Student Studies Soil Management and Organic Blueberry Production in Michigan

Michigan State University (MSU) graduate student, Jesse Sadowsky, received an NCR-SARE $9,900 Graduate Student Grant to conduct an observational study to examine the effects of both organic and conventional management on plant health and soil biology in blueberry fields in Michigan. According to USDA-ARS, Michigan is the number one blueberry producing state in the U.S. However, according to Sadowsky, organic blueberries represented less than 0.3% of the total blueberry acreage in Michigan at the start of Sadowsky’s project.

“We realized that comparing already existing certified organic blueberry fields with conventional fields was a great opportunity to address the extent to which microbial communities and some of the biological processes they carry out diverge after organic transition of mature blueberry fields,” said Sadowsky. Sadowsky added that because of their ability to break down and transfer organic nitrogen to the plant, ericoid mycorrhizae may be of particular importance in organic blueberry production, but their presence in blueberry fields has not been assessed under MI conditions. Ericoid mycorrhizae are a symbiotic relationship between fungi and the roots of plants, possessing properties that can improve nutrient uptake.

Sadowsky carried out an observational study to describe characteristics of plant and soil health on commercial organic and conventional blueberry production fields in Michigan. His team located a representative sample of certified-organic production fields and matched each with a conventional field. Pairings were based primarily on USDA National Resource Conservation Service (NRCS) soil type, but cultivar and field age were considered and matched as closely as possble. Weed management, fertilization practices, and types of soil amendments applied differed sharply between the organic and conventional fields. Soil, roots, and fruit were collected from each of the growers’ fields over two years and five sampling dates. Measured variables included levels of blueberry root colonization by ericoid mycorrhizal fungi, activity of several soil enzymes involved in carbon and nutrient cycling, size of labile soil carbon and nitrogen pools, and incidence of blueberry stem dieback and fruit rot diseases. In addition, a six month greenhouse experiment investigated how different species of ericoid mycorrhizal fungi and dairy compost or feather meal as nitrogen sources interact to influence growth and mycorrhizal colonization of young blueberry plants.

According to Sadowsky, organically grown fruit had a higher incidence of anthracnose rot while conventionally grown fruit had a higher incidence of alternaria rot. Mycorrhizal colonization levels were significantly higher in organic blueberries.

“To the surprise of some organic growers, ericoid mycorrhize were abundant on many of the conventional blueberry farms in our study,” said Sadowsky. “Because ericoid mycorrhizae are present at moderate to high levels on both organic and conventional blueberry farms, they may play an appreciable role in nutrient uptake on both types of management systems. However, the precise role of ericoid mycorrhizae in intensively managed highbush blueberries remains to be determined in future studies.”

Sadowsky also discovered that organic and conventional blueberry production practices differed in their effects on soil processes such as carbon and nitrogen cycling. Although organic and conventional blueberry soils were similar in terms of total organic matter content, pH, and other standard chemical measures, the labile soil carbon cycled more rapidly on organic farms, and both nitrogenacetylglucosaminidase activity (which acts to mobilize nitrogen from cell walls of dead fungi and many other soil organisms) and the size of the potentially mineralizable nitrogen pool were enhanced by organic management.

“This finding suggests protein-based organic fertilizers could be more likely than compost to elevate late-season nitrogen (N) levels in blueberry fields,” said Sadowsky.

A greenhouse experiment demonstrated that inoculation with ericoid mycorrhizal fungi increased shoot growth of plants fertilized with feather meal, while compost enhanced the survival of mycorrhizal fungi. At 165 days after compost and feather meal application, the N-supplying capacity of compost was nearly exhausted while feather meal continued to release N. This finding suggests protein fertilizer is more likely than compost to elevate late-season N levels in blueberry fields, according to Sadowsky.

Sadowsky says that, recognizing that organic and conventional blueberry soils differ on a biological basis has implications for understanding how soils in each management system will respond to management inputs. For example, enriching the pool of potentially mineralizable nitrogen and activity of nitrogen-mobilizing enzymes may be valuable for satisfying crop nutrient needs in organic fields, but may be relatively less important on conventional fields where nitrogen is applied in forms that are immediately available to plants.

“Additionally, we are certain that our interactions with growers throughout the study, including individualized reports that summarized the measures we recorded in their fields, were informative and demonstrate that microbial communities and biological processes in blueberry soils are influenced both by inherent field characteristics and the grower’s approach to management,” said Sadowsky.

Want more information? See the related SARE grant(s) GNC08-099, Student Studies Soil Management and Organic Blueberry Production in Michigan.