Transactions of the ASABE Vol. 56(1): 155-166 © 2013 American Society of Agricultural and Biological Engineers ISSN 2151-0032 155 THRESHOLDS FOR IRRIGATION MANAGEMENT OF PROCESSING TOMATOES USING SOIL MOISTURE SENSORS IN SOUTHWESTERN ONTARIO F. Jaria, C. A. Madramootoo ABSTRACT. Processing tomato (Lycopersicon esculentum Mill.) is an economically important vegetable crop in southwestern Ontario. Processing tomato (cultivar H9553) fruit yield and quality were evaluated in field experiments in southwestern Ontario over a three-year period (2008-2010). A split-plot randomized complete block design with four blocks was used in 2008 and 2010. Irrigation types (buried and surface drip) served as the main plots, while four moisture depletion levels constituted the split plots. In 2009, a 2×4 factorial complete randomized block design with four blocks was used, with the same two factors. The moisture treatments represented the lower soil moisture triggers, which initiated irrigation scheduling. Irrigation was terminated for each treatment when field capacity was reached. Continuous soil moisture status over the growing season was monitored with a combination of volumetric and tensiometric sensors. Seven fruit quality parameters were monitored: fruit weight, color, pH, size, firmness, Brix yield, and soluble solids. In each year, the most stressed treatment produced the highest soluble solids (6.0, 4.8, and 5.2 °Brix for 2008, 2009, and 2010, respectively). Total and marketable fruit yields ranged from 91.9 to 121.1 Mg ha -1 and from 91.4 and 119.7 kg ha -1 , respectively. Statistical significance was obtained among treatments and irrigation types in 2008 only. Irrigation water use efficiency was also not statistically significant over the three years. Seasonal irrigation depth ranged from 58 to 196 mm, and statistical significance among the moisture treatments was obtained in 2008 and 2010. Keywords. FDR, Irrigation scheduling, Irrigation thresholds, TDR. griculture is a key driver for the Canadian economy, providing one in seven jobs within the country. The agri-food sector accounts for 8.3% of Canada’s gross domestic product, USD $26.5 billion of which comes from exports and employing nearly 2.1 million persons (AAFC, 2006). Canadian vegetable growers reported sales of USD $659 million in 2010, with two provinces (Ontario and Quebec) accounting for more than 80% of the vegetable sales (Statistics Canada, 2011). Vegetable and dry bean production is a critical part of the food and agriculture industry in Ontario. Virtually all of the tomatoes grown in Canada for processing are produced in Ontario, with the counties of Essex and Kent being the main producing areas. In 2008, 0.62 million tons of processing tomatoes were produced by 150 growers, generating over USD $60.5 million (OHCRSC, 2006). In most years, rainfall during the growing season is insufficient to attain optimum production (Warner et al., 2007). Tan et al. (2003) noted that, through the 1990s, rainfall during the growing season decreased by about 25 mm year -1 . The 30-year (1971 to 2000) climate normal rainfall for Windsor and London averaged 254.3 and 251.3 mm, respectively. Over the growing season, an average cultivar requires 400 mm of water (LeBoeuf et al., 2007). Thus, intensive tomato production in these two counties necessitates the use of supplemental irrigation to offset the deficiencies in rainfall to maintain high levels of production (Warner et al., 2007). There is increasing pressure for more judicious utilization of limited water resources to reduce negative environmental impacts. Shock et al. (2001) identified economic competition in marketing produce, competition for water, and political pressure as the three forces to minimize off-site impacts of irrigation-induced runoff and leaching. It is desirable to optimize crop yield and quality under the constraints of reducing water use and increasing the efficiency of the use of agricultural chemicals. Irrigation scheduling is a technique for timely and accurately application of water to a crop and is key to conserving water, improving irrigation performance, and ensuring the sustainability of irrigated agriculture (Thompson et al., 2007b). Several irrigation scheduling methods based on water budget, soil, and plant indicators have been used for different crops, with the water budget method probably the most widely used technique (Fareres et al., 2003). However, over the past decade, a new generation of soil moisture sensors based on electrical properties, such as resistance, capacitance, and time Submitted for review in June 2012 as manuscript number SW 9792; approved for publication by the Soil & Water Division of ASABE in January 2013. The authors are Felix Jaria, ASABE Member, Graduate Student, and Chandra A. Madramootoo, Professor and Dean of Agricultural and Environmental Sciences, Department of Bioresource Engineering, McDonald Campus of McGill University, Ste. Anne de Bellevue, Quebec, Canada. Corresponding author: Felix Jaria, Department of Bioresource Engineering, McDonald Campus of McGill University, 21111 Lakeshore Rd, Ste. Anne de Bellevue, Quebec, Canada H9X 3V9; e-mail: felix.jaria@mail.mcgill.ca; phone: 514-398-8785. A