Collaborative Organic Orchard Research with the National Park Service
The Carrell peach orchard trial was tilled and then planted with alfalfa seedlings in 2012. The seedlings were then promptly eaten by voracious marmots, requiring a treatment revision.
Scenic and unique locations beckon field researchers. So it was no surprise when an opportunity to work in the historical fruit orchards of Capitol Reef National Park (CRNP) in south-central Utah was seized by a team of USU organic researchers. A USDA Organic Research and Extension Initiative grant garnered by the team included on-farm trials to test, implement, and demonstrate innovative organic practices.
CRNP is one of the few national parks to maintain orchards as part of its cultural heritage. Early settlers planted orchards as a cash crop and for subsistence. Today, the historical orchards contain over 3,100 fruit and nut trees. A few years ago, the orchards were converted to organic production practices. The tree-ripened fruit is sold through u-pick to park visitors, and is a main attraction in the summer months.
Wayne Hanks, CRNP orchard manager, has had success with pheromone-based mating disruption and microbial insecticides in managing key insect pests, but his major challenge is to provide adequate tree nutrition without the use of synthetic fertilizers. After several years without fertilization, orchard trees were noticeably stunted. The cost of hauling compost into the remote area was too expensive, and frankly, unsustainable. So Hanks thought, why not grow nutrients on-site?
The research team considered novel nitrogen-fixing legumes; however, because alfalfa already grew in old production fields and was approved for planting in the park, CRNP opted to focus on alfalfa. The collaborators designed the following objectives to test in the Carrell peach orchard (park orchards are named for the settler families who originally planted them):
- Determine whether peach tree nutrition can be enhanced by growing alfalfa on the orchard floor,
- Assess competition of alfalfa and grass ground covers with weeds and peach trees, and
- Assess effects of ground covers on insect and mite populations with a goal of minimizing pests and enhancing biological control.
In May 2012, following tillage of the established perennial grass cover, alfalfa seed was drilled into the soil of four 23 ft × 25 ft plots. Unfortunately, a nearby colony of yellow-bellied marmots foraged heavily on the exposed alfalfa seedlings, greatly diminishing the stand and biomass. A new treatment was added in late summer 2013: drill alfalfa into undisturbed grass cover to "hide" the seedlings from the marmots. Ground cover density and biomass, soil and peach leaf nutrition, peach tree growth, and arthropod abundance and diversity were sampled in spring and late summer of 2013 and 2014 to address the study’s objectives.
In 2014, the alfalfa-grass mix had the greatest biomass and was resistant to invasion of weeds, as good as the undisturbed plots. The tillage/alfalfa plots were heavily invaded by weeds, especially dandelion, field bindweed, prostrate vervain, and showy milkweed. Soil nitrate and ammonium were very low, reflecting nitrogen deficiencies seen in the peach trees.
Collecting soil samples for nutrient analysis.
Sampling ground cover biomass
Tillage/alfalfa increased soil nitrate levels; however, this was likely due to release of nitrogen following tillage of the grass and not to nitrogen fixation, as the marmots kept alfalfa biomass low. In the soil, readily mineralizable carbon, microbial biomass, and phosphatase (enzyme produced by plants and microbes used in phosphorus mineralization) responded positively to planting alfalfa. Peach trees were deficient in nitrogen (N), magnesium (Mg), calcium (Ca), iron (Fe), and manganese (Mn). The tilled alfalfa treatment temporarily relieved N deficiency.
Arthropod abundance and diversity was low in all ground covers, less than 2.5 arthropods per gram dry weight. Thrips, flat mites, and springtails were the most abundant arthropods in the ground cover; the first two groups being primarily herbivores, and the latter group, detritivores. Western flower thrips was the most abundant insect on peach leaves, and densities were greatest in plots with alfalfa. Few other insects and mites were found on peach leaves. Tree trunk circumference was greatest in tillage/alfalfa plots demonstrating that grass is competitive with trees for nutrients and water.
Our preliminary results show that tillage before planting alfalfa stimulated nitrogen release from the grass cover; however, this effect is likely to be short-lived without the addition of cover crops or compost. We are hopeful that the no-till alfalfa stand will thrive and provide a long-term source of nitrogen. The heavy herbivory of marmots on alfalfa in the tilled plots was unanticipated, but despite challenges in establishing and growing alfalfa in the Carrell peach orchard, the addition of a nitrogen-fixing legume shows promise for improving soil and peach leaf tissue nitrogen levels. We recognize additional research collaborators, Drs. Brent Black and Corey Ransom.
-Diane Alston, Entomologist; Jennifer Reeve,
Organic and Sustainable Agriculture Researcher,
and Esther Thomsen, graduate student