2017 ASHS Annual Conference

Abstract. This study evaluates the relative capacity of two soilless root support substrates (SRSS) to reduce nitrate and phosphate leaching, and compared leaf yield of Swiss chard (Beta vulgaris L.) cv. Acelga from both SRSS in a small-scale nonrecirculating hydroponic system (NRHS). The SRSS comprised of 100% perlite and DM6-G^® rockwool propagation blocks (RPB; 4-inch square x 2.5 inch deep). A drip system delivered the nutrient solution (15N–2.2P–12.5K plus micronutrients, supplied at 200 mg nitrogen/liter) at pH 6.5 – 7.2 during the cropping cycle. Leachate from the SRSS flowed to drainage with or without subsequent passage through a 2-liter layer of granulated horticultural charcoal, respectively. Mean temperature and relative humidity were 22.9^oC and 63.2%, respectively. We grew Swiss chard seedlings from the third true-leaf stage until harvesting at 74 days after transfer into hydroponic culture. Weekly leachate sampling from the NRHS for determination of nitrate and phosphate concentration started only when the seedlings were fully established and capable of active mineral nutrient absorption. We analyzed the samples for nitrate and phosphate using the Dionex™ ICS-5000⁺ Capillary High Performance Ion Chromatograph System equipped with conductivity detector. Compared with the inflow nutrient solution, both perlite and rockwool significantly reduced leachate nitrate concentration by ~48.2% (p = 0.0008) and 46.7% (p = 0.0266), and leachate phosphate concentration by ~45.9% (p = 0.0004) and 44.6% (p = 0.0001), respectively, over a 49-day sampling period. Rockwool removed 1.83% more nitrate (p = 0.0564), but 11.16% less phosphate (p = 0.0557) from the leachate compared with perlite. Charcoal filtration of leachate had no significant (p = 0.05) effect on the concentration of nitrate or phosphate leached from either perlite or rockwool. Crop grown in perlite had significantly (p < 0.05) greater marketable yield: leaf count (LC) and leaf fresh weight (LFW) per plant, as well as leaf dry weight (LDW) and leaf water content (LWC) than rockwool. Leaf count, LFW, LDW and LWC were 12.1%, 39.2%, 23.5 and 1.8% greater for crop grown in perlite than rockwool. The DM6-G^® RPB had smaller volume for root growth (40 cubic inch [0.655 liter]) compared with 1-Gal™ (~210 cubic inch [3.44 liter]) of perlite. Apparently, the smaller volume of the rockwool blocks negatively affected the water relations of the crop, and consequently reduction in growth. Although there was no significant (p = 0.05) SRSS effect, the two SRSS reduced nitrate and phosphate leaching by over 40% during the 49-day sampling period. This quantity amounts to significant savings from reductions in fertilizer losses, and minimization of potential nitrate and phosphate pollution from small-scale nonrecirculating hydroponic installations.