Robert G. Anderson, Extension Floriculture Specialist
Kentucky has over 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.
Materials and Methods
Plants were grown in two ways in this study. In the first system, plants floated in holes (35) cut in six 90 cm x 55 cm x 2.5 cm polystyrene sheets. The holes were 4 cm in diameter and spaced 8 x 9 cm. In the second system, plants were placed in contact with capillary mat draped over small rectangles of 2.5 cm polystyrene (5 cm x 5 cm). The capillary mat absorbed water from the hydroponic solution to satisfy the plants' needs. The capillary mat system was used to determine if sufficient oxygen would be available to the plants' root system in non-aerated hydroponic ponds.
Six 1.08 m2 wooden hydroponic ponds or tanks were built in two rows of three on one side of a 9 x 18 m naturally ventilated sidewall plastic greenhouse. Tanks were lined with black polyethylene and filled with water to a depth of 15 cm to make a tank volume of approximately 164 L. Electric water pumps were placed in three tanks to oxygenate the water; previous work demonstrated that oxygen levels would be maintained at 4-6 ppm with this procedure. Brassica plants were grown in 29 ml plastic soufflé cups (Solo Cup Company, Urbana, IL) that had holes drilled in the bottom. A commercial inorganic fertilizer (Peter's 20N-4P-16.6K, Scotts, Maryville, OH) 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 'Mei Qing Choi' Pac Choi (Brassica rapa Chinensis group) and 'Tatsoi' (Brassica rapa Narinosa group) were purchased from Johnny's Selected Seeds, Albion ME. A single crop was grown in February 2001. Seeds were sown (Jan. 15) in the cups and germinated at an average daily temperature of 25 C. Seedlings were fertigated twice per week with 150 ppm 20-4-16.6 inorganic fertilizer before placement in the hydroponic tanks. The plants were placed in the hydroponic ponds on Feb. 5 and grew under natural light conditions. The greenhouse had a heat set-point of 16C and a ventilation set-point of 24C.
Plants were harvested from the tanks on Mar. 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.
Results and Discussion
Thirty days was sufficient to grow high quality heads of 'Tatsoi' and 'Mei Qing Choi' pac choi. 'Mei Qing' has a relatively typical pac choi head with large, nearly-white, thick petioles. On the other hand, 'Tatsoi' forms a lose 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 similar to 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 a non-aerated treatments, typical of a tobacco "float" bed (Table 1 ). Aeration is just as important for lettuce in tank or "float" bed production (Anderson and Schmidt, 2001; Goto, et al, 1996). In planning this experiment, it was thought that plants grown on the capillary mat would be similar in aerated and non-aerated treatments and that these treatments would be similar to the floating plants in the aerated solution. This did not occur (Table 1 ). The plants were too top heavy to maintain good contact with the capillary mat to receive sufficient water for normal growth. This system needs to be redesigned to re-evaluate the use of capillary mats with non-aerated hydroponic solutions. 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.
Table 1. Mean shoot dry weight (g) and standard error of 'Mei Qing' and 'Tatsoi' brassica grown with inorganic fertilizer.
|Brassica cultivar||Floating||Capillary Mat||Floating||Capillary Mat|
|'Mei Qing'||3.97 ± .22||2.54 ± .16||1.98 ± .10||1.79 ± .11|
|'Tatsoi'||3.55 ± .20||2.19 ± .17||1.84 ± .10||
1.34 ± .08