Replacing animal manure with plant-based fertiliser in organic blackcurrant (Ribes nigrum L.) production

Growing nitrogen (N)-fixing green manure, such as red clover (Trifolium pratense L.), in blackcurrant (Ribes nigrum L.) plantation alleys for use as a plant-based fertiliser could potentially reduce the need for animal manure fertilisation in organic farming. Knowledge about nutrient release from plant-based fertilisers is limited. Therefore, a blackcurrant field experiment was conducted in Denmark (2016–2019) to investigate the effects of using red clover (Clover) as a plant-based fertiliser, compared to poultry manure (Poultry), on soil nutrient availability, plant nutrient status and blackcurrant yield. Four weeding/fertiliser placement strategies were included to assess the combined effects of weed suppression and fertiliser placement, forming the following four field experiment conditions: no weeding, surface-applied fertiliser (NO); no weeding, fertiliser incorporation (NOᵢₙc); weeding with a brush cutter, fertiliser incorporation (Brushᵢₙc); and weeding with a rotary harrow, fertiliser incorporation (Rotaryᵢₙc). Nutrient release of surface-applied and incorporated fresh clover (Clover), ensiled clover (Silage), and poultry manure (Poultry) was studied from litterbags subjected to controlled conditions in a 70-day plant-free incubation experiment based on changes in nutrient concentration and dry matter contents in remaining litter; data were collected at 0, 14, 35, 49, and 70 days. Levels of released N and potassium (K) from litterbags were comparable between Poultry and Silage treatments during the 70-day incubation period. Meanwhile, soil mineral N and K levels were similar (or higher) with Clover in the field than with Poultry, indicating that Clover can be substituted for Poultry. However, phosphorus (P) release from litterbags was faster from Poultry than from Clover and Silage in the 70-day incubation experiment, while soil P levels were higher in the field with Poultry, suggesting that Clover should be a preferred fertiliser option for fields with high P levels. Generally, compared to surface-applied fertilisation, incorporation of fertilisers increased N, P, and K release from litterbags in the 70-day incubation experiment, and increased blackcurrant leaf N and K concentrations in the field. Fertiliser types did not affect blackcurrant yield, supporting the replacement of imported poultry manure with a plant-based fertiliser from alley-grown clover.