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Abstract
Annual hill plasticulture systems are the primary production method for strawberry (Fragaria x ananassa) in North Carolina (NC). Approximately 1,200 acres of strawberries are grown in NC in plasticulture systems, mostly for direct-to-consumer markets. Young strawberry plants are transplanted every year in September/October in order to produce fruit the next Spring. Fertilizers for this system are usually applied in two steps: as fall applied pre plant fertilizer and in the following spring as liquid fertilizer. It is common practice for smaller growers to apply full-spectrum fertilizer as pre-plant, often at rates that match currently recommended 67 kg/ha (60 lbs/ac) Nitrogen (N). However, questions remain whether or not such practice leads to optimal plant nutrition during the first months of establishment. Polysulphate is a certified organic fertilizer which consists of 48% sulphate, 14% potassium, 6% magnesium, and 17% calcium. We hypothesized that Polysulphate co-applied with a standard full spectrum fertilizer will lead to higher yield and fruit quality, and when applied with lower full spectrum fertilizer will reduce N concentrations in strawberry beds. To investigate this, two field trials were conducted using F. x ananassa cv. Camarosa. Studies were conducted between September 2019 - May 2020 and September 2020 – June 2021. Six pre-plant fertilizer treatments were established at two research stations in North Carolina: Piedmont Research Station (clay loam soil, non-fumigated) and Central Crops Research Station (sandy loam soil, fumigated). Each trial was set up as demonstration plot in a split plot design, with one plot per treatment, and four replicates per plot. Following treatments were established: High N (1334.93 kg/ha 6-6-18); Grower Standard (1120.85 kg/ha of 6-6-18); Low Polysulphate (902.29 kg/ha 6-6-18 + 280.21 Polysulphate); Medium Polysulphate (684.84 kg/ha 6-6-18 + 560.43 Polysulphate); Half-Grower Standard (560.43 kg/ha 6-6-18); High Polysulphate (446.27 kg/ha 6-6-18 + 840.64 Polysulphate); and a non-treated control (NTC). To assess N concentration in soil, samples were taken at 0-12.7 cm and 12.7-25.4 cm depth for the first 5 months after transplanting. To assess nutrient content in plants, tissue samples were collected during the first 5 month after transplanting. All field trials were subject to spring fertigation at 17 kg N /ha/week. Marketable and total strawberry yields were assessed over 6 weeks in the spring of 2020 and 2021. The co-application of Polysuphate did not lead to a significant increase in yield or fruit quality. However, especially in sandy soil, N rapidly declined under plastic within the first 8 weeks after pre-plant fertilizer application. Treatments that contained Polysulphate and lower amounts of full spectrum fertilizer showed significantly lower N concentrations in the soil, while maintaining similar yields compared to grower standard per-plant fertilizer treatments. To evaluate the impact of different N dosages on plant growth, a greenhouse study was conducted at the Horticulture Field Lab. In the greenhouse a complete randomized plot design was set up in a soilless container system (n=12 per treatment). Four N rates: 1.19 mmol/L of N; 2.36 mmol/L of N; 4.76 mmol/L of N; or 9.50 mmol/L of N were applied before planting and mixed into Coconut Coir: Perlite (50:50) substrate. Treatments were assessed in a soilless potted system. F. x ananassa cv. Chandler were planted as plug plants and grown over 14 weeks under greenhouse conditions. The following traits were assessed over the growing period: the number of flowers, crowns, and leaves, crown diameter, and chlorophyll. At the end of experiment, nutrient content of plant tissue, substrate, and dry weights of above and below ground plant organs were assessed. In this study we observed that N is left over in the substrate of the highest N treatment, while nitrogen content was generally similar in petioles and crowns across all treatments. Our greenhouse study shows that the highest N rate did not result in the complete use of N by the plant in the first 14 weeks after planting. Based on our studies, it is likely that the current recommendations of pre-plant N rates for NC plasticulture strawberry are too high, especially for highly aerated soils. We successfully reduced the amount of full-spectrum fertilizer applied as pre-plant and used Polysulphate to serve as source for sulfur, calcium, magnesium and phosphorous for plant establishment for the strawberry cultivar Camarosa. However, while our study was focused on the investigation of Nitrogen distribution in soil, future research on the impact of fertilizer on the overall nutrition requirements of strawberry plants is necessary for comprehensive fertilizer recommendations.