Management practices that build soil organic matter (SOM) are valuable for improving soil water-holding capacity, nutrient retention, aeration, and infiltration rates (Magdoff and van Es, 2000). For carrots (Daucus carota subsp. sativa), these functions are particularly important, because fluctuations in soil moisture and nutrient availability can have negative impacts on carrot quality and yield (Batra and Kalloo, 1990; Bienz, 1965). Increased biological activity associated with SOM may also reduce the prevalence of plant parasitic nematodes and fungal pathogens, both important factors reducing quality and yield in carrot production (Abawi and Widmer, 2000; Widmer et al., 2002).
Vegetable growers can increase SOM by adding organic amendments including compost and cover crops and by minimizing SOM losses through reduced tillage practices (Magdoff and van Es, 2000). Although organic matter inputs are common in vegetable production systems, adoption of reduced tillage practices has been limited. Indeed, most vegetable production systems till extensively to suppress weeds, to incorporate residues, and to promote mineralization and soil-warming. For small seeded crops like carrots, tillage is critical for creating the fine seedbed necessary for good stand establishment. Although elimination of tillage may not be practical or desirable in many vegetable crops, the extent of tillage can be reduced using hybrid tillage systems including ST (Luna et al., 2012). Under ST, a narrow band is tilled where the crop is to be planted, leaving an undisturbed strip between crop rows to protect and improve soils. Strip tillage can also reduce energy costs and equipment wear by reducing the number of tractor passes and horsepower required to prepare fields (Luna and Staben, 2002).
Strip tillage in combination with cover crops provides a valuable windbreak for carrot producers. Currently, several growers in western Michigan use pre-established cover crops in combination with ST primarily to reduce the risk of stand loss of vulnerable carrot seedlings resulting from wind and rain early in the growing season (personal observation). In this system, pre-established wheat or barley is allowed to grow in the non-tilled between-row strips until carrot seedlings reach the two- to three-leaf stage and then is killed with standard post-emergence grass herbicides.
Strip tillage has been used successfully in several vegetable crops including sweet corn, snap beans, and cabbage (Luna et al., 2012; Mochizuki et al., 2007). However, to our knowledge, the impact of ST on carrots has not been previously evaluated. Holmstrom et al. (2008) evaluated the potential for soil conservation practices including full-width reduced tillage and mulches for Canadian carrot production but found that these practices resulted in poor seedling establishment or increases in parasitic nematodes.
Despite the potential value of ST and compost for vegetables, adoption has been slowed in part by uncertainties about the effects of these practices on crop establishment and yield and concerns about the potential difficulty in managing weeds when tillage is reduced (Luna et al., 2012). In some cases, the prevalence of perennial and winter annual weeds increases when tillage is no longer used to physically disturb soil (Brainard et al., 2012; Luna and Staben, 2002). On the other hand, reduced tillage systems may reduce the prevalence of certain weed species through a variety of mechanisms, including reduced stimulation of seed germination, increased rates of seed predation and decay, and through the suppressive effects of cover crop residues, which remain on the soil surface under reduced tillage systems (Liebman and Mohler, 2001; Luna et al., 2012; Teasdale, 1998). Evaluation of the impact of management practices on weeds is particularly important in carrot production as a result of the poor competitiveness of carrots with weeds; the limited number of registered broadleaf herbicides available for carrot growers; and the development of herbicide-resistant weeds in carrot production systems including linuron-resistant common purslane (Portulaca oleracea) (Masabni and Zandstra, 1999) and Powell amaranth (Amaranthus powellii) (McNaughton et al., 2005).
The primary objective of this research was to assess the effects of ST and compost on establishment, quality, yield, and profitability of three processing carrot varieties. Secondary objectives included evaluating the impacts of tillage and compost on weed populations. We hypothesized that ST and compost would maintain or improve carrot yield and profitability but that these effects would differ by cultivar. We further hypothesized that ST and compost would influence the density of important weeds in carrot production systems.
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