Rootzone cooling, water productivity and economics of intensified vertical grown red butterhead lettuce (Lactuca sativa Linn)

Abstract

To alleviate the effects of high temperatures and high air humidity to lowland tropical crop production, resulting to poor quality marketable produce, rootzone cooling systems were applied to vertical recirculating hydroponic–drip irrigated towers for lettuce production. Two treatments, ground heat exchanger (GHE) and water chiller cooling systems, were installed to lower the temperature of the nutrient solution supplied to the towers. Harvest periods were 20, 25 and 30 days after transplant (DAT). Data gathered were analyzed using combined analysis for the strip plot design and Tukey Honestly Significant Difference (HSD) test for the comparison among means. For 30-DAT, using the GHE and chiller rootzone cooling systems, the average daily electrical conductivity was 2.10 dS/m and 2.14 dS/m, the pH levels were 6.14 and 6.27; and dissolved oxygen were 5.28 mg/L and 6.06 mg/L, respectively. Lettuce harvested at 30DAT using the water chiller rootzone cooling method had the highest water consumption of 2.13L/plant while those lettuce harvested at 20DAT using the ground heat exchanger rootzone cooling method had the least water consumption of 1.16L/plant. The highest water productivity using the ground heat exchanger rootzone cooling method was 31.18 g/L at 25 DAT while using the water chiller rootzone cooling method the highest water productivity was 33.45 g/L at 20 DAT. The lettuce harvested at 30DAT planted at the top level of the towers had the highest yield (89.42 g), the most number of leaves (with a mean of 59.50), and the widest leaves (73.58 mm). Chiller rootzone cooling method at 30DAT and planted at the middle level of the hydroponic tower had the longest leaf with 129.17 mm. Economic analysis showed that the GHE rootzone cooled nutrient solution at 25DAT is most feasible with benefit to cost ratio of 1.73, average net income Php166,963.48, return on investment of 23.39%, and payback period of 0.81 years