2.1. Experiment 1—Potting Media Comparison
The first experiment was carried out in a heated greenhouse and a field at Berea College Farm between February 2021 and June 2021. The potting-media substrates evaluated in this study are listed in
Table 1, which included: Food-residuals compost processed at Berea College Farm (C); a commercial peat-based potting medium (Pro-Mix MP Mycorrhizae Organik, Premier Tech, QC, Canada); and a commercial peat-based seed starting mix (Baccto Organic Optimum Seed Starting Mix, Michigan Peat Company, Houston, TX, USA). The commercial peat-based potting medium (P) contained 60–70%
Sphagnum peat moss, horticultural coir, perlite, mycorrhizae, and limestone. The commercial peat-based seed starting mix (Baccto) contained
Sphagnum peat moss, perlite, organic slow release fertilizer (derived from aerobically composted turkey litter, hydrolyzed FM, and sulfate of potash), and limestone. Additional fertilizers used in the treatments included FM (NPK, 13-0-0), a commercial slow-release fertilizer for micronutrients (Azomite, Azomite Mineral Products, Inc., Nephi, UT, USA), a commercial pelleted fertilizer derived from chicken manure (Italpollina Organic Fertilizer Pellets 4-4-4, Hello Nature USA, Inc., Anderson, IN, USA). The eight treatments were prepared as shown in
Table 1. For T3 (25% C), T4 (50% C), and T5 (75% C), potting-media substrates were mixed by volume based on the respective ratio. For treatments that included Azomite, granulated Azomite was applied at the recommended rate of 4.2 kg/m
3 (7 lbs/yard
3). For T2 (P + FM), which was included to replicate the procedure currently used at Berea College Farm to grow transplants, 60 mL of FM were added to each flat 21 days after seeding. Macronutrient contents of each potting medium were calculated based on the soil testing results of the food-residuals compost (A & L Western Agricultural Laboratories, Modesto, CA, USA) and on the product information of the commercial materials.
The experimental unit was a plastic 50-square-plug horticultural flat (50 cm by 26 cm by 6 cm). The volume of each plug was 85.5 mL. On 21 February, 2021, 3–6 ‘Boro F1’ beet seeds (Johnny’s Selected Seeds, Winslow, ME, USA) were sown in each plug to a 13 mm (0.5 inch) depth and covered with horticultural vermiculite. For each of the eight treatments, five replications were made. All flats were placed on a table in the greenhouse in a completely randomized design.
Two plugs randomly selected from each flat were used as samples 36 days after seeding. Soil around the roots was gently removed with water. The following morphological attributes were recorded for each sample: The number of plants per plug; fresh and dry weight of shoots (g); fresh and dry weight of roots (g); the number of true leaves per plug, leaf width (mm), and shoot length (mm). For leaf width, a medium-sized leaf was selected from each plug, and the widest part of the blade was measured. For shoot length, a well-developed true leaf was selected from each sample plug, and the length from the bottom of the stem to the tip of the blade was measured. The mean values derived from two sample plugs from the same flat were used to represent the single replication. The number of weed seedlings in each flat was recorded 45 days after seeding. Qualitative observations of growth and leaf colors were made weekly.
Beginning 36 days after seeding, all treatments received liquid fertilizer (Allganic Nitrogen Plus 15-0-2, SQM S.A., Santiago, Chile). Stock solution was prepared at the rate of 363 g/3.8 L (0.8 lbs/gal), and applied through the irrigation water once a week. Fifty-seven days after seeding, one plug was randomly selected from each flat and transplanted into the field plot in a completely randomized design with a 30 cm by 30 cm (1 ft by 1 ft) spacing per plant. The size of the experimental plot was 6.3 m by 1.6 m (20.7 ft by 5.2 ft). The plot was irrigated with a single drip-irrigation tape. The plants were harvested 55 days after transplanting (114 days after seeding). After removing soil from the roots, the number of plants per sample, total weight of plants per sample (g), weight of roots (g), and weight of leaves (g) were recorded. The harvest index was calculated based on the total weight per sample and the weight of roots. For ease of comparison with other studies, mean root weights of each treatment are presented as yield per hectare.