Organic farmers need a diverse toolbox of weed management tactics. Tillage is currently the most common method of weed management in organic systems (Baker and Mohler, 2015), but wet soil conditions, crop growth stage, and overuse can limit its effectiveness. For example, if rotary hoeing is used too frequently, it can cause crop injury and a decline in yield, although too few passes can also cause yield loss due to poor weed control (Kluchinski and Singer, 2005; Leblanc and Cloutier, 2001; Taylor et al., 2012). Because tillage disturbs the soil surface, there is a potential for soil degradation including the loss of soil aggregate structure and organic matter, and increased erosion (Munkholm et al., 2013). Tillage can also increase weed seedbank density by distributing and burying newly shed weed seeds; similarly, soil disturbance can bring buried weed seeds near the surface to facilitate germination (Bond and Grundy, 2001; Melander and Rasmussen, 2000).
Current alternatives to tillage for weed management in organic systems include mulching with plastic films, straw, or cover crops and organic herbicides, mowing, flame weeding, laser treatment (Mathiassen et al., 2006), hot oil microdosing (Zhang et al., 2012), and steam and hot water (Kristoffersen et al., 2008). Mulching can provide season-long weed control along with improved crop performance through increases in available water and enhanced efficiency of irrigation systems (Sarkar et al., 2007; Sarkar and Singh, 2007). Mulching with straw or cover crop residues increases soil moisture and reduces weed growth, but it is usually less effective than black plastic mulch film for weed management (Anzalone et al., 2010). The volume of organic mulch required for effective weed management can be cost-prohibitive and logistically challenging to transport if not produced on or near the farm. Organic mulch residues are biodegradable, but decomposition during the growing season can result in untimely microbial immobilization of essential plant nutrients (Bond and Grundy, 2001). Moreover, straw and hay mulches can carry weed seeds and increase seed bank density (Schonbeck, 1999).
Plastic and biodegradable plastic mulch films are commonly used for weed management in vegetable production, but weeds can emerge through any uncovered space, including crop holes or tears in the plastic (Schonbeck, 1999; Wortman, 2015). Plastic films increase soil temperature (Lamont, 2005) and increase water use efficiency (Moreno and Moreno, 2008), especially when integrated with drip or subsurface drip irrigation (Anzalone et al., 2010). The biggest drawback to plastic mulch film is that it must be removed from the field after harvest, and disposal has economic and environmental consequences (Kasirajan and Ngouajio, 2012; Miles et al., 2012).
Concerns about plastic mulch film disposal have led to the development of biodegradable alternatives. Biodegradable plastic mulch film can degrade into nontoxic compounds, although more stable degradable polymers may remain in the soil as microfragments for extended periods of time (Fontanelli et al., 2013; Kasirajan and Ngouajio, 2012; Miles et al., 2012). Most biodegradable plastic mulches provide similar agronomic services as polyethylene plastic, though polyethylene is more effective for increasing soil temperature (Moreno and Moreno, 2008). The main barrier to on-farm adoption of biodegradable mulch films is the higher cost, which is a result of the higher costs of raw materials (Fontanelli et al., 2013). Paper mulch (e.g., WeedGuardPlus; SunShine Paper, Aurora, CO) is the only biodegradable mulch currently allowed as a soil input on U.S. Department of Agriculture (USDA) certified organic farms, but it can be difficult to install, is susceptible to damage from high winds, and often degrades before the end of the growing season, leading to weed establishment (Anderson et al., 1996; Anzalone et al., 2010; Schonbeck, 1999).
Despite recent innovations in nonchemical weed management tactics, flame weeding remains among the most common alternatives to tillage on organic farms (Baker and Mohler, 2015). Flame weeding with propane gas does not disturb the soil, which can help to minimize weed seed germination; however, flaming has been shown to increase germination of some weed species (Ascard, 1995; Bond and Grundy, 2001; Taylor et al., 2012). Flame weeding can be done in wet soils, and application timing is generally more flexible than tillage because it does not require soil disturbance. One drawback of flame weeding is the potential for crop injury and yield loss when applied in the row (Ulloa et al., 2010). However, crops like garlic (Allium sativum) can tolerate up to three flame treatments for in-row weed management without any reduction in productivity (Chehade et al., 2018). Despite its effectiveness in field crops, flame weeding cannot be used to manage in-row weeds (e.g., those in the crop hole) in mulched or plasticulture vegetable production because the mulch or films could catch fire or melt.
Abrasive weeding is a relatively new approach to organic weed management that uses compressed air to propel agricultural grits, including fertilizers, at weed seedlings to physically destroy emerged structures (Forcella, 2009a; Wortman, 2014). A diverse range of materials have demonstrated effectiveness as grits for abrasive weeding, including granulated corn gluten meal, corn cobs, greensand fertilizer, walnut shells, and soybean meal (Wortman, 2014). Abrasive weeding applications may also be used to supplement in-season crop nutrition if organic fertilizers (e.g., soybean meal) are used as the grit source, which could lead to additional gains in crop growth, yield, and profitability of this weed management tactic. Moreover, delaying plant available soil nitrogen early in the growing season could shift the outcome of crop-weed competition in favor of crops and better synchronize nutrient mineralization with peak crop demand (Liebman and Davis, 2000; Wortman et al., 2011).
Abrasive weeding has been studied in corn and soybean (Carlson et al., 2018; Erazo-Barradas et al., 2019), but less research has been conducted in vegetable crops considering interactions with unique cropping system attributes such as mulch films (Wortman, 2014, 2015). Although abrasive weeding has been used successfully to manage weeds in plasticulture tomato and pepper (Wortman, 2015), it is not known if this technology is compatible with other types of agricultural mulch or if it is suitable for use in systems without mulch. There are also questions about the effects of abrasive weeding on crop health. Stem and leaf injury after abrasive weeding in vegetables may increase susceptibility to disease by providing an entry point for pathogens (Wortman, 2014), but that potential risk has not been quantified. Lastly, the opportunity for integrating nitrogen and weed management with abrasive grit applications has been proposed (Forcella et al., 2011), but actual soil nitrogen contributions and potential plant nitrogen uptake from abrasive weeding have not been quantified.
The overall aim of this study was to determine the effectiveness of abrasive weeding in organic sweet red pepper (Capsicum annuum L. ‘Carmen’) production. Specific objectives were to 1) quantify the effects of different grit types on weed suppression, disease severity, soil nitrogen availability and potential crop uptake, and crop yield and 2) determine the compatibility of abrasive weeding with different types of mulch for nonchemical, in-row weed management.
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