Romaine lettuce (Lactuca sativa) is one of the most important fresh market vegetable crops in the United States. The annual lettuce consumption in 2017 was 24.5 lb per person, of which 45% (11 lb per person) were romaine and leaf lettuce. Ninety-five percent of the romaine lettuce consumed in the United States is grown in the United States, primarily in California and Arizona [U.S. Department of Agriculture (USDA), 2019]. The cost of shipping by refrigerated truck for distances between 1501 to 2500 miles in the second quarter of 2018 was $2.45 per mile (USDA, 2018); transporting a load of lettuce from California to New England costs more than $6000. Due to the high cost and environmental impact of shipping, loss of freshness, and desire for fresh and healthy foods, the demand for local produce is increasing rapidly in New England.
Generally, lettuce is considered a cool-season crop with an optimum production temperature of 23 °C during the day and 7 °C at night, while 28/12 °C day/night or higher can cause bolting, bitterness, tip burn, and poor heading (Jackson et al., 1996). Lettuce is particularly sensitive to root zone temperatures and can tolerate higher than optimum air temperatures if the temperatures in the soil or rooting media are optimal (Jie and Kong, 1998). New England has cool spring and summer temperatures suitable for lettuce production. Lettuce can be harvested from outdoor plantings in Rhode Island from May through October, making it a good crop for local markets.
Farmers commonly use black plastic mulch to increase the yield and quality of vegetable crops (DoVale and Medeiros, 2012). Black polyethylene mulch is known to increase soil temperatures (Tarara, 2000) and decrease weed competition (Lamont, 1993). Nutrient availability often increases under black polyethylene mulch as it prevents leaching and accelerates mineralization of soil organic matter (Lamont, 1993; Steinmetz et al., 2016). Plastic mulch has been shown to increase soil moisture in irrigated production systems by decreasing evaporation from the soil surface (Lamont, 1993), but it can decrease soil moisture in rainfed systems by inhibiting the penetration of rainfall (Schonbeck and Evanylo, 1998a). However, the impact of plastic mulching on the environment is a critical issue. Worldwide, plastic mulch consumption on farms has increased dramatically in recent decades (Kasirajan and Ngouajio, 2012). Few recycling facilities will accept plastic mulch because it is contaminated with soil and agricultural chemicals (Steinmetz et al., 2016); therefore, most plastic mulch is disposed of in landfills. Many small-scale and organic farmers opt not to use plastic mulch because of disposal problems and concerns about soil contamination from partially degradable plastic mulches (Cowan et al., 2013; Kasirajan and Ngouajio, 2012).
Many organic materials can be used as mulching materials to control weeds. Organic mulches can improve soil quality, structure, and nutrients over time by increasing soil organic matter (Sinkevičienė et al., 2009). They increase soil moisture in both irrigated and rain-fed systems by reducing surface evaporation (Schonbeck and Evanylo, 1998a). Effects of organic mulches on soil temperatures vary depending on the material used as mulch, the soil type, and the season. A 3-inch-thick mulch of oak (Quercus sp.) and maple (Acer sp.) leaves reduced soil temperature by 10 °F relative to bare soil during the summer in Connecticut (Hill et al., 1982). Numerous other studies have reported that mulching with coarse organic materials such as straw, hay, shredded newspaper, pine (Pinus sp.) needles, and bark mulch reduces soil temperatures, particularly in the spring (Munn, 1992; Schonbeck and Evanylo, 1998a; Sinkevičienė et al., 2009; Skroch et al., 1992). Gheshm and Brown (2018) reported that a 1-inch-thick layer of multisource compost applied to soil in a high tunnel in Rhode Island increased soil temperatures relative to bare soil during fall lettuce production but had no effect on average soil temperature during the spring. Schonbeck and Evanylo (1998a) reported that soil temperatures in plots mulched with dark-colored hardwood leaf compost were an average of 1.4 °C cooler than soil temperatures in bare ground plots. Pinamonti (1998) reported that two mixed-source composts used as mulch in a vineyard in Italy increased soil temperature relative to bare ground in June and September but decreased soil temperature in July and August. In Florida, various dark colored mixed-source composts are used in increased soil temperatures relative to white-on-black plastic mulch; no bare ground treatment was used (Roe et al., 1993).
All mulch materials provide benefits and have negative effects; farmers must weigh these factors when selecting a mulch material. In southern New England, the soil-warming properties of black polyethylene mulch and ease of mechanical application are attractive, but negative environmental effects and cost of disposal of plastic mulch may outweigh the benefits for lettuce. Organic mulches can be incorporated at the end of the growing season to add organic matter to the soil. Both leaves and compost can be spread with manure spreaders, thus reducing labor costs (Roe et al., 1993). Compost has been extensively researched as a mulching material; however, in peri-urban areas, competition from homeowners and landscapers drives the cost of finished compost higher than what many farmers can afford. Hardwood leaves are a popular mulching material on peri-urban farms in New England because the leaves are available free of charge from local yard care services. However, leaves have been reported to significantly slow soil warming in the spring in New England (Hill et al., 1982). The objective of this study was to compare compost and shredded leaves as mulching alternatives for spring romaine lettuce production in southern New England. Bare ground was included as a control, and all treatments were compared with black polyethylene mulch because it is the most widely used mulching material in vegetable production.
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