Researchers have increasingly been investigating cover crop–based reduced tillage systems, such as NT and ST, as methods for mitigating some of the adverse effects of CT. In these systems, cover crops are grown before cash crop establishment and are ended without incorporating residue into the soil, thus leaving a surface mulch into which the subsequent cash crop can be planted. Although much of this research has been conducted on agronomic crops, such as corn and soybeans, more research is needed to investigate such systems in vegetable production. Application of cover crop–based reduced tillage in vegetable production systems face challenges due to 1) necessity of producing large cover crop biomass, 2) managing cover crop residue to provide maximum weed suppression and reduced N immobilization, and 3) the ability of cover crop–based cropping systems to align with conservation tillage approaches.
Challenges also exist in effective cover crop termination strategies in organic vegetable production. Many cover crop–based conservation tillage studies relied on herbicide for cover crop termination and supplemental weed control (Abdul-Baki et al., 1996; Brainard and Noyes, 2012; Haramoto and Brainard, 2012; Peachey et al., 2004). Organic producers, however, typically end cover crops mechanically—using a tool such as a roller-crimper or flail mower—and have few options for chemical control of weeds that come up through the cover crop mulch. Weed management through rolled cover crops has long been an attractive proposition for organic producers. The thick mulch between rows can drastically reduce weed pressure, especially pressure from annual weeds, compared with bare ground, saving time on cultivating and weeding, and reducing the need for herbicides (Nair et al., 2015). Cover crop residues on the soil surface can affect light availability, temperature, and moisture levels, reducing the number of germinating weed species (Creamer et al., 1996b). This is true for the BR region of the crop, but weeds that grow in the IR region could be a challenge. Studies in cover crop–based ST systems suggest a need to develop weed management strategies that target distinct zones while balancing crop and soil management tradeoffs (Brainard et al., 2013).
To make NT and ST systems viable options for growers, it is crucial to identify yield-limiting factors and develop strategies to mitigate them. There has been considerable variability in vegetable crop yield under cover crop–based reduced tillage in prior organic research. Some studies found that NT treatments yielded as much or more than CT in production of tomatoes (Creamer et al., 1996a; Delate et al., 2012), bell pepper (Delate et al., 2008), spinach (Lounsbury and Weil, 2015), and onion (Vollmer et al., 2010), while others found that NT treatments yielded poorly (Díaz-Pérez et al., 2008; Leavitt et al., 2011). Schellenberg et al. (2009) found that an NT system using flail mowed warm-season legume cover crops [lablab (Dolichos lablab L.), soybean (Glycine max L.), sunn hemp (Crotalaria juncea L.), and a mixture of sunn hemp and cowpea (Vigna sinensis Endl.)] had similar yields for spring broccoli, but reduced yields for fall broccoli, compared with CT.
The ST production system with rolled and crimped cereal rye has been shown to promote soil health, resulting in significant increases in soil aggregate stability, potentially mineralizable N, active soil carbon (C), and microbial activity when compared with a conventionally tilled production system (Pieper et al., 2015). However, it was not effective for production of vegetables, with the ST plots having the lowest yields potentially due to failure of the cereal rye mulch to suppress weeds throughout the growing season and reduced decomposition and N mineralization due to cooler soils. Similar challenges exist in cover crop–based NT production systems. Nitrogen is often a limiting factor in rolled cover crop systems (Wells et al., 2013) and in organic systems (Berry et al., 2002), where plant-available N is typically derived from mineralization of particulate organic matter, legume-fixed N, and supplemental purchased fertilizers (Gaskell and Smith, 2007). In a study using agronomic crop residue, Aulakh et al. (1991) showed that nonincorporated residue is mineralized more slowly than incorporated residue, and residue with a high C:N ratio causes a greater rate of N immobilization. Cereal rye (S. cereale), a cover crop commonly used in conjunction with reduced tillage, has a high C:N ratio at the time of termination when mowed or rolled, increasing the likelihood of reduced N availability in these systems. An additional challenge in organic NT systems is the difficulty of sidedressing organic granular fertilizers with typical equipment, because fertilizer cannot be effectively incorporated into the soil without disturbing the cover crop mulch. Use of fertigation may be one strategy to provide inorganic N to the crop in otherwise N-limited conditions.
Lower soil temperature, caused by the insulative effects and high albedo of surface residue (Baker et al., 2007), may also contribute to reduced yields in cover crop–based NT systems. Soil temperature has been shown to be lower in NT than in CT (Johnson and Lowery, 1985; Nyborg and Malhi, 1989), and in mulched as compared with bare ground systems (Creamer et al., 1996a; Mochizuki et al., 2008). The heavy residue left on the soil surface in cover crop–based NT systems is thus likely to reduce soil temperature.
In ST, untilled cover crop residue protects soil between rows, whereas tillage, which is confined to the planting row, minimizes the negative effects of tillage while promoting N mineralization and soil warming. As compared with NT, ST may produce higher IR soil temperature (Licht and Al-Kaisi, 2005) and may increase IR N availability because the cover crop in that region is incorporated rather than left on the soil surface. As compared with CT, ST conserves soil moisture (Licht and Al-Kaisi, 2005) and retains many of the soil health benefits of NT in the BR region. To our knowledge, no studies have compared NT, ST, and CT side by side in an organic vegetable production system. Additionally, there could be differences in economic returns between CT, ST, and NT systems due to differences in tillage costs, machinery and fuel costs, yield, and labor involved. Strip tillage system has been shown to reduce tillage costs and machinery operating time by an average of $36.50 and 0.47 h·ha−1, respectively, as compared with CT (Luna and Staben, 2002).
Our objectives were to determine the effects of tillage system and split fertilizer application on crop yield, plant health, weed biomass, and economic returns, and in particular, to evaluate ST as a tool for increasing soil temperature and improving N availability to the crop compared with NT. Our hypothesis is that 1) the split application of fertilizer (preplant and successive applications of fish emulsion) would be most effective in supplying plant-available N than a one-time preplant application and 2) weed suppression property of rolled cover crop mulch will depend on the region (IR or BR), with higher degree of weed suppression in the BR than IR region.
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