Nitrogen supply from soil organic matter: Predictors and implications for recommended nitrogen application rates in northern highbush blueberry

Authors: SLOAN, CHEYENNE - Washington State University; DEVETTER, LISA - Washington State University; GRIFFIN-LAHUE, DEIRDRE - Washington State University; BENEDICT, CHRIS - Washington State University; Bryla, David; LAHUE, GABRIEL - Washington State University

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2024
Publication Date: 4/25/2024
Citation: Sloan, C., DeVetter, L.W., Griffin-Lahue, D., Benedict, C., Bryla, D.R., Lahue, G. 2024. Nitrogen supply from soil organic matter: Predictors and implications for recommended nitrogen application rates in northern highbush blueberry. HortScience. 59(6):725-735. https://doi.org/10.21273/HORTSCI17632-23.
DOI: https://doi.org/10.21273/HORTSCI17632-23

Interpretive Summary: Like most crops, blueberries require nitrogen (N) fertilizer for good growth and fruit production. However, multiple studies over the past four decades have shown that insufficient or excessive use of N fertilizer can negatively affect both yield and fruit quality in blueberry. Nitrogen is also available from soil organic matter, but the amount released is difficult to predict and is usually not considered in N fertility programs. We conducted a series of laboratory and field experiments to identify when and how much N is available from soil organic matter. The amount released ranged from 15 to 110 lbs of N per acre in soils collected from mature commercial blueberry fields in northwest Washington. Availability peaked in June, which is when blueberry plants take up most of their N during the growing season. Consequently, high rates of N fertilizer were not needed to maximize yield or fruit quality. In fact, rates as low as 30–45 lbs of N per acre were sufficient to sustain production for three years at sites with 3% to 28% soil organic matter. This combined with longer-term observations from other studies suggests that N fertilizer rates could be reduced to lower input costs with no consequences to plant productivity in mature blueberry plantings.

Technical Abstract: Nitrogen (N) is key to maintaining high productivity in northern highbush blueberry (Vaccinium corymbosum L.) and is often applied using ammonium fertilizers. Nitrogen is also available through mineralization of soil organic matter (SOM), but the amount released is difficult to predict and is not considered in most fertility programs. A laboratory experiment was conducted for 6 months using soil samples collected from 10 commercial blueberry fields in northwest Washington, USA to estimate the timing and magnitude of N mineralization from SOM and to identify soil properties that can be measured to predict mineralization in soils containing 3–40% organic carbon. Net N mineralization peaked after 3-4 months of incubation in each soil, corresponding to the period in which N uptake reaches its maximum in northern highbush blueberry. Results indicated that total N may be a useful predictor of N supply from SOM. An experiment was also conducted in four mature, commercial fields to determine whether N requirements in blueberry vary across soils with different amounts of SOM. The fields contained 3-28% SOC and were each fertilized with 33–50, 67–84, or 102–118 kg/ha N per year. While soil inorganic N levels suggested that N mineralization was substantial at sites with higher SOM, higher N fertilizer rates did not increase yield or improve fruit quality in any field, including those with lower SOM. Under the conditions of this experiment, even the lowest N rates were sufficient to sustain production for at least 3 years. Findings from this study indicate that SOM may be an important contributor to N fertility in managed blueberry systems, and yield and fruit quality can be maintained in many cases with relatively low rates of N fertilizer.