Glasshouse hydroponic horticulture is an environmentally friendly method for producing high-quality crops, but its commercial success depends on effective control of the plant pathogens that thrive in warm, moist conditions. Hydroponic growing media lack the vast diversity of microbes in soil, from which plants normally recruit beneficial organisms. To fill this gap, Costa growers apply probiotic supplements to promote growth and to protect plants against pathogens and abiotic stress. However the efficacy of such supplements varies, and the activity of biotic agents is difficult to monitor with conventional tools. This project seeks to characterise the behaviour of probiotic organisms in the root zone of hydroponic tomato plants, and to develop a practical method for routine monitoring of the root microbiome.