Improving Soil Health and Microbial Activity through Zone Tillage and Innovative Cover Cropping Strategies
Organic growers face a challenging set of guidelines, both prohibiting them from using synthetic herbicides and encouraging them to utilize cover crops in order to maintain ground cover and meet crop nutrient needs. The resulting intensive tillage that organic farmers often use to control weeds, incorporate cover crop residue, and prepare seedbeds is detrimental to soil structure and biology, as well as to the greater environment by accelerating nutrient losses and soil erosion to sensitive water bodies. Adopting no-till practices has proven difficult for organic growers, and the long, cold winters in the Upper Midwest exacerbate these difficulties with periods of decreased soil microbial activity and delayed soil warming. Zone tillage is an overlooked reduced tillage strategy where rows are tilled and interrows are maintained with ground cover, and may help address problems organic growers face with both weed suppression and crop nutrient needs. While gaining traction in agronomic settings, an adaptation of reduced tillage using wider row spacing is also sometimes informally used by vegetable growers. The need to optimize this system for vegetable growers in the Upper Midwest is especially important because of the possibility increased cover crop growth where left to grow between rows, leading to greater biomass production, as well as nitrogen contributions when legumes are used. The purpose of the proposed research is to evaluate both crop productivity and soil biological responses to zone tillage methods in organic vegetable production systems with varying preceding winter annual cover crops.
In 2015, a randomized complete block design was used with three by two factorial treatments: cover crop mix (winter rye + hairy vetch and oat + field pea), tillage (conventional/full-width and strip), and soil sampling location (in rows and between rows). Soil and cover crop parameters thus had six treatments and four blocks, whereas crop yield was modeled only on the first two factors. Results were originally expected to enable organic growers to maximize cover crop advantages as ground cover and fertility sources, as well as conserve soil structure, tighten the N cycle, and reducing environmental pollution from sediment and nutrient losses.
Due to lack of snow cover in the 2014-2015 winter, cover crop stands were terminated and re-planted on April 17, 2015. On June 16, cover crops underwent the first (early) termination. Raised beds were formed and covered with plastic mulch, and yellow crookneck summer squash were direct seeded into rows on July 11. Shortly after, between-row cover crops in the strip till treatments were flail mowed on July 15 since legumes in the mixes were flowering.
In total, cover crop biomass samples were collected on June 15 (“early”) and July 15 (“late”) by combining two 0.1 m2 quadrats of biomass material from each plot and dividing species into cover crop grass (rye or oat), cover crop legume (vetch or pea), and weeds. Upon collection, cover crops were placed into 95F ovens for 48 hours before grinding to 1 mm and C/N analysis on an elemental analyzer (VarioMAX, Elementar).
Soils were collected at 4 points throughout the growing season: June 15 (“pre”), June 25 (“post”), July 25 (“mid”), and October 23 (“harvest”) by taking a composite sample of eight cores from the top 6″ of soil. Field soils were divided into two; one was dried at 105F for 48 hours, ground and sieved to 2 mm, and used for C/N analysis (VarioMAX, Elementar), permanganate oxidizable carbon (POXC), and particulate organic matter (POM). The other composite soil half was kept field moist at 40F for up to one week before being analyzed for microbial biomass C/N and inorganic N.
Extractions from microbial biomass and inorganic N assays are currently being analyzed on a TOC Analyzer fitted with a TN Unit (TOC-L, Shimadzu). Soils are still in the process of being analyzed for particulate organic matter, after which POM C/N will be measured as normal soil C/N.
Extractions from microbial biomass and inorganic N assays are currently being analyzed on a TOC Analyzer fitted with a TN Unit (TOC-L, Shimadzu). Soils are still in the process of being analyzed for particulate organic matter, after which POM C/N will be measured as normal soil C/N. All other analyses have been performed and are in the process of being statistically analyzed (SAS, Cary, NC).
Several deviations from the original proposal were made in the first year of the project, with more planned for the second year. First, the fall seeding of cover crops, though planted in September in accordance with UMN Extension, was unsuccessful due to a relatively dry winter and thus lack of ground cover. Legumes did not survive, although terminating the grasses in the mix (particularly rye) proved difficult, causing two “flushes” of winter rye. This likely confounded cover crop biomass production in some plots. Second, because of the need to spring seed cover crops, red clover as a treatment was discontinued, and field pea + oat was used in its place due to its quick growing nature. Third, in the original proposal we planned to use a nightshade vegetable as a cash crop. Eggplant transplants were started in spring 2015 and transplanted to the field in late June. However, the majority of the transplants were eaten in the field. Therefore, yellow crookneck summer squash were direct seeded in the place of eggplant. This switch proved useful, as it seemed a better use of the wide row spacing necessitated by the unit used for bed preparation and mulch laying with space between for cover crops to continue growing. Still, row covers were laid over the direct seeded squash for 3-4 weeks, and a fence was built around the experimental area. Finally, row spacing for conventionally tilled treatments were exceptionally wide, and thus squash yield were not representative. This will be addressed in 2016.
Field plans for 2016 differ, in that we will use a randomized complete block design with only three treatments and four blocks. The treatments include red clover (strip tilled), winter rye + hairy vetch (strip tilled), and oat + field pea (to be spring planted and conventionally tilled, full-width). The latter (conventional till) will be on a narrower row spacing (50″ as opposed to 100″ for strip till treatments).
Impacts and Contributions/Outcomes
Preliminary findings were presented at the Midwest Organic & Sustainable Education Service’s February 2016 Organic Farming Conference held in La Crosse, WI. Students from the UMN HORT 3131 (Student Organic Farm) course have been exposed to the concept of strip tillage, in conjunction with lectures and laboratory sessions on soil carbon, cover crops, crop rotations, and soil management practices. In addition, students have been able to utilize some of the equipment necessitated by the use of strip tillage at the end of the 2015 spring semester. Students in the 2016 course will be exposed to the same material.