CHANGES IN CHEMICAL AND PHYSICAL PROPERTIES OF COMPOST-AMENDED GROWING MEDIA DURING STRAWBERRY CROPPING: MONITORING DEPLETION OF NUTRIENTS WITH PETIOLE SAP MEASUREMENTS

Compost addition to growing media might be a valuable tool for increasing plant fitness in respect to disease defense. Compost is a source of nutrients and affects the structure of the growing medium, and both physical properties and nutrient levels in the growing media are known to affect susceptibility for plant diseases. Therefore we monitored the evolution of nutrient release and physical properties in compost-amended growing media for strawberry (‘Elsanta’) in greenhouse culture with drip irrigated “Meerlebakken” system, for autumn cultures with continued culture in the next spring. Several composts were tested.
In the first year of the experiments, we compared three N fertigation levels in the nutrient solution (33, 66 and 100% of the concentration of current practice) and 2 substrates (peat substrate versus substrate with 50% amendment of compost 1). Substrates were sampled monthly for chemical characterisation. Samples of petioles were analyzed with plant sap meters for K and NO₃. Several alternatives for white peat (e.g., 100% compost 2 versus mixtures with coco fibre, bark, ...) were tested in the second year. In the third year, mixtures of white peat with 20% compost for 3 compost types (3A, 3B and 3C) with variable properties were compared with white peat and coco fibre. For these trials, chemical and physical substrate properties were monitored at 3 times during the culture, and petioles were sampled for plant sap measurements.
In general, the compost-amended substrates with lower N fertigation performed well as growing medium during the autumn culture, but in the spring culture, these substrates needed an adapted fertigation regime for N, P and K. Physical properties changed during the culture, but easily available water and water volume at pF 1 remained in the optimal range for growing strawberries. It is concluded that plant sap measurements allow for fast and relatively reliable determination of K and NO3 in strawberry plants, and thus for correction of fertigation regimes during the culture in case of applying compost in growing media.