said the next phase in determining the nutritional needs for avocadoes is to develop a nutrient model for nitrogen and the other elements. With it, growers can learn from analysis of leaves and of water how much fertilizer to apply when the trees need it for highest returns with reduced potential for pollution.