My crops are showing salt deposits on the leaf edges. Can you give any advice on how to avoid this problem?
A: The type of damage you are seeing is termed “outer leaf marginal necrosis,” also known as leaf burn, and is a physiological disorder that is different from the more commonly seen tip burn, which occurs on the inner, young leaves of lettuce under warm, high-humidity conditions. With outer leaf marginal necrosis, which tends to occur on the lower leaves of the plant, guttation is the main cause of leaf burn.
Under conditions of high root pressure, water containing salts and many other compounds is pumped up the plant and out to the ends of the leaves where hydathodes (specialized glands that secrete water) are located. These droplets, or guttation, is typically seen in the early morning as root pressure is generally much higher at night. As this water evaporates from the leaf, it leaves behind salts that accumulate and cause cellular damage known as marginal leaf burn.
This type of guttation induced salt damage can be more severe when using organic nutrients than with conventional hydroponic fertilizers as the balance of salts may not be optimal, or levels of unwanted salts such as sodium can be higher and accumulate over time.
Prevention or minimization of guttation is the key to preventing the marginal necrosis. Guttation occurs when plants have a strong and vigorous root system creating a high degree or root pressure at night that forces large volumes of water up into the plant and out of the hydathodes. This typically occurs when the air temperature and root temperature are cooler at night combined with humid conditions around the leaf surface. Since stomata are closed at night, this restricts moisture loss from the foliage, leading to a further buildup of turgor pressure within the plant.
Prevention of guttation involves ensuring there is a good rate of air movement across the plants and ventilation in the growing area and restricting root pressure by increasing the EC in the nutrient solution. A higher EC restricts the amount of water plants can take up and thus reduces root pressure and guttation. Pak choi is also a plant that requires higher EC levels than lettuce (around 1.8–2.0) and would benefit from bringing the EC up gradually.
Warmer night temperatures can also assist with this problem as guttation is higher where night temperatures are much lower than day temperatures. There can be a lot of genetic variation in guttation and marginal leaf necrosis severity, so if the problem continues it would be worth trialling other cultivars of lettuce and pak choi, as those varieties with less vigorous root systems often don’t suffer from as much guttation and leaf burn.
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