Oxalis regnellii, the shamrock plant, is a niche crop produced primarily for the St. Patrick's Day holiday (De Hertogh and Le Nard, 1993; Dole and Wilkins, 2005). Fertilization recommendations for oxalis are limited. De Hertogh and Le Nard (1993) recommend using 14N–4.2P–11.6K Osmocote® (no rate specified) after visible growth or a weekly liquid application of 200 mg·L−1 N of 20N–8.8P–16.6K in the irrigation water. Leaf chlorosis has been reported during greenhouse production and has been hypothesized to be Fe deficiency (De Hertogh, 1996; De Hertogh and Le Nard, 1993; Dole and Wilkins, 2005; Hammer, 2006). Iron deficiency is a common disorder that affects many plant species and is often the first micronutrient that becomes limiting in greenhouse media as pH rises (Nelson, 1994). In addition to high substrate pH, another potential cause of Fe deficiency is poor root system growth attributable to disease, poor substrate aeration, and excessive watering (Nelson, 1994).
Many oxalis species are found in nature in the understory (Brickell and Zuk, 1996). Under high light conditions, leaves fold downward, a response similar to water stress. In O. montana, Comerro and Briggs (2000) suggest this leaf folding is a hydropassive response as the plant reorients its leaves as a result of lower water potential from increased transpiration. If greenhouse growers do not assess the cause of wilted leaf appearance, they may unwittingly overwater oxalis, causing root damage and subsequent induced nutrient deficiencies, including Fe. Therefore, a better understanding of a more efficient watering practice would be beneficial.
There are a vast number of species and cultivars produced in the floriculture sector, and often times, cultural guidelines are lacking for specialty niche crops. A major drawback to developing cultural guidelines is that species and even cultivars do not all respond similarly to nutrition and irrigation rates and practices. Overhead irrigation (e.g., hand, boom, drip) and subirrigation (e.g., ebb and flow) are two irrigation methods commonly used in the greenhouse industry and each system has advantages and limitations. A major advantage to overhead irrigation compared with subirrigation is the ability to control soluble salt levels through leaching. Overhead drip irrigation and boom watering systems, as compared with hand watering, can also reduce labor costs and can improve efficiency of water delivery (Hall, 1980; Harbaugh et al., 1986). Major drawbacks to overhead irrigation, particularly hand irrigation, include labor costs and the potential for poor water use efficiency. Another drawback to overhead irrigation is the potential for unsightly mineral residue on foliage from fertilizer applications, compromising crops marketed primarily for their foliage characteristics such as oxalis. Other drawbacks to overhead irrigation include potential for clogged emitters, increased susceptibility for foliar diseases, and foliage deflecting water, reducing penetration to the media.
Like with overhead irrigation, there are positive and negative implications associated with subirrigation (e.g., ebb and flow, tray, trough, floor flooding). There is the potential to reduce labor costs because many plants can be irrigated at the same time and often with the push of a button. Subirrigation also has the potential to improve water and fertilizer use efficiency (Dole et al., 1994; Holcomb et al., 1992; Uva et al., 1998) because the irrigation water is often collected and recirculated, reducing input costs (i.e., fertilizer) and reducing runoff and nutrient leaching. Currently, with increasing social concerns regarding water use and nutrient runoff, it is imperative to conduct, at minimum, small-scale trials to determine efficient but effective irrigation practices for individual species. Another benefit to subirrigation is reduced susceptibility for foliar diseases with little to no water contacting the foliage. However, there is an increased risk for spread of disease between plants, particularly soilborne pathogens, when using an ebb and flood system with recirculated water. A major production challenge with subirrigation is that soluble salts can accumulate, because leaching does not occur (Kang et al., 2004), which can reduce plant growth and development (Todd and Reed, 1998). The costs associated with installing or retrofitting a greenhouse can also be a limiting factor for implementing subirrigation.
Greenhouse fertilization of floriculture crops typically uses fertilizers dissolved in and delivered through the irrigation water. Selecting the appropriate fertilizer composition is important to ensure adequate and not deficient or toxic amounts of nutrients. An important consideration in selecting a fertilizer is the ratio of ammonium to nitrate-N, because the N source of the fertilizer affects its potential for increasing or decreasing substrate pH (Marschner, 1995; Reed, 1996). Moreover, the N source and growing temperature interaction is important to consider (Barker and Mills, 1980). For example, under cool greenhouse conditions, the propensity for toxic ammonium-N to accumulate in plants is greater. This would be important with O. regnellii production in northern greenhouses, because the crop is primarily forced during the time of year in which growers often try to save energy by using as little heat as possible, which could potentially result in ammonium toxicity. Other important considerations for fertilizer selection include the fertilizer trace element charge, the content of Ca and Mg, and potential acidity or basicity (Biernbaum, 1997; Reed, 1996). Careful fertilizer selection not only aids in optimal plant growth, but can also reduce production costs and nutrient runoff (Elliot, 1990; Uva et al., 1998).
With little information regarding fertilizer recommendations and irrigation practices of greenhouse production of oxalis, two experiments were designed with the following objectives: 1) to determine the effects of fertilizer concentration and irrigation method on growth of O. regnellii; and 2) to investigate the effect of fertilizer formulation on growth and development of O. regnellii and O. triangularis.
Cabrera, R.I. 1997 Comparative evaluation of nitrogen release patters from controlled-release fertilizers by nitrogen leaching analysis HortScience 4 669 673
Comerro, H.K. & Briggs, G. 2000 Effects of leaflet orientation on transpiration rates and water potentials of Oxalis montana SUNY Geneseo Jour. Sci. Math. 1 7 10
De Hertogh, A.A. & Le Nard, M. 1993 Oxalis 764 767 De Hertogh A.A. & Le Nard M. The physiology of flower bulbs Elsevier, Amsterdam The Netherlands
Dole, J.M., Cole, J.C. & von Broembsen, S.L. 1994 Growth of poinsettias, nutrient leaching and water-use efficiency respond to irrigation methods HortScience 29 858 864
Haley, T.B. & Reed, D.W. 2004 Optimum potassium concentration in recirculating subirrigation for selected greenhouse crops HortScience 9 1441 1444
Hansen, C.W. & Nielsen, K.L. 2001 Reduced phosphorus availability as a method to reduce chemical growth regulation and to improve plant quality Plant Nutr. 92 314 315
Harbaugh, B.K., Stanley, C.D. & Price, J.F. 1986 Interactive effects of trickle irrigation rates, cultivars, and culture on cut chrysanthemum HortScience 21 94 95
Haver, D.L. & Schuch, U.K. 1996 Production and postproduction performance of two New Guinea Impatiens cultivars grown with controlled-release fertilizer and no leaching J. Amer. Soc. Hort. Sci. 5 820 825
Holcomb, E.J., Gamez, S., Beattie, D. & Elliott, G.C. 1992 Efficiency of fertigation programs for Baltic Ivy and Asiatic Lily HortTechnology 1 43 46
James, E.C. & van Iersel, M.W. 2001 Fertilizer concentration affects growth and flowering of subirrigated petunias and begonias HortScience 36 40 44
Kang, J.G. & van Iersel, M.W. 2001 Interaction between temperature and fertilizer concentration affects growth and flowering of sub-irrigated petunias and begonias J. Plant Nutr. 24 753 765
Kang, J.G. & van Iersel, M.W. 2002 Nutrient solution concentration affects growth of subirrigated bedding plants J. Plant Nutr. 2 387 403
Kang, J.G., van Iersel, M.W. & Nemali, K.S. 2004 Fertilizer concentration and irrigation method affect growth and fruiting of ornamental pepper J. Plant Nutr. 27 867 884
Kent, M.W. & Reed, D.W. 1996 Nitrogen nutrition of New Guinea Impatiens ‘Barbados’ and Spathiphyllum ‘Petite’ in a subirrigation system J. Amer. Soc. Hort. Sci. 121 816 819
Klock-Moore, K.A. & Broschat, T.K. 1999 Differences in bedding plant growth and nitrate loss with a controlled-release fertilizer and two irrigation systems HortTechnology 2 206 209
Lea-Cox, J.D., Ross, D.S. & Teffeau, K.M. 2001 A water and nutrient management planning process for container nursery and greenhouse production systems in Maryland J. Environ. Hort. 4 230 236
Masson, J., Tremblay, N. & Gosselin, A. 1991 Nitrogen fertilization and HPS supplementary lighting influence vegetable transplant production. I. Transplant growth J. Amer. Soc. Hort. Sci. 116 594 598
Medina, L.C., Obreza, T.A., Sartain, J.B. & Rouse, R.E. 2008 Nitrogen release patterns of mixed controlled-release fertilizer and its components HortTechnology 18 475 480
Poole, R.T. & Conover, C.A. 1992 Fertilizers and medium affect foliage plant growth in an ebb and flow irrigation system J. Environ. Hort. 10 81 86
Rideout, J.W. & Overstreet, L.F. 2003 Phosphorus rate in combination with cultural practices reduces excessive growth of tomato seedlings in the float system HortScience 38 524 528
Todd, N.M. & Reed, D.W. 1998 Characterizing salinity limits of New Guinea Impatiens in recirculating subirrigation J. Amer. Soc. Hort. Sci. 123 156 160
Uva, W.L., Weiler, T.C. & Milligan, R.A. 1998 A survey on the planning and adoption of zero runoff subirrigation systems in greenhouse operations HortScience 33 193 196
Yelanich, M.V. & Biernbaum, J.A. 1993 Root medium nutrient concentration and growth of poinsettia at three fertilizer concentrations and four leaching fractions J. Amer. Soc. Hort. Sci. 118 771 776