AAFC Gets the Dirt on Organic Fruit Crops in BC

Organic Agriculture Centre of Canada

Every organic grower knows that a delicious piece of organic fruit isn’t just free of chemical pesticides.  It is the product of an agricultural system that is fundamentally different from conventional production.  One foundation of organic agriculture is the maintenance of healthy soil which in turn will nourish and protect healthy plants. 

As demand for locally and sustainably grown organic fruit continues to grow in British Columbia and across Canada, researchers at Agriculture and Agri-Food Canada (AAFC) Summerland and Agassiz are working hard to understand and improve on the relationship between soils and organic fruit production.  Drs. Gerry Neilsen, Denise Neilsen, and Tom Forge have been looking at ways to use organic materials to supply nutrients to three perennial fruit crops important in BC:  apple, blueberry, and raspberry.  The first is grown mostly in the dry interior Okanagan Valley and the latter two in the wetter coastal Fraser Valley. 

In conventional agricultural systems, growers can apply calculated amounts of nitrogen (N) and other nutrients when the crop most needs them.  In organic farming, nutrients are slowly released from soil organic matter as it is broken down by the soil community.  Soil organic matter also affects the soil’s structure, aeration, moisture retention, and populations of soil organisms, all of which in turn affect the growth of plant roots and their ability to take up nutrients. 

To enhance soil fertility, organic growers can supplement the soil with manure, compost, other organic materials, cover crops, and unrefined inorganic materials such as rock phosphate.  As an added challenge, in established perennial fruit crops, soil amendments can be applied only to the top of the soil as mulches.  Moisture and the actions of soil organisms, such as bacteria, fungi, microfauna (nematodes and tiny arthropods), and macrofauna (earthworms), are needed to release the nutrients and carry them to the roots.  “Fertilizing” organic fruit crops is a complicated science!

In a study on orchard floor management in organic apples, Gerry Neilsen is looking at the effects of mulches on soil fertility.  He is comparing poultry manure compost combined with tillage, bark mulch, or polyethylene landscape fabric, and a practice called “mow and blow” which involves growing alfalfa between the rows of trees and blowing the cuttings onto the rows.  Mulches were found to affect the levels of nutrients in the soil and in the leaves.  Early results indicate that bark mulch may suppress yield.

In other long-term studies on apple, the researchers have compared mulches of compost, shredded paper, spray-on paper, alfalfa hay, polyethylene fabric, and “living mulch” cover crops of dwarf white clover, sweet clover, hairy vetch, and annual rye.  While cover crops reduced growth and yield, other mulches tended to have a positive effect.  Soil nutrient levels and turnover by soil organisms increased under the mulches, and plant uptake of potassium and phosphorus was increased, probably because of improved soil moisture and root growth.
Blueberries are usually grown with a sawdust mulch that helps maintain moisture and the acidic conditions that blueberries like, but requires the addition of high amounts of N fertilizer.  “We were hoping to find a compost mulch that had all the good properties of sawdust, but also supplied nitrogen” says Forge.  “Manure-based composts are usually too high in pH and salts for blueberry, so we tried yard-waste compost”.  In one study, after 3 years of applying compost, there was no difference in yield compared to sawdust with fertilizer.   

Another study, now underway in cooperation with UBC, aims to examine the feasibility of using composts and sawdust mulches in combination with organic sources of additional N such as “mow and blow” alfalfa, feather meal, and composted poultry manure. 

Concerns over consumer safety have stopped the use of manure for fertilizing raspberries, so Forge is also looking at composts as alternatives to inorganic fertilizers for raspberry.  Promising results have arisen from this project - after three years comparing composts to conventional fertilizer, crop yields were higher in the compost plots.

Results to date look good for maintaining healthy, nutrient rich soils for organic fruit production in BC.  Mulches not only add nutrients but improve soil quality.  But Gerry states “We still have a challenge understanding N availability for various organic amendments.”  He and his collaborators plan to continue exploring the influences of organic amendments on soil fertility.

This article was written by Andrea Muehlchen on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).  The Organic Science Cluster is a collaborative effort led jointly by the OACC, the Organic Federation of Canada and industry partners. For more information: oacc@dal.ca or 902-893-7256.

Posted May 2011