Regional Considerations for Nitrogen Timing

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.

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5 thoughts on “Regional Considerations for Nitrogen Timing

  1. Great Post! Thank you for noting that nitrogen applications will differ from one farm or county to another. Sometimes it seems that cultural practices are too generalized. One question: if April leaf samples were around 3.6% for nitrogen how safe is it to say that there is enough N on the trees already (i.e. not deficient in July)? How much can the levels fall from mid-April to Mid-July? I looked at Patrick Brown studies and it seems everything should be fine. Just not confident I’m interpreting the information correctly.

  2. The research conducted by Dr. Brown (UC Davis) has shown that a leaf Nitrogen analysis of at least 3.5% is sufficient and will be lead to a sufficient nitrogen level (~2.5%) in mid-July. This is assuming that the nitrogen needed by the crop is applied to the trees. The nitrogen budget – and therefore the amount needed to be applied to maintain sufficient nitrogen levels within the tree – can be calculated by taking the estimated kernel pound yield and multiplying it by 0.085 (or 85 pounds N for every 1000 kernel pounds). If nitrogen is not applied, tissue levels will drop, leading to deficiency in mid-July as the crop will draw the needed nitrogen from the leaves for development. I hope to have an entry on this soon.

  3. Depends on the timing. Nitrogen applied in July before the complete formation of the hull abscission layer will mostly be directed into the hull. Not only will this will be removed at harvest, but will increase the risk for hull rot. Nitrogen applied after the formation of the abscission layer will stay within the tree and will be used for growth and bud development. The formation of this layer occurs sometimes in early to mid-august, but is dependent upon the variety and orchard location. Since this is tricky to determine the exact timing of this layer, we generally recommend to apply N as soon as the harvest is completed.

    1. I should add that nitrogen that is directed to the hull is termed ‘luxury consumption,’ and is not as clear as the above comment indicates. What we do know if that N applied in June, July over tree growth demands will tend to be sequestered away in the fruit, potentially flaring hull rot. It also may be stored in other tissues. Generally these applications should be avoided unless the tree is significantly deficient.

      Due to shorter days and lower water use, it has been estimated that around 50 lbs of nitrogen/acre can be “pulled” into the tree during the postharvest period. This is even less for later harvesting varieties. With this in mind, it may be important to spoon feed on very small amounts of N in August/September in order to meet the postharvest requirement.

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