Use of unheated plastic film-covered high tunnels to extend the growing season of high-value fruits and vegetables is increasing rapidly in the United States (Lamont, 2009). Driven by strong consumer demand for local and organic produce, there is growing interest in high tunnels for both organic and conventional production (Conner et al., 2009). Expanding the window in which fruits and vegetables can be produced locally allows growers to better meet this emerging market. High tunnels can be constructed cheaply and easily from readily available materials, and cool-season vegetables can even be produced year round in some regions without the need for supplemental heating. High tunnels are routinely used to enhance early-season growth and productivity (Lamont, 2005). They are used to increase the average daily temperature and protect plants from wind, rain, snow, hail, insects, and diseases. In the high mountain valleys of the Intermountain West, warm-season vegetables like tomatoes are generally planted outdoors in mid- to late May with harvest beginning in late July or early August. High tunnels are used to advance the planting dates by 4–6 weeks and the harvest dates by a month or more.
Maintaining soil quality and fertility in a sustainable manner is challenging in intensively cropped systems such as high tunnels (Montri and Biernbaum, 2009). Intensively tilled soils can develop hard pans, cloddy structure, surface crusting, and loss of organic matter. These problems negatively affect plant growth and are indicators of reduced soil quality (Celik et al., 2004; Shepherd et al., 2002). Conventional farming is often criticized for low nutrient use efficiency of highly soluble fertilizers, which are susceptible to loss to the environment (Byrnes, 1990). Fertility management is also a challenge in organic production as a result of the high variability in nutrient content and availability of organic fertilizers (Gaskell and Smith, 2007). Organic tunnel house producers often rely on plant- and/or manure-based compost to provide their nutrient needs because it is relatively cheap and often readily available locally. In a survey of high tunnel growers in the central Great Plains, 35% of respondents used organic amendments as their sole nutrient source (Knewtson et al., 2010a). Organic additions were made on an annual basis by 62% of the growers and 13% said they applied them more than once per year. In addition to providing nutrients, plant- and manure-based composts also build soil organic matter and soil quality, which is particularly important in highly disturbed, intensively managed systems (Rosen and Allan, 2007). However, if compost is relied on to supply all crop N needs, or applied in large amounts as a soil conditioner, yields can suffer, at least in the short term, as a result of variability in N mineralization rates (Hartz et al., 2000). In addition, when composts are applied continuously, growers risk excessive P, K, and trace element buildup, posing risks for nutrient uptake imbalances and loss to the environment (Gaskell and Smith, 2007; Montri and Biernbaum, 2009). Manure-based composts can be particularly high in salts and trace elements; however, heavy use of plant-based composts can also present these risks because N is reactive and prone to loss during the composting process while salts accumulate. Opportunities for growing N-fixing cover crops or living mulches can be limited in intensively managed systems such as high tunnels when continued production is often needed for adequate return on investment and to supply year-round markets (Montri and Biernbaum, 2009). This presents a significant conundrum, particularly for nutrient management in organic production systems.
Matching peak crop demands with available nutrients is particularly critical in vegetable crops as a result of their rapid growth and relatively short growing cycles (Gaskell and Smith, 2007). The uncertainty over the availability of nutrients from composts and potential for yield reductions produces doubts in the minds of many growers on how best to transition to organic high tunnel production. At issue is how to manage the tradeoffs between supplying adequate plant-available nutrients and building soil organic matter and soil quality without causing nutrient imbalances or excessive P buildup. Although growers are concerned about soil quality and want to carefully manage the soil environment in high tunnels, 45% of growers surveyed responded that they did not conduct soil tests (Knewtson et al., 2010a). Interestingly, soil quality (measured as clods, surface crusting, mineral deposition, hardpans, and particulate organic matter) in high tunnels does not appear to be adversely impacted by most growers practices (Knewtson et al., 2010b).
Compost has a strong carryover effect and yields could be expected to improve once a steady state is achieved between inputs of marginally available organic N and carryover from applications made in previous years (DeLuca and DeLuca, 1997; Endelman et al., 2010). Whether this actually occurs is debatable as a result of a potential mismatch between peak crop requirements and compost mineralization rates (Gaskell and Smith, 2007). Various organically allowable soluble fertilizers produced from plant, fish, and slaughterhouse byproducts are available, but these are very expensive and are likely as susceptible to leaching as conventional fertilizers. Many of these products provide a good source of readily available N but have the same narrowly defined N:P ratio that many composts do. Overfertilization in high tunnels can reduce plant health, increase disease or insect problems, and lead to the buildup of excessive soluble salts and nutrient levels (Magdoff and van Es, 2000; Montri and Biernbaum, 2009).
Tomatoes are one of the more commonly grown fresh vegetables and widely grown in high tunnels (Carey et al., 2009; USDA-ERS, 2010). In studies comparing organic with inorganic fertilizers for the production of highly managed greenhouse tomatoes, most showed that productivity levels were similar (Kumar et al., 2007; Rippy et al., 2004) or lower than the yields from plants receiving inorganic fertilizer sources (Heeb et al., 2005, 2006). Heeb et al. (2005) noted that even with a 20% increase in total N inputs, lower yields with organic fertilizer were probably the result of insufficient nutrient supply because the availability is low and difficult to predict.
This study was initiated to compare the yields of transition organic and conventional tomatoes grown at varying N rates in unheated high tunnels and to compare crop productivity, soil quality, and soil fertility. In particular, we were interested in the economic/soil quality tradeoffs of using compost vs. commercial fertilizers in the transition to organic high tunnel production.
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