We have been studying various production systems in a certified organic field from planting establishment (Oct. 2006) to present at Oregon State University’s North Willamette Research & Extension Center. The long-term goal of this project is to develop organic production systems that maximize plant growth, yield and fruit quality, facilitate weed, water and nutrient management, and provide economic benefit to blueberry growers. This one-acre trial was in its seventh growing season in 2013.

Bernadine Strik in organic blueberry research trial, 2012. Photo courtesy Oregon State University.

We are comparing production on raised beds to flat ground, ‘Duke’ and ‘Liberty’, two fertilizer sources (fish emulsion or feather meal) at a low (25 lb N/acre from 2007-09; 50 lb N/a  from 2010-12; 65 lb N/a 2013-present) or a higher rate (50 lb N/acre from 2007-09; 100 lb N/a  from 2010-12; 125 lb N/a from 2013-present) and three weed management treatments: “Sawdust” (3” deep sawdust mulch with only hand weeding); “compost+sawdust” (1.5” of yard debris compost topped with 2” sawdust with organic contact herbicides and hand weeding); and weed mat (porous, polyethylene landscape fabric). Feather meal was applied as a granular, split into half in March and half in May. Fish emulsion was diluted 1 part product:10 parts water with the total rate of nitrogen (N) split into 7 applications from mid-April to early July. Plants were fertigated starting in 2011 (under the mulches). Plants are set at 30” in the row with 10 ft. between rows. This article summarizes key aspects of what we’ve learned so far.

Is planting on raised beds recommended?

Plants are performing better on raised beds than on flat ground, even though we have a very good “blueberry soil” at the research site. Cumulative yield, over six fruiting seasons (2008-2013), was 22% higher on raised beds than on flat ground. It seems clear from this study that planting on raised beds improves plant growth and yield.

What’s the best fertilizer source and rate?

Plant growth. Visually, ‘Duke’ plants fertilized with either rate of feather meal or the low rate of fish emulsion are larger than those fertilized with the high rate of fish emulsion. It’s hard to visually see differences among ‘Liberty’ fertilizer treatments at this stage.

Yield. Cultivars have responded differently to fertilizer source and rate, and 2013 was no exception. To date, there has been no effect of fertilizer source or rate on yield of ‘Liberty’ (Figure 1). However, in ‘Duke’, fertilization with feather meal has produced the highest yield and fertilization with the high rate of fish emulsion the lowest, in each year of the study.

Figure 1. Effect of fertilizer source and rate on cumulative yield (2008-13) of ‘Duke’ and ‘Liberty’ (averaged over mulch type and raised vs. flat ground); 0.5 kg/plant = approx. 1 ton/acre.

Fruit quality. Fertilizer has had no consistent effect on berry size or weight in either cultivar over the years of the study. However, fertilizer has affected fruit firmness and Brix (soluble solids).  Firmness and Brix were higher in general when plants were fertilized with fish emulsion than with feather meal, and were also higher in general when more fertilizer (fish or feather meal) was applied.

Leaf nutrients. Leaf nutrient concentrations, especially N (%N), have been affected by both the source and rate of organic fertilizer used. None of the concentrations, however, including %N, have been correlated to yield. The high rate of fish emulsion, for example, resulted in the highest leaf %N in ‘Duke,' but has led to the lowest yield each year. Fish emulsion also increased leaf %N compared to feather meal in ‘Liberty,' but has produced no difference in yield to date.

Soil pH. High rates of fertilizer, particularly with fish emulsion, have reduced soil pH over time. This may cause problems in the long-term if soil pH drops too low for optimal plant performance. It is often difficult to increase soil pH in established plantings and may be even more troublesome when the planting is organic.

Overall, our results have illustrated that establishing blueberry plants can be “over-fertilized” (the higher rate is either wasted or reduces yield) in organic systems (similar to what we’ve shown in conventional systems). Fertilizer source may impact fruit quality, but more work is needed to understand why and to determine if the treatment effects persist as the planting matures. We may need to establish new standards for nutrient management (leaf testing) in organic blueberry.

What mulch type has worked best?

Weed control. We have controlled weeds, as needed, to make sure weeds have not competed with the blueberry plants. There was relatively little weed pressure in the first growing season, but weed presence has increased as the planting has aged. Only hand weeding has been used in the sawdust mulch and weed mat (around plant crown) treatments. In the compost+sawdust mulch we have tried to use organically-approved contact herbicides on weeds when they are still small. In the early years, we used Weed Pharm® (20% vinegar) and more recently Avenger AG® (formerly GreenMatch Burndown Herbicide®; citrus oil) as a targeted direct application. The contact herbicides were generally effective when weeds were small and when the application was followed by one or two days of dry, hot weather. When the contact herbicides were ineffective, hand weeding was used to remove weeds.

Weed mat has been the best option for weed management while compost+sawdust mulch has resulted in the most weeds and the highest weed-control cost so far. The compost we used did not contain weed seeds. While we expected the sawdust on top of the compost to minimize weed seed germination, the sawdust eroded off of the compost in places over time (wind, weed pulling, etc.); we suspect that weed seeds germinate quite readily in the nutrient-rich compost.

Soil pH and organic matter. Soil pH has declined over time in most treatments and was lowest in the sawdust mulched plots and highest in the weed mat in 2013; the soil pH is still within acceptable range for blueberries. Mulching with compost+sawdust increased soil organic matter relative to all other treatments, a desirable trait for a blueberry compost. In contrast, organic matter has declined over time under the weed mat.

Soil nutrients. Yard debris composts are often high in potassium (K); little is known about any possible effects of long-term use of compost on soil and plant health in blueberry production. In our study, soil K in the compost+sawdust mulch was almost 60% greater than in the sawdust or weed mat treatments – a dramatic effect considering we only have a 1½ inch depth layer of compost (applied in 2006 and in 2011). We still need to determine if the higher soil K under compost mulch may have detrimental effects on uptake of other nutrients (e.g. Ca and Mg).

Plant growth. We found that plants grown with weed mat had less root growth and larger canopies than the other mulch treatments after two growing seasons (we have not dug up plants since). Plants mulched with weed mat also required additional irrigation to maintain the same soil water content as those mulched with sawdust or compost+sawdust, especially when grown on raised beds. More research is needed on the long-term effects of weed mat on root growth and sustainability of the planting.

Yield. Plants mulched with compost+sawdust or with weed mat produced greater yield than those mulched with sawdust in many years (Figure 2).

Figure 2. Effect of mulch type on yield from the second through the seventh fruiting season (averaged over Duke and Liberty, fertilizer source and rate;and raised vs. flat ground)

Cultivars

‘Liberty’ has consistently had a higher yield than ‘Duke’. Yields for the best treatments have been similar to what might be expected for hand-picked, conventional production. However, it is clear that the lowest performing treatments would have low grower returns (Figure 3). Note that in 2013 we had problems controlling birds in the early ripening ‘Duke,' leading to loss in yield for this cultivar, but not for ‘Liberty.'

Figure 3. Yield in 2013 by cultivar for the best and worst performing production systems (as indicated on the bars). Note, yield was lower than expected for ‘Duke’ in 2013 due to some bird depredation.

Our plan is to continue our organic research study until the planting is mature and to work toward answering questions that our results have raised.

Thank you!

We’d like to thank our other collaborators on this study, including Emily Vollmer (former FRA, Strik) and Gil Buller (Senior FRA, Strik), Luis Valenzuela-Estrada (Post-doctoral Assoc., Bryla and Strik), the students who have worked on this project and our advisory board members. We appreciate the support of the organizations that have provided funding: The Oregon Blueberry Commission, the Washington Blueberry Commission, the Northwest Center for Small Fruits Research, the NIFA-OREI program and industry contributors.