Updated Levels Are Lower Than Previously Available Thresholds
It is part of our routine. We collect leaves in the middle of the summer and submit them to a laboratory (like the UF/IFAS Analytical Services Laboratory) to find out how our plants are doing. We get a result back: 1.2% N, 1.5% N, 2.0% N. But what does this mean? Like a doctor taking a patient’s temperature, test results are only meaningful when compared to a reference standard. 104°F is a fever. 97°F is normal. Knowing where the bar is set makes all the difference in how we interpret test results.
So, we had to ask the question: Where do the leaf nitrogen recommendations for southern highbush blueberry come from? We were able to put together a timeline tracking old publications. To the best of our knowledge, the first blueberry leaf N recommendations were generated in New Jersey in the 1960s. Then, research in Michigan and Oregon expanded the original findings to include more varieties in the 1990s. Eventually, extension publications in Florida and Georgia adopted these recommendations in the 2000s. However, there are some key differences between New Jersey, Michigan, Oregon, and Florida. In the north, they grow northern highbush blueberry varieties that go dormant for 3 to 4 months of the year. They have different soils and different environmental conditions. Here, in Florida, we grow southern highbush blueberry varieties that have little to no dormancy – we have functional leaves for most of the year. This made us think that the leaf nitrogen standards we were using might be wrong.
Recent research in the Small Fruits Horticulture Laboratory confirmed this idea (https://doi.org/10.21273/HORTSCI19136-25 ).
We used a data set of nearly 1,200 leaf test results from research and commercial farms throughout the state. Sentinel, Arcadia, Falcon, Optimus, and Farthing were among the sampled cultivars. Using data science and artificial intelligence, we determined new leaf nitrogen sufficiency ranges for southern highbush blueberry. We were able to develop tailored recommendations for evergreen and deciduous systems and for different times of the year. As expected, the recommendations we came up with are different than those available for northern highbush blueberry.
In the summer, optimum leaf nitrogen concentrations ranged between 1.48% and 1.88% for deciduous plants and 1.47% and 1.77% for evergreen plants. These new leaf nitrogen sufficiency ranges are lower than the previously available thresholds. When leaf test results show concentrations below these standards, it is advisable to increase fertilization. When leaf test results show concentrations above these ranges, it is advisable to reduce or delay fertilization. Our research also produced leaf N standards for the flower bud initiation, bloom, and fruit development periods. Because nutrient uptake and mobilization shift significantly when reproductive organs are present, we recommend using those thresholds cautiously and consulting with an advisor before making fertilization decisions.
In the future, we plan to look at other nutrients to try to develop similar crop-specific thresholds. Until then, these newly developed optimum leaf nitrogen concentration thresholds can help us answer the ever-present question “Are my plants getting enough nitrogen?”
CREDITS:GERARDO H. NUNEZ & HANNA I. DE JESUS
