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Does soil organic matter influence N management in blueberry production?

Volume 12 Issue 3

Gabe LaHue, Cheyenne Sloan, Lisa DeVetter, Deirdre Griffin LaHue, Chris Benedict, and Dave Bryla1


Soil organic matter (SOM) is critical to a wide range of soil functions, including providing nitrogen (N) for crop growth. Interestingly, there is a huge range of SOM content in blueberry fields in Whatcom County – from <5% to >50% – resulting from the processes of soil formation rather than from any particular management practice. With such a range of SOM, does that mean that blueberries planted in high SOM peat soils need less N fertilizer than their counterparts on lower SOM mineral soils? And how much nitrogen do mature blueberry plantings need anyway? These are some of the questions we set out to answer with funding from the Washington Blueberry Commission.

First, we took soil samples from 5 locations per field in 10 commercial blueberry fields, which ranged from 3 – 40% soil organic carbon (SOC), roughly corresponding to 6 – 80% soil organic matter (note that soil organic matter is approximately half carbon, so you can convert by multiplying or dividing by 2 as appropriate). We incubated these soils at representative field soil temperatures for 6 months and measured soil available N (ammonium and nitrate) each month. We looked at how well the amount of N released from SOM after 4 months could be predicted by SOC, soil total N, and other measures of SOM quality (this timing corresponds to July, when most N uptake by blueberry roots has already occurred).

Second, we conducted a 3-year field trial in 4 commercial ‘Duke’ blueberry fields that ranged from 3 – 28% SOC. Experimental plots in each field received 20, 50, or 80 lbs. N per acre as broadcast granular fertilizer, plus uniform liquid N fertilizer applications across the whole field, for combined N application rates of 30–45, 60–75, or 90–105 lbs. N per acre. We measured soil available N, leaf tissue N, fruit yield, fruit quality, vegetative growth, and cold hardiness. So, what did we find?

Is there evidence that SOM is supplying N in blueberry fields?

Yes! Our laboratory incubation showed that available N increased considerably under representative soil temperatures and optimum soil moisture conditions, particularly for high SOM peat soils (> 20% SOC). In these peat soils, SOM supplied 344 ± 81 lbs. N per acre on average, though there was considerable variation between fields. Furthermore, soil available N generally increased throughout the growing season at all field sites regardless of the N fertilizer application rate, but this increase was much more pronounced at the field sites with higher SOM soils.

Figure 1: Cumulative N release from soil organic matter (ammonium and nitrate) during a 6-month laboratory incubation at temperatures representative of field soil temperatures for Skagit and Whatcom Counties. Soils with high soil organic carbon (SOC) are commonly referred to as peat soils (> 20% SOC or > 40% soil organic matter), whereas soils with < 20% SOC are mineral soils. Note that one site is removed for easier visualization.

N release from SOM depends on soil temperature, so won’t it be available too late in the season for the plants?

Surprisingly, our data suggest that N release from SOM is well-timed to coincide with plant N uptake. Research shows that N uptake by blueberry plants peaks in May and June, with much less uptake later in the season (Throop and Hanson, 1997). We initially thought that there would be considerable N release from SOM, but that this N release would be in August and September, coming too late to supply the N needs of the growing plant. However, the figure above shows that there is substantial N release from SOM in May and June, as soon as the soil temperature reaches 58°F.

Can we predict how much N is released from SOM during the growing season?

Yes and no… the best predictor of N release from SOM that we measured was soil total N. Soil total N is essentially the N content of SOM plus any mineral N (soil ammonium and nitrate), though the N content of SOM is usually much greater than the mineral N. On average, for each 0.1% absolute increase in soil total N, the soil is expected to supply 5.7 lbs. N per acre by the end of July. For context, the range of total N in the soils we sampled was 0.2 – 2.0%. However, soil total N only explained 43% of the variation in N release from SOM, meaning that there is considerable uncertainty in how much N will be supplied in a particular field, even if you measure the soil total N. This could be due to a number of factors though it is likely that the quality of the SOM, such as the carbon-to-nitrogen ratio, accounts for some of this unexplained variability.

Figure 2: Mean fruit yield from 2019 to 2021 for 3 N application rate treatments (30–45, 60–75, or 90–105 lbs. N per acre) at 4 commercial ‘Duke’ blueberry fields that varied in soil organic carbon (SOC) content. Note that the differences between sites cannot be assumed to be due to differences in SOC given other differences in management practices. Error bars represent the standard error of the mean and values with the same lowercase letter or no letter are not statistically separable from each other.

Can I apply less N fertilizer if I have a peat soil or a field with a high SOM content?

Given the substantial N supply from SOM, particularly in fields with a high SOM content, there may be opportunities to save on N fertilizer costs and reduce N applications. We initially thought that fields with a lower SOM content would require more N fertilizer to maximize fruit yield and quality. However, as shown in Figure 2, we found no response to N application rates over 3 years at 3 of the 4 sites, suggesting that there was sufficient N supply at all sites even with the lowest N application rate (30–45 lbs. N per acre). Interestingly, at the lowest SOM site (3% SOC), the lowest fruit yield was observed with the highest N application rate (90–105 lbs. N per acre). The idea that there was sufficient N supply regardless of N application rates is further supported by the leaf tissue N data, which showed levels within the sufficiency range across all sites and N application rates, based on recently published leaf tissue N thresholds for western Washington (Lukas et al., 2022).

How much N fertilizer do I need to apply?

As mentioned above, we saw no negative effects of applying only 30–45 lbs. N per acre to mature blueberry plantings for 3 consecutive years. This is considerably lower than publicly available N application rate recommendations for the Pacific Northwest, which suggest at least 100 lbs. N per acre for mature blueberry plantings (Bryla and Strik, 2015). However, it is consistent with several recent studies conducted in Oregon and British Columbia. Nevertheless, it must be stressed that this was only a 3-year study and it is not known how these plantings would respond to lower N application rates beyond 3 years. Therefore, any producers who reduce N application rates should monitor leaf tissue N carefully. On the flip side, with a few notable exceptions, we did not find negative effects of the highest N application rate (90–105 lbs. N per acre), suggesting that mature blueberry plantings are relatively tolerant to a range of N application rates, at least within the range of application rates used in this study.

Where can I learn more?

For questions or suggestions on this work, please reach out to Gabe LaHue (, an Assistant Professor of Soil Science at the Washington State University Mount Vernon Northwestern Washington Research and Extension Center (NWREC). We’d also love to hear your feedback on this work! Please take the time to fill out this brief survey. It will take less than 3 minutes of your time, and it really helps inform our work going forward!