high tunnels, crops are often grown Organic Vegetable Crop Production in directly in the soil, and fertility can Controlled Environments Using Soilless Media be maintained through inputs such as composts, cover crops, and com- mercially available organic fertilizers Mary A. Rogers1 (Montri and Biernbaum, 2009). In aquaponics, wastewater from fish pro- duction provides nutrients to crops in ADDITIONAL INDEX WORDS. hydroponics, substrate, controlled-environment agriculture, compost a media-filled raised bed or floating system (Tyson et al., 2011). Other SUMMARY. Organic vegetables produced in greenhouses and other controlled CEA systems such as vertical farms environments may fill a unique market niche as consumers demand local, high and passive solar heated greenhouses vegetables year round. However, limited technical information supports these (Fig. 1) use a hydroponic or soilless- production systems and more research is needed to provide recommendations for media-containerized system, where appropriate substrate mixes and nutrient management. Compost can be used as a substitute for peat-based media, and research results vary widely based on regular applications of nutrients from feedstock, compost method, and proportion used in mixes. Most studies consider external sources are required to main- compost in terms of peat-substitute or replacement and not as a source of fertility in tain plant growth and quality. Both soilless systems. Common challenges in using compost in soilless media are due to water culture and soilless substrate immaturity of the compost, poor water holding capacity, and unbalanced salinity culture are considered ‘‘hydroponic’’ and pH. It is possible to certify organic soilless production systems; however, the systems; however, this review will fo- National Organic Program (NOP) of the U.S. Department of Agriculture has not cus on the latter, as these are typically yet provided clear rules and requirements supporting these systems. The objective the systems of interest to orga- of this article is to review the literature on soilless organic vegetable production, nic producers. Although hydroponic summarize results from the more widely studied topic of vegetable transplant production is not a new concept production, and point to future research for organic agriculture. (Jensen and Collins, 1985), fitting this system into an organic framework rganic vegetable production grown under protected culture, so the has been somewhat controversial. under glass or in other pro- actual increases in vegetables grown The European Commission prohibits Otected environments, hereto under protection are likely larger than hydroponic production in organic referred as controlled-environment ag- 28%. In 2014, the highest value of agriculture (International Federation riculture (CEA) is growing, according salesinmillionsofdollarswasfresh of Organic Agriculture Movements, to the 2014 census of organic agricul- market tomatoes [Solanum lycopersi- 2014). In the United States, the ture reported by the U.S. Department cum ($18.1)], followed by fresh cut National Organic Standards Board of Agriculture (USDA, 2015). Accord- herbs ($5.9), lettuce [Lactuca sativa (NOSB) recommended in 2010 that ing to this recent census, there were ($5.4)], bell peppers [Capsicum ann- the NOP prohibit hydroponic pro- about 1500 certified or exempt orga- uum ($1)], and spinach [Spinacia duction in organic agriculture, stating nic farms producing vegetables, pota- oleracea ($0.5)]. In millions of dollars that ‘‘.based on its foundation of toes (Solanum tuberosum), and melons in sales, the top three leading states in sound management of soil biology (Cucumis sp.) under glass or plastic, protected culture of organic vegeta- and ecology, it becomes clear that representing $76 million in sales. Al- bles are California ($31.3), Pennsyl- systems of crop production that elim- though still a niche compared with the vania ($12.2), Massachusetts ($6.2), inate soil from the system, such as entire specialty crop industry in the New York ($2.7), Oregon ($2.6), and hydroponics or aeroponics, cannot be United States, this represents a 28% Florida ($2.6) (USDA, 2015). The considered as examples of acceptable increase in the number of farms using increase in farms, acreage, and sales of organic farming practices’’ (USDA, protected culture since 2008 (USDA, CEA reflects increasing consumer de- 2012, 2015). The previous 2008 cen- mand for local and organic fresh pro- sus pooled organic floriculture, bed- duce. Simultaneously, there is increasing ding, and nursery crops, as well as interest and development of CEA in mushrooms and all other food crops urban settings, including high tunnels, passive-solar-heated greenhouses, verti- cal farms, and aquaponics (Despommier, Department of Horticultural Science, University of Minnesota, 1970 Folwell Avenue, St. Paul, MN 2011; Ford and Waibel, 2009; Specht 55108 et al., 2014; Tyson et al., 2011; This paper was part of the workshop ‘‘Soil Health and Wortman and Lovell, 2013). Faced Implication in Organic Nutrient Management on with a variable and changing climate Vegetable Production’’ held 5 Aug. 2015 at the ASHS Annual Conference, New Orleans, LA, and sponsored that contributes to instability in horti- by the Organic Horticulture Working Group. cultural crop production, increasing We thank Sue Wika and Tom Prieve of Paradox Farms the capacity for localized food produc- Fig. 1. Lettuce and mustard greens and Elizabeth Perkus for the photo used in Figure 1. tion will increase food system resiliency We also thank Carl Rosen, Heidi Anderson, Andrew growing in a vertical gutter system in Petran, Jared Rubinstein, and Aimee Talbot for (Hodbod and Eakin, 2015) and CEA soilless, organic media in a passive reviewing the manuscript. can help meet this need. solar heated greenhouse at Paradox 1Corresponding author. E-mail: [email protected] Many different production sys- Farm, Ashby, MN (photo by E. doi: 10.21273/HORTTECH03352-16 tems can be categorized as CEA. In Perkus).
166 • April 2017 27(2) 2010). The implication is that soilless soilless culture; however, peat substi- better under conventional practices; systems are exclusively reliant on syn- tutes have also been evaluated due to however, using the highest rate of thetic fertility sources and do not concerns with the availability and long- Sea Tea with organic growing medium benefit from ecosystem functions me- term sustainability of mined peat (Sunshine; Sun Gro Horticulture, diated by microorganisms associated (Raviv, 2005). A wide variety of sub- Bellevue, WA) produced equivalent with soil. However, emerging re- strates can be used for vegetable trans- quality transplants. Quality of onion search shows that soilless hydroponic plant production, including coir (Cocos transplants was equivalent regard- systems using white peat and coconut nucifera), rice (Oryza sp.) hulls, pine less of fertilizer used (Russo, 2005). fiber substrate may support a diverse (Pinus sp.) bark, switchgrass (Panicum Transplant quality will vary by crop, community of microorganisms that virgatum), posidonia (Posidonia sp.) substrate, and fertility source. With mediate important functions in these residues, and composts (Kuepper and pure plant-based fertilizers, an incuba- systems, including disease suppression, Everett, 2004). The physical and chem- tion period of 2 weeks is recommen- and contribute to productivity and ical properties of the growing media are ded to avoid phytotoxicity associated plant quality similar to soil-based sys- important for germination, growth, with mineralization (Koller et al., tems (Grunert et al., 2016). Hydro- and quality of vegetable transplants. A 2004). ponic systems relying on carbon-based high-quality substrate has a carbon: Compost can be used successfully substrates may fit within the organic nitrogen ratio between 15:1 and 20:1, as a peat substitute or as a substrate definition. The NOP has not yet estab- pH range of 6.5 to 7, bulk density component in vegetable transplant lished rules and regulations based on within 40 to 60 lb/ft3 (0.64to0.96 production (D ıaz-P erez et al., 2008; the NOSB advisory board recommen- gÁcm–3), and adequate moisture reten- Raviv, 2005; Reis et al., 1998; Ster- dation, but certifying agencies may tionandaerationtosupportplantde- rett, 2001) and high-quality organic allow hydroponic organic operations velopment (Nelson, 2012; Sterrett, commercial mixes often contain basedoninterpretationofthecurrent 2001). Although it holds nutrients compost, but these can be expensive regulations (Dixon, 2015). Vegetable well, peat is inert and does not contrib- and difficult to access (C. Ford, per- seedlings grown for field transplant ute to crop fertility needs. Organic sonal communication). Growers may using soilless media can be marketed fertility sources for transplant produc- create their own compost from lo- as organic without these reservations, tion include fish and seaweed based cally available materials, but if done provided they are grown in accordance fertilizers, guano, poultry litter, cotton- improperly can result in high salinity, to NOP regulations. Therefore, many seed (Gossypium sp.), soybean (Glycine storage problems, and odors and vegetable growers in CEA wishing max), and alfalfa (Medicago sativa) flies (Matkin and Chandler, 1957; to adhere to organic standards are meal, bone, feather and blood meal, Sterrett, 2001). Using locally avail- estimating fertility rates based on con- and rock phosphate (Greer, 2005; able materials as feedstock in com- ventional fertility recommendations Kuepper and Everett, 2004). Tomato post often results in inconsistent (Zheng et al., 2011). Currently, there transplants grown in a peat-compost plant quality compared with com- are few practical, research-based rec- growing medium performed equally mercially available mixes, due to the ommendations supporting organic well (57% to 83% increased shoot dry wide range raw materials, the com- vegetable production in greenhouses weight compared with unfertilized posting method, and how compost using soilless media. The objective of control plants) when fertilized with is stored. The majority of problems this article is to review the literature on blood, feather, meat, crab shell, fish, with incorporating compost into soilless organic vegetable production cottonseed meal, and dried whey media for vegetable transplants are systems, summarize the results from sludge, but poorly when fertilized with associated with immature compost, themorewidelystudiedtopicofvege- wheat bran (Triticum aestivum), alfalfa high electrical conductivity (EC) and table transplant production, and point meal, and canola (Brassica napus)meal poor water-holding capacity (Sterrett, to future research needs for organic (Gagnon and Berrouard, 1994). In 2001). To be acceptable for contain- agriculture. another study bell pepper and onion erized production, composts must (Allium cepa) transplants were grown be stable and mature so that active Organic transplant production organically using three different sub- microbial decomposition is com- Inputs and media mixes vary strates approved for organic produc- plete and the compost is free of as organic growers may use a combi- tion and containing various materials phytotoxic compounds. If compost nation of plant- and animal-based including peatmoss, compost, and pas- is immature, it can immobilize N, amendments depending on what is teurized topsoil and fertilized with inhibit germination, or be phyto- available locally (Clark and Cavigelli, organic 2.1N–3.3P–2.2K (Sea Tea; toxic. Nitrogen mineralization rates 2005; Treadwell et al., 2007). Peat- Garden-Ville, San Antonio, TX) con- of the feedstock and environmental based media became widely adopted to taining compost tea (Russo, 2005). factors such as moisture, tempera- avoid some of the historic problems The organic transplants were compared ture, salinity, and pH determine the using soil, such as limited availability of with a conventional control, grown amount of nitrogen available to quality topsoil, poor drainage, presence using commercial medium (Reddi- plants (Sikora and Szmidt, 2001). of weed seed and disease and herbicide Earth; Scotts-Sierra, Marysville, OH), High salinity is indicated by EC and residues. These concerns prompted the fertilized with half-strength 20N– organic fertilizers contribute to sa- development of ‘‘peat-lite’’ mixes con- 8.7P–16.6K water-soluble fertilizer linity as they mineralize. In sensitive taining peatmoss, vermiculite and/or (Peters 20–20–20; Spectrum Group, crops, high EC can inhibit germina- perlite (Boodley and Sheldrake, 1977). St.Louis,MO).Theresultsshowed tion and cause stunting (Peet et al., Peat-based substrates are common in that bell pepper typically performed 2008). High EC levels have been
• April 2017 27(2) 167 WORKSHOP reported from studies incorporating Organic vegetable production In addition to yield, fruit quality composts as organic amendments in CEA with soilless media in organic vegetable production is of (Clark and Cavigelli, 2005; Russo, interest. There is potential to market 2005; Rippy et al., 2004; Sikora and Compared with transplant pro- these products as functional foods Szmidt, 2001; Zhang et al., 2013). duction, few studies focus on organic with higher nutritional value to offset This may or may not result in a neg- vegetable production in greenhouse the costs of CEA (Falguera et al., ative plant response based on the environments, where crops are 2012; Kubota et al., 2006). Organic particular crop, environment, and grown for a longer time period and amendments may result in increa- duration of production, as leaching require greater amounts of nutrients sed quality. For example, in a green- will mitigate negative effects. Addi- to achieve satisfactory yield. Re- house study with tomato, bone and tionally, high pH has been reported search on peat alternatives in con- blood meal amendments improved with organic fertility sources (Rippy ventional systems may provide useful fruit color, and compost as a fertility et al., 2004; Zhang et al., 2013), and information for organic vegetable amendment resulted in higher vita- can limit nutrient uptake. production in soilless systems (Arancon min C content but poorer yields Transplant performance in media et al., 2004; Atiyeh et al., 2000; due to lower nitrogen availability containing compost is variable and may Hardgrave and Harriman, 1995; (Montagu and Goh, 1990). Further- be based on species of crop, the feed- Shaw et al., 2004). Although the more, tomatoes grown with vermi- stocks used in the compost, and the same organic compliant substrates compost as the nutrient source had relative proportion of compost in the and fertility sources can be used as increased levels of calcium and vita- media. Most studies involve tomato in transplant production, the chal- min C compared with conventional and report better results when green lenge is adjusting the fertility rate controls (Premuzic et al., 1998). For waste composts are used for transplant and timing to maximize production. leafy greens such as lettuce and bras- production rather than manure-based Media containing compost as sub- sica crops (Brassica sp.), nitrate accu- composts. Ceglie et al. (2015) showed strate may continue to mineralize mulation is a concern and is related to that the best media for overall trans- and release nutrients over time, source of fertilizer, timing of nitrogen plant quality across three crops tomato, thereforeappropriateratesandtim- release, light intensity and duration, melon (Cucumis melo), and lettuce was ing of higher solubility organic fer- temperature and type of crop, and a mixture of 20% green compost {cre- tilizers may be difficult to discern. part harvested (Anjana and Iqbal, ated from waste from olive (Olea euro- One study investigated performance 2007). Excess nitrate consumption paea), conifer [pine and spruce (Picea of greenhouse-grown tomatoes pro- can have toxic effects on humans sp.)] tree prunings, and perennial rye- duced with different substrates and and high nitrate accumulating veg- grass (Lolium perenne) clippings}, 39% organic vs. synthetic fertilizer sour- etables include leafy greens, celery, palm (Phoenix sp. and Washingtonia ces. The organic treatments had high and radish [Raphanus sativus sp.) fiber, and 31% peat (v/v). An- EC levels, attributed to the amend- (Santamaria, 2006)]. Studies have other study found that a mixture of ments of dolomitic limestone, blood been done measuring nitrate accumu- compost [20% posidonia waste, 40% and bonemeal, and potassium sulfate lation in organic vs. conventional pro- grapevine (Vitis sp.) prunings, and added to the media (Rippy et al., duction systems and generally find 40% vegetable residues from broccoli 2004). However, after 4 weeks the lower nitrate content in vegetables (Brassica oleracea var. italica), fennel EC levels were within acceptable grown under organic practices (Woese (Foeniculum vulgare), and celery levels and the authors suggested an et al., 1997). However, this might not (Apium graveolens) scraps], contain- incubation period would alleviate the be the case in CEA as the environ- ing either 25%, 50%, or 75% (by problem. A 4-week incubation pe- ments are different in these systems. volume) peat performed equally well riod for a compost-peat based mix Greenhouse and CEA is valued in the for both melon and tomato trans- resulted in improved quality of or- winter months in cool climates when plants (Mininni et al., 2013). To- ganic tomato transplants (Nair et al., field production cannot take place. mato transplants grown in 50% v/v 2011), but in this case, EC gradually Nitrate accumulation has been found compost made from food residue increased over the study period, pre- to increase under conditions of low plus yard waste performed as well as sumably due to potassium sulfate light, shorter photoperiod, and peat-based media with synthetic fer- and as a result of mineralization-N warmer temperatures (Santamaria tilizer, but transplants grown in 50% in the compost. This illustrates the et al., 2001), which are realistic (by volume) compost made from need for more data on mineralization environmental conditions for green- horse bedding did not perform well, rates of various organic fertility sour- houses in northern climates. More re- duetohighsalinity(ClarkandCav- ces in a range of crops at different search is needed to determine nitrate igelli, 2005). In tomato transplant pro- temperatures in controlled environ- accumulation on leafy greens pro- duction, compost containing yard ments. Mineralization rates for vari- duced organically in a variety of CEA trimmings and biosolids as a peat sub- ous organic fertilizers were assessed and soilless systems to ensure levels stituteandfertilizedwithastandardcom- under different temperature regimes are within safe limits (Santamaria, mercial soluble fertilizer (200 mgÁL–1 and incubation periods (Hartz and 2006). of nitrogen per week) resulted in high Johnstone, 2006) and serve as a help- quality transplants, but compost qual- ful guideline; however, the fertilizers Conclusions ity varied and salinity was problematic were incubated with field soil and Although market opportunities in one replicate (Ozores-Hampton results may be different with soilless exist, there are few practical guide- et al., 1999). substrate. linesandrecommendationssupporting
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• April 2017 27(2) 169 WORKSHOP organic mixes: Role of EC and mix com- Sikora, L.J. and R.A.K. Szmidt. 2001. media/NOP%20Final%20Rec% ponents. Acta Hort. 797:393–398. Nitrogen sources, mineralization rates, 20Production%20Standards%20for% and nitrogen nutrition benefits to plants 20Terrestrial%20Plants.pdf>. Premuzic, Z., M. Bargiela, A. Garcia, A. from composts, p. 287–305. In: P. Stofella Rendina, and A. Iorio. 1998. Calcium, U.S. Department of Agriculture (USDA). and B. Kahn (eds.). Compost utilization in iron, potassium, phosphorus, and vitamin 2012. Organic production survey, 2008. horticultural cropping systems. CRC C content of organic and hydroponic to- 28 Jan. 2016. 170 • April 2017 27(2)