Robert G. Anderson, Department of Horticulture
Kentucky has more than 30 acres of greenhouses with modified pond or tank hydroponic beds for "float" tobacco transplant production. These facilities could be used to grow other crops during the fall, winter, and spring. Previous work has demonstrated that lettuces can be easily grown in such production systems (Anderson and Schmidt, 2001; Thompson et al., 1998). This study evaluated production of two types of pac choi, `Mei Qing Choi' and `Tatsoi', that could be grown in the same system and sold in Asian vegetable markets.
In this study, six 12-square-foot wooden hydroponic ponds or tanks were built in two rows of three on one side of a 30-foot x 60-foot naturally ventilated sidewall plastic greenhouse. Tanks were lined with black polyethylene and filled with water to a depth of 6 inches to make a tank volume of approximately 38 gallons. Electric water pumps were placed in each tank to oxygenate the water in the aeration treatments; previous work demonstrated that oxygen levels would be maintained at 4 to 6 ppm with this procedure. Holes (35) were cut in six 36-inch x 22-inch x 1-inch polystyrene sheets. The holes were 1.5 inches in diameter and spaced 5 x 6 inches. Plants were grown in 1-ounce plastic soufflé cups (Solo Cup Company, Urbana, Illinois) that had holes drilled in the bottom. A commercial inorganic fertilizer (Peter's 20N-4P-16.6K, Scotts, Marysville, Ohio) was added to the water in each tank and maintained at an EC of 1.2 dSm-1 (approximately 160 ppm NO3-N).
Seed of Chinese cabbage varieties `Mei Qing Choi' pac choi (Brassica rapa Chinensis group) and `Tatsoi' (Brassica rapa Narinosa group) were purchased from Johnny's Selected Seeds, Albion, Maine. A single crop was grown in February 2001. Cups were filled with a peat-based germination medium (Scott's Redi-Earth, Marysville, Ohio) and placed in trays. Seeds were sown (January 15) in the cups and germinated at an average daily temperature of 76°F. Seedlings were fertigated twice per week with 150 ppm 20-10-20 inorganic fertilizer before placement in the hydroponic tanks. The plants were placed in the hydroponic ponds on February 5 and grew under natural light conditions. The greenhouse had a heat set-point of 60°F and a ventilation set-point of 75°F.
Plants were harvested from the tanks on March 7, and dry weights were measured for nine plants in each replicate. Plants were grown with and without aeration with three replicates in a randomized complete block.
Thirty days was sufficient to grow high quality heads of `Tatsoi' and `Mei Qing Choi' pac choi (Chinese cabbage). `Mei Qing' has a relatively typical pac choi head with large, nearly white, thick petioles. On the other hand, `Tatsoi' forms a loose head of long, thickened petioles with dark green leaf blades. It seems both would be fine for stir fry cooking and salads, but petioles of `Tatsoi' are more like celery in form rather than a pac choi.
Aeration of the hydroponic solution is clearly necessary for the production of these plants. Dry weights were nearly double for those plants in aerated treatments compared to those in non-aerated treatments typical of a tobacco "float" bed (Figure 1). Aeration is just as important for lettuce in tank or "float" bed production (Anderson and Schmidt, 2001; Thompson et al., 1998). Although aeration is somewhat difficult to arrange for "float" beds, it is critical to the success of vegetable plant production in this type of hydroponic system.
Figure 1. Mean shoot dry weight (oz.) of `Mei Qing Choi' and `Tatsoi' Chinese cabbage grown in aerated and non-aerated hydroponic ponds with inorganic fertilizer.
Robert G. Anderson and L. Stephanie Schmidt, Department of Horticulture
The market for organic produce continues to expand (Thompson, 2000). Kentucky
has more than 30 acres of greenhouses with modified pond or tank hydroponic
beds for "float" tobacco transplant production. The development of
a certified organic greenhouse production system for lettuces and greens could
allow Kentucky tobacco farmers access to a new market for their facilities as
the tobacco market changes. In this study, commercial organic fertilizers were
used to grow `Ostinata' Bibb and `Red Sails' leaf lettuce in a pond, tank, or
"float" production system. Plant growth was evaluated, and the fertilizer
solutions were analyzed for nutrient amounts and compared to recommended standards
for inorganic fertilizers (Muckle, 1993,
Thompson et al., 1998).
In this study, nine 12-square-foot wooden hydroponic ponds or tanks were built in three rows of three on one side of a 30-foot x 60-foot naturally ventilated sidewall plastic greenhouse. Tanks were lined with black polyethylene and filled with water to a depth of 6 inches to make a tank volume of approximately 38 gallons. Electric water pumps were placed in each tank to oxygenate the water; previous work demonstrated that oxygen levels would be maintained at 4 to 6 ppm with this procedure. Holes (35) were cut in eighteen 36-inch x 22-inch x 1-inch polystyrene sheets. The holes were 1.5 inches in diameter and spaced 5 inches x 6 inches. Lettuce plants were grown in 1-ounce plastic soufflé cups (Solo Cup Company, Urbana, IL) that had holes drilled in the bottom.
Inorganic fertilizer (Peter's 20-10-20, Scotts, Marysville, Ohio) or one of three commercially available, water-soluble organic fertilizers (Peaceful Valley Farm Supply, Grass Valley, California) was added to the water in each tank. Algamin, 0.2N-0P-3.3K, (18 percent cold processed kelp, Ascophyllum nodulosum from Norway) was applied at the label rate and three times the label rate, approximately 1 Tbs/gal. and 3 Tbs/gal., respectively. EcoNutrients, 0.2N-0.4P-0.8K (21 percent digested bull kelp, Nereocystis luetkeana, from Northern California) was applied at the label rate, 2 Tbs/gal. Omega 6N-2.6P-5K (microbe-digested organic fertilizer derived from blood meal, bone meal, and sulfate of potash) was applied at the label rate and one-half the label rate, approximately 2 Tbs/gal. and 1 Tbs/gal., respectively.
Three crops of lettuce were grown sequentially, one in September, another in October, and another in November of 2000. Cups were filled with a peat-based germination media (Scott's Redi-Earth, Marysville, Ohio) and placed in trays. Bibb lettuce `Ostinata' and Grand Rapids lettuce `Red Sails' seeds were sown in the cups and germinated at an average daily temperature of 76°F. Seedlings were grown for 14 days and fertigated with 150 ppm 20-10-20 inorganic fertilizer before placement in the hydroponic tanks. The plants grew under natural light conditions, and the greenhouse had a heat set-point of 60°F and a ventilation set-point of 75°F.
Plants were harvested from the tanks after 30 days, and the fresh and dry weights were measured. Five water samples were taken during each crop and analyzed as standard greenhouse water samples (available nutrients) and as organic samples (total nutrients). The September crop evaluated the label rate of Algamin, EcoNutrients, and inorganic fertilizer; the October crop evaluated the 3X label rate of Algamin, the label rate of Omega, and inorganic fertilizer; and the November crop evaluated inorganic fertilizer, the one-half label rate, and the label rate of Omega in randomized complete block experiments.
Water soluble materials derived from algae (Algamin and EcoNutrients) had little value as an organic fertilizer for lettuce. Dry weight of lettuce grown with these materials was only 10 to 18 percent of those grown in inorganic fertilizer, depending on the cultivar (Table 1). Nitrate nitrogen and phosphorus levels were less than 1 percent, and potassium was 5 to 20 percent of the recommended levels when used at the label rate or the 3X label rate for these fertilizers (Table 2). These results are comparable to our previous unpublished trials with fish waste (fish emulsion and fish powder). However, the poor plant growth with fish waste was attributed to the high biological oxygen demand from the fertilizers that prevented root penetration into the nutrient solution for three or more weeks, despite moderate nutrient levels.
Dry weight of lettuce grown with a formulated organic fertilizer (Omega) was similar or significantly lower than lettuce grown in inorganic fertilizer, depending on the cultivar (Table 1). Although dry weights were similar, head size was visually smaller with the organic fertilizer. Nitrate levels were 50 percent, P levels were 300 percent, and K levels were 100 to 120 percent of recommended levels (Table 2).
Production of lettuce in this study was simplified when compared to the sophisticated practices evaluated by Thompson et al. (1998). A commercial fertilizer was used as a control, rather than a formulated fertilizer. Plus, the inorganic fertilizer was supplemented only with additional fertilizer as the conductivity decreased and pH was not manipulated. The pH dropped dramatically throughout this study (Table 2), yet no apparent effects on lettuce growth were noted. Additionally, the commercial fertilizer did not match recommended nutrient amounts precisely. Fresh weights in this study did not reach the commercial goal of 5 ounces per head (Thompson et al., 1998). The heads of lettuce grown in the inorganic and Omega treatments averaged approximately 3.9 ounces. The dry weights, however, were generally similar to dry weights reported by Thompson et al. (1998), but difficult to compare because of different temperatures and light levels used in these studies.
In conclusion, this study indicates that it may be possible to formulate an organic fertilizer for the hydroponic production of lettuce in the greenhouse. It is unknown if state and federal agencies would certify such production practices as organic production.
UK New Crop Opportunities Center <http://www.uky.edu/Ag/NewCrops/>
A Handbook for the Production of CEA-Grown Hydroponic LettuceCornell
Hydroponic Food Production. A Definitive Guide to Soilless Culture. 1994. Howard M. Resh. 5th ed. Woodbridge Press, Santa Barbara, Calif.
|Table 1. Mean shoot dry weight (oz) of 'Ostinata' (O) and 'Red Sails' (RS) lettuce grown with inorganic and selected commercial organic fertilizers in 2000.|
|September Crop||October Crop||November Crop|
|Inorganic||0.134 a1||0.141 a||0.067 a||0.049 a||0.067 a||0.061 a|
|Omega||50||0.062 b||0.058 a|
|Omega||100||0.067 a||0.039 b||0.063 b||0.057 a|
|Algamin||100||0.027 b||0.049 b|
|Algamin||300||0.005 b||0.015 b|
|EcoNutrients||100||0.034 b||0.048 b|
|1||Means followed by the same letter in a column are not significantly different at p = 0.05 according to the Least Squares Means procedure.|
|Table 2. Recommended amounts (Muckle, 1993; Thompson, 1998) and measured amounts of macronutrients and pH in inorganic and selected commercial organic fertilizers used during September, October and/or November crops of lettuce in a pond culture system.|
|Inorganic||20-4-16.6||12||90||6||34||140||10 x||10 x||4.8||1.2|
|x - Municipal water used for the nutrient solutions added a mean of 52 ppm Ca and 27 ppm Mg and negligible amounts of N, P, and K.|
Introduction ! Tree Fruits ! Small Fruits ! Vegetables ! Greenhouse Production ! Diagnostic Laboratory ! Appendix