By Bob Curtis, Associate Director, Agricultural Affairs and Gabriele Ludwig, Associate Director, Environmental Affairs

Properly managing nitrogen fertilizers in almonds means paying attention to the “Four R’s”, said UC Davis Pomologist Patrick Brown: Applying the right rate of the right fertilizer source in the right place at the right time.

“This sounds relatively simple, but the devil is in the details,” Brown told an audience at The Almond Conference in December.

This second in a three-part series on nutrition, particularly nitrogen, and almonds will look at the latest research the Almond Board is funding to help growers hit the Four Rs of N application to meet tree demand, optimize crop yield and avoid environmental impacts from applied nitrogen fertilizers.

The right rate means that the amount of applied nitrogen is correlated to the needs of the tree and developing crop. The right source involves selecting the best form of nitrogen for current conditions and compatibility with other elements in the system. The right place means getting the nitrogen to the wetted root zone where it can be absorbed by the tree. And the right time involves applying nitrogen when it can best be taken up from the soil.

Best management practices

A 2007 survey of almond growers published in 2010 in California Agriculture revealed that the vast majority of almond growers are adopting best management practices related to nutrient management. It also identified areas where growers wanted more guidance related to fertilization to optimize yield and meet environmental standards. The latest Almond Board–funded research is providing specific guidance to growers to help them improve their sampling methods, respond better to nutrient leaf status results earlier in the season and implement a refined nutrient budget approach.

Brown said an important first step in developing the right rate is to understand the supply of N available to trees through all inputs in the system, including the N status in the soil, the N in the irrigation water and the nitrogen status in the tree going into the season.  Nitrogen use efficiency (NUE) can be greatly enhanced by timing spoon-fed applications concurrent with demand. To get applied fertilizers in the right place growers must recognize the root zone dynamics within the wetted zone.  Brown is also studying the NUE of a number of commercial sources of N fertilizer to better understand the best source of nitrogen under different conditions.

Already, these ongoing trials have produced some valuable findings. In this replicated trial, from 55 pounds N (at a leaf N of 2.0 percent in July) to 70 pounds N (at a leaf N of 2.4 percent in July) is removed for every 1,000 pounds of kernels harvested from the orchard.  And 8 pounds of P and 80 pounds of K were removed with every 1,000 kernel pounds harvested.  About 80 percent of all N and 75 percent of both P and K accumulate in the fruit before July (120 days after bloom).   

One high yielding test orchard during the 2010 and 2011 seasons produced 3,481 to 4,833 kernel pounds per acre with an N-rate of 275 pounds per acre.  There was no benefit from higher N applications. An NUE of 75 percent was observed for the 275-pound N rate and this is among the most efficient measured in agriculture. Also losses through leaching and off-gassing were very low in this orchard, which is fertigated by either drip or fan-jets, making several runs from February through June to match demand during tree growth and fruit development.

Predictive models

Brown and graduate student Sebastian Saa are developing predictive models for leaf sampling nitrogen and other major nutrients.  Sampling models for leaves on either fruiting or non-fruiting spurs are under investigation. Initial results show that samples in April can predict July nitrogen leaf content and the percentage of trees below the July critical value. For instance, one model suggests that leaf nitrogen content above 3.3 percent in April will result in more than 95 percent of all trees being above 2.2 percent in July. These data will be further validated in 2012. Unfortunately, K concentration in leaves has been highly variable and not well correlated to yield, making leaf sampling difficult to interpret in this study.

Research is also looking at ways to help growers deal with variability in leaf sampling and the research aims to develop improved protocols for more meaningful interpretation. Nutrients vary greatly in the field and the approach of taking just a few samples from a large orchard does not accurately measure this variability. For instance, research indicates accurate N composite samples can be taken in April (43 days after bloom, +/- 6 days).  Depending on sample area and accuracy desired, the composite for one analysis will come from 18 to 28 trees, at least 90 feet apart, sampling at least eight leaves per tree at the 5- to 7-foot level.

Brown said the ultimate goal will be to help growers fold updated information into management decisions based on a nutrient budget being developed by Brown’s research associate Saiful Muhammad that hits the Four R’s to optimize production and minimize environmental impacts in fertilizer management.

For more information and access to research posters and annual reports related to almond nitrogen research, go to AlmondBoard.com/farmpress24 and choose Air Quality and Water Quality.

This is the second in a series on nitrogen fertilization in almonds. Part 1 is here.