Over the past few weeks, I have attended a few events that discussed the application timings of nitrogen. The results that were discussed at these events were based on an extensive five year study conducted by the University of California at a trial located in Kern County.One of the many findings of this study included the development of a nitrogen application schedule that maintains crop productivity.
From this trial, the researchers have determined that 80% of the nitrogen should be applied before the completion of kernel fill (mid-June), while the remaining 20% should be applied in the post harvest. In the study, the researchers applied the spring nitrogen doses beginning in mid-February prior to bloom, and continued monthly until mid-May. They roughly followed a 10%-25%-25%-20% application schedule.
The question remaining, especially with impending groundwater and nitrate regulations, is this applicable to all parts of California? The short answer is “Of course not,” as differing climates and soils create different challenges for growing almond efficiently. For example, in Merced County, we receive an average of 10-12 inches of rain and have some areas of very sandy soils. This is in contrast to the trial’s location in Kern County, which receives 6 inches of annual rainfall and is located on a sandy loam soil. Practices of nitrogen application, therefore, will vary by location – especially the timing of the first application.
In a previous entry, I wrote about the movement of nitrate and the rationale for applying after leaf out. Although this is still the most efficient timing to apply nitrogen, I have since learned that there is some level of root uptake of nitrogen during the period of delayed-dormancy (bud-swell). This occurs due to nitrate, being found in a greater concentration outside of the root moves into the root to establish equilibrium. The higher the concentration of nitrate within the soil, the higher the concentration that moves into the root. After movement into the root, however, the nitrogen is not able to move into the vegetative parts of the plant until some level of transpiration occurs.
So considering all of this, should we apply 10% (~25 lbs/acre) of our nitrogen in mid-February, prior to bloom? In Merced County, with the exceptions of the heavier soils (Clay/Clay Loam), applications this early would most likely be inefficient. February (the rainiest month) and March rains have a great potential to leach the nitrogen applied below the rootzone, preventing root uptake and tree use, leading to waste and nitrate contamination. This is especially of concern in areas with sandy soils or a high water table. I encourage all growers within these areas to wait until leaf out and then to spoon feed on small doses until the threat of heavy rain is reduced. This strategy may be more useful for areas of the state that receive higher amounts of rain (i.e. Sacramento Valley).
Keep in mind that your nitrogen applications and rates should be based upon your local conditions. This will differ across the state, across the county, and across the farm. Only by learning to farm these differences will we be able to create a more efficient, higher yielding almond orchard.