The idea of minimizing pecan production costs in a low-yield, off-year by reducing fertilizer rates may seem to make sense.
But, the phenology of a pecan tree — the sequence of biological events that occur in the tree throughout the year — reveals a fallacy in this thinking, says Monte Nesbitt, Extension Pecan and Fruit Crops Specialist with Texas A & M University.
Phenology includes such developments as bud break, the appearance of new shoots and leaves, shell hardening, etc. It begins each year with pistillate flower initiation, normally the latter part of January, and ends with leaf abscission, which typically occurs in the first half of December. Phenological events are well documented, while timing of a particular event is dependent on temperature and variety of tree.
Understanding the phenology of pecan trees can help determine the best time and rates for applying water and nutrients to support biological events, Nesbitt notes.
Think of the pecan tree as developing in regular and overlapping two-year cycles. During this time, the tree buds out, grows, produces nuts and stores carbohydrates and other nutrients that affect both the current and the following year’s crops.
The tree takes up the water and nutrients that are applied now, stores them in various structures throughout the tree, and meters them out as needed for different processes. Some are used this season and some are reserved for use next season.
“Once you understand this process, you realize that you can’t take a year off from your fertilizer program when a crop is small,” Nesbitt says. “To meet this perennial tree’s perpetual nutrient needs, you have to think about the impact of your fertilization program this year on the crop you may have next year.”
August is especially critical for fertilizer management. More distinct phenological events — 13 of them — take place this month than any other month.
For example, vegetative growth is continuing, carbohydrate levels are fluctuating, shuck and shell are separating and nuts are filling.
However, Nesbitt notes, research shows that only about 4 percent of any nitrogen applied when the kernels are filling (August-September) will be used to produce nuts that year. The rest will help sustain growth of leaves, catkins and woody tissues the following year.
Meanwhile, other studies have looked at how late-season fertility management can be adjusted to reduce competition for nutrients between nut development and the induction and differentiation of pistillate flowers. This competition lowers nut quality during the current season and hinders bloom the succeeding season.
“Researchers showed quite some time ago that a heavy crop of pecans can deplete the tree’s supplies of nitrogen, potassium and magnesium between July and September,” Nesbitt says. “For that level of cropping, supplementing these nutrients during late summer depletion may improve crop production the following year, if other growing requirements are met.”
One team of scientists found that applying nitrogen in October instead of March improved long-term production of certain cultivars, but not all. Another group of researchers improved yields the following season by applying 50 or more pounds of nitrogen per acre, plus minor nutrients, between Aug. 1 and Oct. 1 in the current season.
“In developing a comprehensive fertility management program, it’s important to consider tree phenology when determining the best time for application,” Nesbitt says.
“Both spring and late-summer fertilization are beneficial, and a combination may be an effective strategy for managing pecan orchard fertility each year.”