The disease citrus greening (HLB, Huanglongbing) continues to decimate Florida’s citrus industry, resulting in the lowest yields since the 1940s. With no cure or tolerant rootstock/scion combinations available, growers must manage the disease. Although increased fertilizer applications have been shown to improve tree health, the additional inputs are expensive and decrease operational profitability. As a result, interest has grown in improving soil fertility parameters such as organic matter, with the goal of reducing inputs and increasing horticultural sustainability. Despite the recognized potential of organic matter, little research has been undertaken to establish the optimal contents required to improve citrus root characteristics. The objective of this study was to answer two specific questions: what are the amounts of organic matter needed to significantly affect citrus root growth and physiology? And how does the incorporation of organic matter influence nutrient availability? To answer these questions, a six-month completely randomized greenhouse experiment was conducted to measure the impact of different soil organic matter contents on potted citrus trees. Six different treatments consisting of different organic matter contents (0% - control, 1%, 2%, 3%, 5%, and >10%) were created by mixing different amounts of locally sourced compost with sand. One-year-old ‘US-942’ (Citrus reticulata x Poncirus trifoliata) rootstock plants (n = 6 per treatment) were grown in the different mixtures for six months under controlled greenhouse conditions. During the study, soil moisture, plant height, and stem diameters were assessed every other month. At the conclusion of the study, total plant and root biomass, root nutrients, average root length, and average root diameter were measured. Results from the study indicated that higher organic matter contents (>2%) led to significantly increased plant biomass and stem diameter, and increased root growth. Significant results were also found at the soil level, where increased organic matter contents led to increased nutrient retention and increased root uptake of nutrients. Additional investigation is needed to better understand the advantages of increasing organic matter content, even by marginal percentages, utilizing grafted citrus trees of various cultivars in field trials. Such trials would provide insights into the practical implications of these findings within the industry.
