Mitigating Salt Injury in the Tobacco Float Transplant Production System in Zimbabwe

C. Koga, D. Rukuni

The tobacco float seedling production system occasionally exposes transplants to salt injury, especially when there is excessive evaporation due to winds or high temperatures. Evaporation causes salt accumulation on the Gromix® soilless media surface. These salts cause physiological damage to young transplants and more often, mortality. Three trials were conducted at Kutsaga Research Station to address this problem that affects mainly the late July, August and September sown float seedbeds. The first trial evaluated four hydroponic fertilizer formulations (Floatfert®, Hydrofert®, Peters® and Formula 1) for potential in reducing salt injury. While the fertilizers had different salt indices (SI), all the formulations exposed the transplants to salt injury. The second trial evaluated the effect of split fertilizer application and delayed application of Floatfert® fertilizer. Floatfert®is normally applied at 7, 21 and 35 days after sowing (d.a.s.). Further splitting or delayed Floatfert®application were tested to determine salt accumulation reduction potential. There were treatment differences in germination and survival at 21 and 28 d.a.s. when fertilizer was split-applied or when application was delayed. A third trial evaluated different mulch types (Vlei grass, [Eragrostis lehmanniana], perforated clear plastic, clear plastic tent and a floating-row cover) for reducing salt accumulation. Vlei grass mulch resulted in the highest germination and survival of transplants. The results from this work indicated that mitigation of salt injury during hot and windy periods requires a holistic approach that includes careful selection of fertilizers, appropriate application rates and timing and the use of suitable mulch.