No-till and Strip-till Systems for Enhanced Soil Health and Profitability in Organic Broccoli and Pepper Production

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Commodities

• Vegetables: broccoli, peppers

Practices

• Crop Production: conservation tillage
• Farm Business Management: agricultural finance, budgets/cost and returns
• Production Systems: organic agriculture, transitioning to organic
• Soil Management: green manures, nutrient mineralization, organic matter, soil analysis, soil quality/health

Organic farmers rely extensively on tillage to incorporate plant residues, prepare seedbeds, and control weeds. However, tillage may have adverse effects on soil health, and conventional no-till production methods, which rely on herbicide for weed control, are not compatible with organic farming, so research was conducted on organic no tillage (NT) and strip tillage (ST), which rely on terminating a cover crop with a roller-crimper. Field research was carried out over two years (2013–14 and 2014–15) to compare two organic, cover crop-based reduced tillage systems (NT and ST) with conventional tillage (CT) in the production of bell pepper and broccoli. As nitrogen has been previously suggested as a limiting factor in organic NT systems, split fertilizer application was also included as a treatment to evaluate the impact of timing of nutrient addition on plant N status and yield. A cover crop mixture of cereal rye (Secale cereale L.) and hairy vetch (Vicia villosa Roth) was seeded in all plots in the fall and either tilled in (CT) or terminated with a roller-crimper (NT and ST) in the spring before planting. Data were collected on vegetable crop growth, yield, crop quality, cover crop biomass, weed suppression, soil temperature and moisture, leachate nitrate concentration, and soil health as indicated by soil microbial biomass and microbial diversity.

            For both crops, the effect of NT and ST on yield varied from year to year. Broccoli yield was reduced under NT and ST in 2014, but was not different from CT in 2015. Pepper yield, on the other hand, was similar among treatments in 2014, but reduced by NT and ST in 2015. While soils under ST had higher soil temperatures compared to NT, there were no differences between ST and NT in yield or crop N status for either crop in either year. Preplant and split fertility treatments produced similar marketable yields of broccoli in both years and for pepper in 2015, but preplant fertility increased marketable pepper yield in 2014. Costs of production varied minimally across treatments, so the highest yielding treatments had best economic performance. Nitrate concentration in leachate was lower under NT and ST compared to CT at three sampling dates in July 2014, but few differences were observed in subsequent samples. While there was a trend toward greater soil microbial biomass and diversity in NT and ST compared to CT plots in 2015, few significant soil health benefits were observed for NT and ST. Soil microbial biomass and diversity were both consistently higher in surface soil (0–7.5 cm) than the deeper soil (7.5–15 cm), but this occurred independently of treatments. While NT and ST did not consistently perform as well as CT, we found sufficient evidence of the potential for high yield and ecological benefits to warrant further study and fine-tuning of reduced tillage organic systems.

Introduction:

Tillage is an important tool used by vegetable growers for soil preparation and weed control. Primary tillage loosens the soil, incorporates crop residues, and enhances soil warming in the spring (Johnson and Lowery, 1985), while secondary tillage is used to form a seedbed to provide good contact between the soil and the seed or transplant roots.  After crop establishment, many organic farmers rely heavily on mechanical cultivation—or shallow tillage of the soil after the crop has been planted—to control weeds since synthetic herbicides are not available for use under organic standards. While tillage clearly has many functions in vegetable production, it has also been shown to have adverse effects on soil and environmental health. It can break down soil aggregates, decrease soil organic matter in surface soil, facilitate soil erosion (Magdoff and van Es, 2009; Moebius-Clune et al., 2008); and reduce water-holding capacity (Zibilske and Bradford, 2007). Thus, systems that reduce the intensity and frequency of tillage use in organic systems would be desirable.

Reduced tillage systems, such as strip tillage (ST) and no tillage (NT), have only recently been tried in organic production. However, an “organic no-till” system was developed in the 1990s by researchers at the Rodale Institute and other institutions. They found that cereal rye (Secale cereal L.) at the anthesis growth stage could be killed without tillage or herbicide by crushing the rye using a heavy roller. This allowed the possibility of growing a weed-suppressive mulch in situ using a high-biomass cover crop such as cereal rye, which spurred the development of a specialized implement called a roller-crimper (Fig. 1.1). While other nonchemical methods of terminating cover crops without tillage exist (mowing, roll-chopping, and undercutting [for raised beds]), there are benefits of using a roller-crimper instead of a flail mower: faster operation, reduced energy usage, improved summer weed suppression, and uni-directional stem orientation, which facilitates unobstructed operation of no-till planting equipment running parallel to the stems (Creamer and Dabney, 2002; Smith et al., 2011; Wayman et al., 2014).

A rolled cereal rye or cereal rye/hairy vetch (Vicia villosa Roth) cover crop mixture of sufficient biomass can effectively suppress emergence of most annual grass and broadleaf weeds (Silva, 2014; Smith et al., 2011). However, surface mulches generally fail to control perennial weeds (Mirsky et al., 2011), so tillage is typically used before seeding the cover crop to control perennial weeds. Because this system has distinct tillage management periods—conventional tillage (CT) before cover crop establishment, but NT from planting through harvest—it has been termed “organic rotational no-till” (Mirsky et al., 2012; Mirsky et al., 2013). This term can also be used to describe several years of organic NT followed by a year of CT production. While periodic use of tillage is assumed for long-term implementation of organic reduced tillage systems, the terms “no tillage” and “strip tillage” will be used to describe the system employed during the growing season.

Prior studies have found variability in the effects of organic NT systems on vegetable crop yields. Some researchers reported yields similar to or exceeding those of CT (Creamer et al., 1996; Delate et al., 2008; Delate et al., 2012; Lounsbury and Weil, 2014; Vollmer et al., 2010), but others observed yield reduction when using NT (Delate et al., 2003; Díaz-Pérez et al., 2008; Leavitt et al., 2011). Risk due to this inconsistency—combined with the need for specialized equipment and different management strategies—has slowed grower adoption of these systems. Reduced nitrogen (N) availability and soil temperature under NT are two factors thought to adversely affect crop productivity compared with CT (Delate et al., 2003; Griffith et al., 1988; Leavitt et al., 2011). However, in ameliorating these constraints, there is an unavoidable tradeoff with soil health: increasing soil temperature and aeration using tillage hastens mineralization of N from soil organic matter (MacDonald et al., 1995), thus supplying plants with available N at the potential cost of soil organic matter loss.

We hypothesized that ST, which integrates a tilled in-row (IR) region with an untilled, mulched between-row (BR) region, would be effective in improving crop growth and yield without sacrificing the soil health benefits of NT. Use of ST has been shown to create a similar degree of IR soil warming (Licht and Al-Kaisi, 2005) and comparable tomato yields (Thomas et al., 2001), compared to CT. Central questions in this research were whether ST would be effective in raising soil temperature and improving plant N status compared with NT, and whether these changes would translate to higher crop yields. To our knowledge, no previous studies have compared cover crop-based NT, ST, and CT in an organic vegetable system.

In addition to testing ST as a method of increasing N availability, split application of organic fertilizer was evaluated as a means of providing N to the crop during the period of N immobilization that can last for 6–8 weeks after rolling a cereal rye cover crop (Wells et al., 2013). Composted or dehydrated poultry manure is commonly used as a fertilizer in organic agriculture for both preplant and sidedress application. However, the rolled cover crop residue in NT and ST systems physically obstructs the incorporation of sidedress fertilizer with tillage, creating a challenge in supplementing the crop with fertilizer during the growing season. An alternative is liquid fish fertilizer, which, although more expensive per unit of N, can be applied through drip irrigation (fertigation), allowing for application of water-soluble N directly to the crop roots at times of peak N demand and low soil N availability. Fertility treatments in this study included only preplant fertilization (poultry manure) and split fertilizer application (poultry manure + liquid fish), both of which contained the same amount of total N. Data were also collected on an unfertilized subplot to measure for an overall fertilizer effect in the event that no differences were found between the two fertilized treatments.

Field studies were carried out in 2013–14 and 2014–15 to evaluate th effects of NT, ST, and CT, along with split fertilizer application, on growth and yield of bell pepper (Capsicum annuum L.) and broccoli (Brassica oleracea L. var. italica). These crops were chosen because one is a warm-season (pepper) and the other a cool-season (broccoli) vegetable; they represent important vegetable crop families (Solanaceae and Brassicaceae); and they are among the top 12 most consumed vegetables in the U.S. (PBH Foundation, 2015). Soil and environmental health data (microbial biomass and diversity, leachate nitrate-N concentration) were collected only in the pepper study because its longer growing season allowed for a longer duration of sampling. We assumed that these environmental data would not be substantially affected by the cash crop being grown in the plot, and thus that results could be applied to the tillage system more broadly, rather than to just the specific crop.

Introduction References

The goal of this research was to develop a broad understanding of the effects of organic reduced tillage systems on yield of warm- and a cool-season vegetable crops, as well as on a range of soil and environmental variables. Because most research in this area has looked at no-till, a specific objective was to determine whether strip tillage (functionally a hybrid of no tillage and conventional tillage) would increase soil temperature and nitrogen availability and lead to higher yields, compared with no-till.