Managing salt injury in tobacco seedbeds
Our Tobacco Research Board, Kutsaga, has issued the following advisory on how to manage salt injury in tobacco seedbeds.
The tobacco float seedling production system occasionally exposes plants to salt injury, especially where there is excessive evaporation due to winds or high temperatures.
By Dr Susan Dimbi
The float system technology offers numerous benefits for tobacco seedling production, including the production of robust transplants with intact root systems, leading to more uniform crops and higher yields of good quality. This method also promotes efficient water and nutrient utilisation, eliminating nutrient leaching and reducing waste. Additionally, the float system minimises weed infestation, reducing competition for resources and labour requirements. Overall, this technology promotes a more sustainable, greener and productive approach to tobacco seedling production, with advantages extending from seedling quality to field-level efficiency and reduced environmental footprint.
However, this production system occasionally exposes transplants to salt injury, especially when there is excessive evaporation due to winds, high temperatures, or both. Under excessive evaporation conditions, the pond water moves by capillary action from the pond to the surface of the growth medium where it evaporates and leaves behind fertiliser salt deposits. If these conditions persist, the process triggers the continuous movement of water and salts through to the seedlings, resulting in an accumulation of salts on the medium and around the seedlings (Fig 1).
Symptoms
Tobacco seedlings affected by salt injury exhibit distinct symptoms, including cupped leaves, mild chlorosis, and browning of lower leaf tips, particularly in the heart of the seedling. If left unchecked, severely affected seedlings may die, especially if this happens when they are still young. In addition, salt stress damages cellular components such proteins leading to metabolic dysfunction. It is also reported to accelerate chlorophyll decomposition thus decreasing photosynthesis.
Notably, salt injury also increases the risk of root diseases like Pythium root rot as sickly and unhealthy seedlings are more susceptible to pathogen infection than when growing healthily. Oftentimes, salt injury is misdiagnosed as damping off or rhizoctonia, which causes brown water lesions on the stem, wire stem, and wilting, or pesticide phytotoxicity, characterised by brown or yellow patches and leaf bronzing.
How seedlings are predisposed to salt injury
This physiological damage can result from over application of both basal and top-dressing fertilizers. Salt injury can also be introduced through the use of a salt-contaminated mulch or saline irrigation water. This, followed by excessive evaporation conditions, caused by high temperatures or wind, then triggers continuous movement of water and salts from the pond, resulting in an accumulation of salts on the medium and around the seedling.
Preventing and correcting salt injury in seedlings
- Water quality
Water quality is a critical factor to consider with float seedling production and only clean fresh water suitable for irrigation must be used. The only way growers can tell if their water is suitable is by having it tested prior use and having the results interpreted for plant production. - Proper fertilisation
To fertilise properly, it's crucial to use the right fertiliser and ensure the correct concentration of the fertiliser solution. Adhere to the correct fertilisation schedule. This prevents seedlings from being damaged by excessive fertiliser salts. To determine the optimal amount of fertiliser for a bed, the volume of water in the pond, fertiliser analysis (its nutrient content) and the desired nutrient levels for the seedlings must be taken into consideration. - Pond water levels
Pond water levels must be maintained at the standard water depth of 10 cm to achieve the required concentration of fertilisers. Low pond water levels hasten the development of salt injury. - Early sowing
Early sown seedlings usually escape salt injury. However, sowing dates may not be easy to play around with, as they are dependent on the scheduled planting dates in the field, given that seedlings should be in the seedbed for at least 10-12 weeks. - Mulching
Covering seedlings with vlei grass mulch or float row cover is also effective in reducing evapotranspiration and subsequently salt injury. - Monitoring the electrical conductivity (EC)
If equipment is available, regularly monitoring the media’s electrical conductivity (EC) to ensure it remains below 1,500 µS/cm can be done. This entails monitoring the EC frequently using a hand-held EC meter, at least every 48 hours, from seedling emergence until the leaves cover the entire cell (approximately 4-6 weeks after sowing). If the EC reading is high immediate action to reduce the EC level, such as flushing the system with fresh water to remove excess salts and adjusting fertiliser application rates downwards, and filling ponds to the correct water level (10 cm). - Flushing
Should seedlings be already affected by salt injury, immediate action is necessary. Drench the trays with water three times using a can or hose with a fine rose until the foliage covers the surface of the media.
For more information, growers should contact Kutsaga on VOIP – 086 8800 2604 or toll-free, 08004511
