Hydroponic cultivation of carrots using modified rockwool blocks

Abstract: Three varieties of carrot (Daucus carota L.), 'Tokinashigosun', 'Hitokuchi' and 'Kurodagosun' were cultured hydroponically with rockwool blocks (56^8^30 cm high) in a greenhouse for 90 days. Two types of rockwool block, with holes and without holes were used in the study. Rockwool blocks with holes had seven rooting holes (10 cm in depth and 2.5 cm in diameter) 8 cm apart, which were filled with vermiculite for promoting root development. Rockwool blocks without holes were used as the control for comparison. Two rockwool blocks were placed side by side in a plastic box (58x18x18 cm high) and 14 plants were grown in each plastic box. The rockwool blocks were automatically sub-irrigated with a nutrient solution containing 35 ppm total N, 14 ppm P, 59 ppm K, 23 ppm Ca, 10 ppm Mg, 0.62 ppm Fe, 0.12 ppm Mn, 0.06 ppb B, 0.02 ppm Cu, 0.04 ppm Zn and 0.01 ppm Mo. The solution was added to the plastic boxes twice a day to keep the depth of the solution at 15 cm. The fresh and dry weights of the storage roots were 2 to 3 times greater in the rockwool blocks with holes than those without holes in each variety. The storage roots produced in the rockwool blocks with holes were 2 times longer than without holes in all the varieties. The diameter of storage roots was also greater in rockwool blocks with holes than without holes. Greater weights of the whole-plant and percent harvest index were obtained in the rockwool blocks with holes than in the without holes in all the varieties. Carbon dioxide concentration inside the rockwool blocks at a depth of 8 cm from the top surface and 1 cm beside the storage roots were lower in the rockwool blocks with holes (0.08%) than in the without holes (0.11%). Carbon dioxide gas diffusion coefficient in the rockwool media was greater in the rockwool blocks with holes than in the without holes. The hardness of the growing media was lower in the rockwool blocks with holes containing vermiculite than in the without holes. Therefore, better aerating conditions inside the rockwool blocks with holes containing vermiculite and lower hardness of the media would partly account for the better growth of storage roots in the rockwool blocks with holes than in the without holes in all the tested varieties.