The challenge of imbalanced nutrient flows in organic farming systems: A study of organic greenhouses in Southern Germany

Organic greenhouse vegetable production is characterized by very high nutrient demands within short growing periods and high nutrient exports via products sold. Therefore, meeting crop nutrient demands and maintaining long-term sustainability in these systems is highly challenging. To gain insight in current practices this study assessed fertilization strategies and nutrient flows of ten organic horticultural farms (22 greenhouses and polytunnels) in Southwest Germany belonging to the two organic farming associations Bioland and Demeter (biodynamic) for a cropping period of three years. Soil samples were taken to analyze plant available phosphorus, potassium, soil organic carbon, pH and salinity. Crop rotations in the greenhouses were very diverse, though focused on tomatoes. Depending on association membership, fertilization was based either on the use of solid animal manures (26.1 Mg ha−1 a−1) and composts (7.9 Mg ha−1 a−1) for the biodynamic Demeter farms or on commercial complementary fertilizers on the Bioland farms (e.g. keratin products, food industry waste products). All farms showed strong imbalances in their nutrient flows with high average surpluses for all nutrients (197 kg ha−1 a−1 for nitrogen, 47.9 kg ha−1 a−1 for phosphorus, 119 kg ha−1 a−1 for sulfur) except potassium with an average deficit of 143 kg ha−1 a−1 and low nitrogen use efficiencies. In addition, a risk for increased soil alkalinity and salinity existed and concentrations of plant available phosphorus in the soil were very high (average 332 mg phosphorus kg−1). The results show that today's fertilization strategies for organic greenhouses are not sustainable, which calls for a thorough revision of the core ideas on soil fertility in the organic horticultural sector.