Towards ecologically-based fertilizer recommendations that improve soil quality in high-density apple orchards

Towards ecologically-based fertilizer recommendations that improve soil quality in high-density apple orchards

Summary

The purpose of this project is to create sustainable soil fertility management practices for Mid-Atlantic and Southern apple growers by integrating the use of carbon-based soil amendments, such as compost and mulch, into new high-density orchard systems, thereby lessening the use of synthetic fertilizers. During the past year, we continued to sample from our study sites at Virginia Tech’s Alson H. Smith, Jr. Agricultural Research and Extension Center (AREC), and at the orchards of two grower cooperators (Silver Creek Orchards and Catoctin Mountain Orchard). Preliminary data from the second year of this project is presented within this report.

Objectives/Performance Targets

Objective 1: Survey commercial apple growers in the Mid-Atlantic region to identify the fertilizer formulations, quantities, and application frequencies most commonly employed.

• The goal of the survey is to better understand current fertilizer practices in order to better effect change in grower behavior.

Objective 2: Use an interdisciplinary approach to examine the benefits of utilizing carbon-based soil amendments in commercial apple orchard systems so that we can investigate the interaction between soil nutrient source and orchard productivity. Our project will investigate microbial community composition, biomass, and soil biological activity in the soil, particularly in the rhizosphere (the soil-root interface). The research will occur on replicated plots located in three separate regional apple orchards.

• Our research utilizes a research farm trial comparing synthetic fertilizers (calcium nitrate), chicken litter compost, municipal yard waste compost, and treatments that integrate synthetic fertilizers with composts when applied to newly planted high-density apple trees.
• We also developed two replicated on-farm trials with cooperating growers.
• Among the three farm sites outlined in this objective, this research project spans 200 miles and puts growers throughout the Mid-Atlantic in close proximity to the test sites, which will encourage faster adoption of the most effective treatments.

Objective 3: Educate stakeholders in orchard nutrition management. Develop printed and web-based outreach materials to support educational activities.

Accomplishments/Milestones

In April and May 2014, apple trees were planted at the three research locations. The treatment rates (pounds of nitrogen per acre) were identical at each location (Table 1). However, the mulch and compost were sourced locally and varied at each site.

Experimental Design and Statistical Analysis

A split-plot, completely randomized design with four four-tree replications for each treatment was used at each location. The main plots are mulch versus no mulch. The four subplots are the fertilizer inputs: compost, calcium nitrate, both compost and calcium nitrate (CaNO₃), and an unfertilized control. There is an untreated buffer tree between each subsplot and two untreated buffer trees between main plots. Mulch was applied at a 1-m width, 10-cm depth and mostly consisted of large-aggregate material that was sourced locally for each site.

Response variables to be measured:

• Tree growth by trunk cross-sectional area on an annual basis
• Leaf nutrient status (20 leaves were collected from each tree in the treated block from current season’s growth)
• Soil nutrient status (taken to 10-cm depth from two locations on either side of each tree approximately 26 cm from the base of the tree)
• Terminal shoot growth and branching
• Fruit yields and quality (in year two, if there is fruit, but more likely not until year three)
• Microbial biomass and community structure

Milestones

• Tree planting dates:
  • 14 May 2014 at AREC
  • 8-10 May 2014 at Catoctin
  • Apr 2014 at Silver Creek
• Treatments applied:
  • 18 Jul 2014 at AREC
  • 22 Jul 2014 at Catoctin
  • 26 Aug 2014 at Silver Creek
  • A second application of CaNO₃ occurred during leaf sample dates in 2014.
• 2015 soil samples dates:
  • 16 July at AREC
  • 23 July at Catoctin
  • 29 July at Silver Creek
• 2015 leaf sample dates:
  • 17 July at AREC
  • 23 July at Catoctin
  • 29 July at Silver Creek

Lastly, we have had some personnel changes that affect this project. PI Peck left Virginia Tech in October 2015 to start a new position at Cornell University. His new responsibilities involve sustainable tree fruit production and he intends to complete the objectives of this SSARE-funded project through to the end of the funding cycle. With his move, graduate student Cody Kiefer chose to stay at Virginia Tech to pursue a different project.

Preliminary Data:

Trunk cross-sectional area (TCSA) was measured in June 2014 for baseline data and again, after tree growth had subsided, in October 2014 and again in November 2015. Baseline measurements were not different among the treatments at the Winchester and Thurmont sites for either date, but at the Tyro site, the Mulch + Compost treatment trees had greater TCSA than the No mulch + CaNO₃ treatment during the June measurement and the No mulch + Control treatment during the October measurement (Tables 2-4). At the AREC site in 2015, the Mulch + Control treatment had greater TCSA than the No Mulch + Compost treatment (Table 2). At the Thurmont site, the Mulch + Compost and the Mulch + Compost + CaNO₃ treatments had larger trees than the No mulch + Compost treatment (Table 3). At the Tyro site, Compost and the Mulch + Compost + CaNO₃ treatments had larger trees than the No mulch + Control, No mulch + Compost, and No mulch + CaNO₃ treatments (Table 4). Additional time is needed to see if the trend of larger trees size in the mulch treatments continue.

Due to a hail storm at the Tyro site, and small tree size at the AREC site, only the Thurmont site had fruit in 2015. Small tree size is not unusual for young Honeycrisp trees, which is known to be a slow growing cultivar. There were no differences among treatments in the yield data at the Thurmont site (Table 5).

Soil tests performed in 2014, prior to the treatment applications indicated uniformity in soil nutrition across the experiments at the Winchester and Tyro sites. The Thurmont site revealed higher iron content in the Mulch + CaNO₃ treatment than all of the non-mulch treatments and the Mulch + Control treatment. In 2015, soil mineral differences were starting to become evident among treatments. At all sites, phosphorus and potassium was higher in the Compost treatments, though differences were not always statistically significant (Tables 6-8). Although statistical analysis was not performed across sites, it is clear that the Thurmont site has much greater levels of phosphorus compared with the other two sites. This is likely due to the farmer’s long-term use of compost and shows the potential negative impact of overuse of compost. Interestingly, leaf phosphorous levels were not greater at the Thurmont site (Tables 9-11). There were few statistical or biologically significant differences in the leaf mineral content at any of the sites.

The importance of belowground plant-microbe associations in the development of apple orchard systems have rarely been tested under field conditions. It is likely that the mulch and fertilization treatments will have a large effect on soil microbial community structure, and the potential for the development of positive plant-microbial feedbacks that can support sustainable orchard production. The research thus sought to determine patterns of change in soil microbial communities associated with fertilizer type in apple orchards at the three field sites. 16S rRNA genes extracted from soil were sequenced to describe bacterial communities. Fungal DNA was also extracted, but the samples have not yet been sequenced.

Soil bacterial communities were analyzed using the software Quantitative Insights Into Microbial Ecology (QIIME). Quality checking of the more than 1.5 million sequence reads needs further scrutiny, however, initial results suggest that bacterial communities differed across treatments, with the greatest differences associated with the compost treatments. The ways in which the community changed, however, differed across locations. The dominant Operational Taxonomic Units (OTUs) were most closely related to Proteobacteria, Acidobacteria, and Actinobacteria. Evidence for fertilizer-induced changes in the relative abundance of ammonia-oxidizing bacterial phyla are also apparent, and suggest that there are ecologically significant and functional differences in nitrogen cycling OTUs across treatments. The changing bacterial community likely has important ramifications for the bioavailability of plant nutrients and therefore orchard sustainability.

Further work into determining fungal community structure and more detailed analysis of both bacterial and fungal communities are expected to reveal further patterns related to fertilizer treatments in apple orchards. Functional analysis using Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) will be conducted to better describe phylogenetic and functional shifts that can help explain changes in soil quality and plant productivity and yield. Of particular interest will be the relative occurrence of mycorrhizal fungal taxa across the fertilizer treatments.

Impacts and Contributions/Outcomes

Study sites have already been a source of discussion and demonstration for the public and commercial apple growers in the Mid-Atlantic region. During an open house at the AREC on 16 Aug 2014, Mr. Kiefer gave two presentations on this project to nearly 85 people. Many of these individuals were very interested to learn about the emphasis on sustainability that is included in this project. On 10 Sep 2014, a group of nearly 50 growers and associated industry personal toured the AREC research site to learn about the first year’s results. On 25 Aug 2015, the AREC plots were used to demonstrate possible organic fertilizer and weed control strategies to 50 growers at a Commercial Organic Apple Production Field Day.

As this project progresses, the project’s results will be used to better inform apple growers’ fertilizer practices. We will do this through conducting an in-service extension training on soil biology and management for Cooperative Extension personnel in the Mid-Atlantic region in the final year of the project. Tree-fruit consultants and representatives from regional fertilizer dealers will also be invited to attend the session. The in-service workshop will be held as part of the annual Virginia Cooperative Extension In-Service training in Blacksburg, and/or as a session at the Alson H. Smith, Jr. AREC. Funds for in-service training are made available through Virginia Cooperative Extension on an annual basis.

Results from this project will be used in the tree-fruit extension program to provide valuable information about high-density apple systems in the Mid-Atlantic. When sufficient data has been collected, Drs. Peck and Williams will publish numbered extension bulletins so the information can be widely disseminated in print and electronic formats. We will also publish our results in a mix of horticulture and soil science themed peer-reviewed scientific journals, such as HortScience, Horticulture Research, Applied Soil Ecology, and Soil Biology and Biochemistry.

Collaborators: