Study of ramial chip mulch and organic fertilizers on wild blueberries

Project Overview

Project Type: Farmer
Funds awarded in 2014: $14,706.00
Projected End Date: 10/17/2017
Grant Recipient: Blue Hill Berry Co.
Region: Northeast
State: Maine
Project Leader:
Nicolas Lindholm
Blue Hill Berry Co.

Annual Reports


  • Fruits: berries (blueberries)


  • Crop Production: nutrient cycling, organic fertilizers
  • Education and Training: farmer to farmer, on-farm/ranch research
  • Production Systems: organic agriculture
  • Soil Management: organic matter, soil analysis, nutrient mineralization, soil chemistry, soil quality/health

    Proposal summary:

    There are very few options for organic wild (low-bush) blueberry growers when it comes to fertility management.  Organic fields, particularly ones that have been organic and not received any herbicide applications for more than a decade, tend to have higher populations of non-blueberry plants (i.e., weeds like grasses, broadleaf perennials, wildflowers, woody and herbaceous plants, and tree saplings) which become established in and amongst the native low-bush blueberry plants.  When an organic grower seeks to promote nutrient uptake and yield for the blueberries with applications of approved fertilizer (be it compost, foliar sprays, rock powders or other soil amendments), it is the weeds who receive the boost, not the blueberries. Weedy fields will have lower yields and show lower levels of important nutrients (like N) in leaf tissue samples of the blueberries; this is due primarily to the ability of many weeds to outcompete the blueberries for soil nutrients and moisture.  Studies have shown that controlling weeds increases the availability of nutrients and moisture to the blueberry plants.  Though there are many factors which affect the uptake of nutrients, including soil pH, soil microbial life, water availability, and the presence or lack of the nutrients themselves, weeds have a competitive advantage over the blueberry, and fertilizing "will stimulate weeds and not improve blueberry yield." (Bulletin 852 "Organic Wild Blueberry Production," Univ. of Maine, 2009). To increase yields in organic wild blueberry fields, managing the soil pH has become the standard approach –using sulfur to get the soil pH down near 4.0 creates an acidic environment which many weeds cannot tolerate while the blueberries still thrive.  This shifts the balance of competition in favor of the blueberries.  However, we have found on our 14-year organic fields, many weeds still thrive and compete in this situation, and we need to examine ways of further tipping the balance in a diverse plant ecosystem in favor of the wild blueberry. The use of mulches has been studied in wild blueberry fields (Fact Sheet 228 "Mulching for improved plant cover," Univ. of Maine, 1990), and shown to have some useful and beneficial effects.  Certain materials work well at smothering certain weeds, inhibiting weed seed germination, while also moderating moisture and temperature fluctuations in the soil.  This all leads to better blueberry rhizome growth and increased fruit production (Bulletin 852). We would like to study the effect of one type of mulch material (ramial chips) in tandem with a selected fertilizer package (fishmeal and aragonite), and follow three separate application rates through two harvest cycles, ultimately looking at nutrient levels and yields over the various plots.  The study will utilize both a standard soil test and a Reams-type soil test, plus the standard foliar nutrient analysis for wild blueberries, in addition to Brix level readings, all in conjunction with harvest data collected from each plot.  This study will yield information for all organic blueberry growers (wild as well as highbush) regarding fertility management and correlating nutrient levels in the soil and plants, and ultimate yield potential. Organic wild blueberry growers struggle with weed control and the need for improved fertility in their fields; typically organic fields are too weedy to allow a farmer to fertilize, as the weeds out-compete the blueberry plants.  We are attempting to find a way to use one of our recurrent weeds (hardwood saplings) as a source of mulch, which would smother weeds, improve soil quality and allow us to apply some select fertilizers.  Over the course of two harvest/prune cycles (four years total), our study will utilize different soil and foliar tests, as well as a refractometer, so that we will, in a way, be looking through different windows at the same view, in an attempt to better understand connections between weed suppression, fertilizing, and crop nutrition.   The goal is to look for corresponding relationships between the use of ramial chip mulch and two different fishmeal/aragonite applications and their impact on plant and soil health, with particular interest in finding their impact on crop yield and potential profitability.  We will create an enterprise budget for those mulch/fertilizer treatments whose costs are justified by the increased income from increased yields.  We also anticipate generating valuable information on the use of refractometers and certain soil and foliar tests in assessing crop nutrients and efficacy of fertilizers, information that could be meaningful, relevant or directly useful to growers of many types of crops.  Working with MOFGA and the Univ. of Maine, we will share our findings through written materials and various presentations at meetings. 

    Project objectives from proposal:

    Our research objective is to look for measurable results between selected applications of ramial chip mulch and organic fertilizer with respective changes in nutrient levels and yield in organic wild blueberry production.  Positive results will help organic blueberry growers make decisions about mulching options and fertility management, which can beneficially affect moisture and nutrient movement in the soil, plant health and efficiency of fertilizer use, and potentially increase yield and farm profitability.  We expect that there will be many divergent findings, both in soil and foliar test results, as well as soil pH, Brix readings, % organic matter and CEC, and anticipate there will be many conclusions to draw, beneficial and detrimental, between this data and the corresponding cost and yield for a given treatment.


    There will be four test treatments in our study, each one replicated four times in 12'x50' plots randomly laid out across our 5 acre blueberry field.  This will give us a total of 16 plots, each one of which will have a buffer zone between it and the next plot; our study area will be close to ¼ acre, or 10,000 sq.ft. in total.  We will manage the entire field as we normally do for organic wild blueberries, in terms of pruning and harvesting techniques.


    One test treatment will be the control, where no mulch or fertilizer is applied.  The second test treatment will have a mulch layer of ramial chips spread 2"-4" deep across its entire surface, with no fertilizer applied –this is a thickness suitable for weed suppression according to Univ. of Maine research (Bulletin 852 and Fact Sheet #228) and we estimate would closely match the rate of 10 tons chips/acre, a rate recommended by Brian Caldwell in his review of several studies of wood chips and soil nutrients ("Wood Chips in Vegetable Production",  The third treatment will have the same layer of ramial chips, plus an application of fishmeal (at a rate of 12 lbs./test plot, or about 70 lbs. N/acre) and aragonite (at a rate of 2 lbs./test plot, or about 150 lbs. Ca/acre)—these rates have been determined by both our own calculations based on nutrient analyses taken from our field in 2010 as well as on suggested rates given by Brian Caldwell in the article cited above.  Our fourth treatment will have the same layer of ramial chips, plus the fishmeal and aragonite applied at half the rates which we apply on the third treatment plots (fishmeal @6 lbs./test plot and aragonite @1 lb./test plot).


    At the start of the study, we will first produce the ramial chips; this process will occur in late spring 2014.  Our tractor and chipper will be used to make the chips, and the tractor used to spread them on the appropriate treatment plots (12 plots in all). 


    The two treatments of fertilizer (covering 8 plots) will be spread just prior to applying the ramial chip mulch.  One person will spread the fishmeal and aragonite at the appropriate rates onto the blueberry plants by hand. 


    Beginning in July 2014 and repeated again in July 2016 and July 2018, the soil tests will require a person taking core samples of the soil in the plots (typically 10-15 core samples taken per plot, mixed together in a bucket, and then measured into the sample boxes and sent to the test labs).  Our study requires two different soil tests from two different labs. 


    Also taken in July of the prune cycle years (2014, 2016 and 2018), the foliar analysis will require a person cutting leaf samples from the plants (during the tip-dieback stage of the pruned plant's growth cycle); 90 stems per plot will be harvested, placed in labeled bags and sent to the test lab. 


    Our refractometer readings of plant tissue will be taken on both the prune and the production years (2014 through 2018), typically on a weekly basis for eight weeks from July through August.  This testing will require a person to cut a stem/leaf sample, squeeze a few drops of sap out of it using an appropriate press (which we will need to purchase), and placing the liquid drop on the glass slide of the tool; a reading is taken simply by looking through the refractometer.  Variables such as temperature and time of day must be taken into account, and the slide wiped clean between samples.  The Brix levels will be recorded and entered into a database on the farm. 


    Refractometer readings of the fruit will be taken weekly on harvest years during our study (2015 and 2017) for up to 5 weeks in July and August.  This will require a person to harvest a small number of berries (from 1 to 6), squeeze a few drops of juice onto the slide of the tool, and take a reading.  These Brix levels will be recorded and entered into our database. 


    Harvest data will be collected during the production cycle years (2015 and 2017); each plot will be completely harvested at one time and the berries processed and weighed with our normal equipment (rakes, field totes, winnower, electronic scale) in our usual manner (for fresh-pack market).  The total lbs. of our top-grade and any lower-grade berries will be recorded and entered into a database on our farm. 


    All of the data collected will show comparisons between the three treatments and our control, with regards to the respective nutrient levels and resulting yields for each plot.  We are interested in comparing and contrasting the results of the two different soil tests side by side, and comparing and contrasting those tests to the foliar tests and the Brix readings, and seeing how they all relate to respective yield statistics.  A final part of our study would be the examination of the specific fixed costs for the treatments (producing and spreading the chips and buying and spreading the fertilizers) in relation to the resultant yields, to give a  picture of the economic impact of the treatments as they correspond to potential yield increases/decreases and profitability.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.