A Kern County study designed to look at the relationship between irrigation and nitrogen application rates and their effect on spur and canopy development — and the resulting effects on yield and other factors — has finished its seventh year. The study is sponsored by the Almond Board of California and the lead researcher is Bruce Lampinen, Department of Plant Sciences, UC Davis.

The researchers looked first at the effect of different rates of irrigation and nitrogen on the development of tree canopies as measured by light interception — the fuller the tree canopy, the more light is intercepted — and then at the effect of fuller tree canopies on yields and on spurs, the fruiting branches on which almonds are produced.

The study began in 2001 with trees planted in 1996. Four treatments regimes were applied to the trees as follows:

  1. T1-High water, high N
  2. T2-High water, moderate N
  3. T3-Moderate water, high N
  4. T4-Moderate water, moderate N

As the trees developed over the years, it was found that those with the densest canopies (and higher rates of light interception) received the highest application rates of both water and nitrogen fertilizer, (T1), followed by T2, T3, and T4. It was also determined that tree canopy development was most strongly influenced by water application.

Furthermore, by adding total yields over the seven years for each treatment, yields ranked in the same order as canopy light interception with the highest yields in treatment T1 followed in order by T2, T3, and T4.

Spur death also followed the pattern of canopy development, with greatest death occurring with the T1 treatment (highest rate of light interception), followed by T2, T3, and T4. Since yields were also greatest with the T1 treatment and least with the T4 treatment, the researchers concluded that spur replacement outside of the inner canopy more than made up for the higher loss of spurs in the inner canopy of the high-water treatments.

Irrigation Scheduling

Irrigation rates in the study were determined by using the mid-day stem water potential technique developed by Lampinen and colleagues. With this technique, one shaded leaf, low on a tree, is bagged about 15 minutes before sampling; then between 1 p.m. and 3 p.m., the leaf is cut off and placed in a pressure chamber with the petiole protruding out. The cylinder is pressurized, and when water just begins to appear at the cut surface, the gauge indicates the mid-day stem water potential. A complete description of this technique is given in “Using Midday Stem Water Potential to Refine Irrigation Scheduling in Almond” found at either the UC Fruit and Nut Information Center (http://fruitsandnuts.ucdavis.edu/crops/Almond_MiddayStemWaterPotential.pdf) or as a link in the December 2007 California Almonds Newsletter article “Full Canopy = Higher Yields” (www.almondboard.com).

Using this technique, a fully watered orchard is about -8 bars, mildly stressed is -12 bars, and moderately stressed is -16 bars.

“The canopy development and yield relationship determined in this study, and verified by data from other trials, can be used to estimate potential returns for an orchard kept at different levels of mid-day stem water potential during the first 10 years after planting,” the study report says. Using a gross return rate of $2 per pound of kernels, the cumulative return in dollars per acre of trees kept fully watered at -8 bars was $47,100, while at -12 bars, and at -16 bars, the returns would only be $38,622 and $27,788, respectively.

While it may be tempting to keep orchards at -8 bars to maximize returns, there are drawbacks, according to Lampinen. “Excessively wet soil, in conjunction with shading early in the year, may play a role in lower-limb dieback,” he said. Furthermore, keeping trees this wet increases potential for tree loss due to lack of aeration and/or root rot (Phytophthora). “In addition,” Lampinen says, “canopy cover above about 70 percent to 75 percent mid-day light interception results in cooler conditions on the orchard floor that might be conducive to Salmonella survival.” Moreover, he says, denser canopy covers will not allow enough light through to dry nuts on the orchard floor, adding to food safety concerns.

“A harvest technique growers can use is, after shaking, to sweep the crop to the center of the driverow to allow a postharvest irrigation. Depending on the irrigation system coverage, the nuts may be able to be dried while the irrigation system is on, or they can be spread out to dry before windrowing.”

Growers should seek a balance between developing the canopy rapidly and preventing tree loss from excessively wet conditions, Lampinen recommends. “The data suggests growers could irrigate to -8 to -9 bars to develop a fuller canopy when trees are young, then irrigate to -10 or -12 bars once a 70 percent canopy has developed to maintain productivity.”

Lampinen also says that growers should choose a planting density that fits their soil conditions and management style to minimize the potential for an overly crowded canopy as the orchard ages.