How do Sawdust and Fertilizer Rates Impact Blueberry Yields and Quality?
Dr. Bernadine Strik, Professor, Dept. Horticulture
Berry Crops Research Leader, NWREC, Oregon State University

The objectives of this study are to determine the long-term impact of pre-plant incorporation of sawdust and surface sawdust mulch and nitrogen (N) fertilizer rate on yield, fruit size, fruit firmness, soil nutrient status and soil properties in “Elliott.” 

We have been studying the following treatments in a planting of “Elliott” established at Oregon State University’s North Willamette Research and Extension Center (NWREC) in October 2003, with or without pre-plant incorporation of sawdust (ten units/acre); with or without a surface sawdust mulch (three inches deep over raised beds); and low, medium or high rate of N fertilization. Nitrogen fertilizer was applied as a granular product (ammonium sulfate in 2004-05; and urea subsequently) as a triple split. The total rate was applied per acre but was concentrated in the row as a broadcast band. The rates used depended on planting age:

There has been no significant effect of N fertilization rate on cumulative yield to date – the sum of machine-harvested yield in years 2006-2011 (Fig. 1). 

Figure 1. Effect of N fertilization rate on yield of “Elliott” blueberries, planted in Oct. 2003, averaged over sawdust incorporation and mulch treatment.  Cumulative yield, from 2006-11, is shown on the right.

Sawdust – Mulch – Fertilizer Interactions
While cumulative yield (2006-11) has not been affected by incorporation of sawdust or mulch alone, there was a significant interaction between mulch and incorporation; plots in which sawdust had been incorporated before planting had a higher cumulative yield if no mulch was used whereas the opposite was true for plots in which no sawdust was incorporated.

Nitrogen fertilization with the high rate of N has decreased fruit size in all years. Averaged, over the six years of this study so far, there has been no treatment effect on firmness.

Plants fertilized with the low rate of N had significantly lower leaf concentration of N in 2010 and manganese (Mn) (likely due to higher soil pH) in both years and higher leaf calcium (Ca), copper (Cu), and Boron (B). While leaf percent N was “below normal” in 2010 this has not yet had a negative effect on yield in “Elliott”; leaf N concentration was within normal range for all treatments in 2011. In 2010 mulching significantly increased leaf B, Mn and Aluminum, but decreased leaf phosphorus (P) compared to bare soil, whereas in 2011 mulching only increased leaf Ca concentration.

Nitrogen fertilization rate had a significant effect on soil pH, iron (Fe) and NO3-N (nitrate) and NH4-N (ammonium) content in late August 2010. The soil pH of plots fertilized with the high rate of N was lower than in plots fertilized with the medium or low rate of N. Soil NO3-N and NH4-N content was four-fold and two-fold higher, respectively, in plots fertilized with the high rate of N than in those fertilized with the low rate of N. Sawdust mulched plots had about half the soil NO3-N and NH4-N content as bare plots. 

There was no effect of N fertilization rate on the weight of prunings removed in January 2011. Pruning removed an average of 14 lbs. N/acre, 1.5 lbs. P/acre, 7 lb. K/acre (potassium) and 3 lbs. Ca/acre. The nutrients in these prunings, even if flailed in the field, may not be available to the blueberry plants as the prunings decompose because there are no roots in the between-row area of a blueberry field.

We analyzed treatment effects on the amount of nutrients removed in the fruit in 2010 and 2011.  About 15 lbs. N/acre, 1 lb. P/acre and 8 lbs. K/acre were removed in the 10 tons/acre of machine-harvested fruit in 2010 and 2011 (or 1.5 lbs. N, 0.1 lb. P and 0.8 lb. K/acre per ton of fruit harvested).

Findings Summary
Based on our results so far, N fertilization rate, surface mulch and pre-plant incorporation of sawdust have had little effect on plant performance (yield, prunings and nutrient removal). However, these treatments have impacted soil nutrient status – we will discuss the possible implications of this at the end of this project.

Estimates indicate that to replace the nutrients removed when pruning and harvesting fruit in a mature “Elliott” planting yielding 10 tons/acre, about 29 lbs. of N, 2.5 lbs. of P (5.7 lbs. P2O5) and 15 lbs. of K (18 lbs. of K2O) would need to be applied per acre. However, some additional nutrient application would be required for plant growth, even in mature plantings. We assume that any nutrients lost at leaf fall would ultimately return to the soil and be available to plants.

In the remaining year of this project we will continue to study the effect of these treatments on yield, fruit quality, soil and plant nutrient status, nutrient removal and soil biology, particularly the level of mycorrhizal infection.

I greatly appreciate the valuable contributions provided by Gil Buller (Senior Research Assistant, NWREC), Denise Nemeth (Ph.D. graduate student), Littau Harvesters Inc. and funding from the Oregon Blueberry Commission and the Agricultural Research Foundation.

Prunings removed from “Elliott” research plots in January 2011

Machine harvest, September 1, 2011 mature “Elliott” at the NWREC

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