Production of lettuce seedlings in substrates with tung compost, carbonized rice husk and earthworm humus

Different properties of substrates based on tung compost (TC), carbonized rice husk (CRH) and earthworm humus (H) were analyzed, besides the effect of the formulations on the production of lettuce seedlings, cv. Veneranda. The experiment was carried out in Pelotas, Rio Grande do Sul State, Brazil, from December 2012 to February 2013, in a greenhouse, using completely randomized design with seven treatments and three replications. The used substrates were formulated on a volume basis: T1 = commercial substrate S10 (R) (control); T2= 90% carbonized rice husk (CRH) + 10% humus (H); T3= 75% CRH + 15% tung compost (TC) + 10% H; T4= 55% CRH + 35% TC + 10% H; T5= 35% CRH + 55% TC + 10% H; T6= 15% CRH + 75% TC + 10% H; T7= 90% TC + 10% H. Seedlings were produced in polystyrene trays with 200 cells, being evaluated 35 days after sowing. The substrate with larger proportion of CRH (T2 with 90% CRH) showed suitable dry density (DD) and total porosity (TP), high pH and aeration space (AS), low easily available water (EAW), electric conductivity (EC), and nutrient content, constituting an inert substrate, not being indicated as substrate due to the lower development of the seedlings. Using 15% TC, an increase was noticed in DD, AEW, EC and nutrients and, a decrease in TP, AS and pH due to, mainly, the size of tung compost particles, which accommodated themselves and altered physical and chemical properties of the substrates. Substrates with 90% and 75% TC (T7 and T6) provided the greatest shoot length, fresh and dry shoot mass and leaf area for lettuce seedlings due to higher nutrient content, also considering physical and chemical properties of these substrates. Pure CRH is not indicated to be used as substrate, but mixed with tung compost (T6), it provides high quality seedlings. Tung compost was effective in producing lettuce seedlings, since the composting is able to eliminate phytotoxic substances from this material which can hinder the seedling growth.