Managing Leaf Burn

As we begin herbicide treatments on our crops this year, I can’t help but wonder, “Do we have the proper amount of nutrients in the plant to help sustain the crop through reproduction?”  The first step is to perform a tissue test on our crops. Sometimes, the soil report we trusted for our fertility needs does not hold true to what the plant roots have obtained for this rapid growth period.  The tissue test will substantiate the amount of nutrients that have actually moved into the plant. Once we have determined the deficiencies and nutrients we need, we can build a blend to apply with our herbicides. Nitrogen is often at the top of the list due to the plants high consumption of this nutrient.  Though, when applying foliar nitrogen, we have to consider the possibility of leaf burn. After all, the number one yield-limiting factor is light, and the process of photosynthesis turning light energy into food energy. We do not want the foliar application of nitrogen to destroy the plants solar panels.

There are two primary reasons for leaf burn: the accumulation of a high concentration of nitrogen on the leaf surface and the process of conversion to NH4 for plant use. Accumulation of nitrogen on the leaf surface will draw moisture from the leaf surface, leaving a burn spot. During the conversion process, nitrogen will pass through the NH3 form due to the urease enzyme on the leaf surface. It is this NH3 (ammonia) that causes leaf burn.

Salt index can also influence leaf burn. However, remember UAN actually has a lower salt index than Urea, but it is much more susceptible to burn than Urea. This is due to UAN’s higher osmolality value. Recent salt index studies rate foliar fertilizers on their osmolality value. This method involves vapor pressure osmometry with lower values indicating lower potential injury. For instance, Urea has a salt index of 74.4 compared to UAN salt index of 64.6 in comparison to their relative osmolality value of 1018 and 1439, respectively. On a burn rating scale of 1 to 9, Urea is 6.3 and UAN is 5.7, with 6 or below being unacceptable injury level.

There are options to help eliminate or decrease leaf burn:

1. Surfactants (Spreaders) will help by eliminating a high accumulation of nitrogen.
2. Megafol® is proven to diminish the stress attributed from herbicide and fertilizer phytotoxicity.
3. ENC® provides balanced foliar nutrition with a blend of micronutrients.
4. An efficient level of calcium helps bind cell walls and improve internal cell strength. The lack of Ca will cause leaf burn due to detrimental cell structure.
5. Nickel is an essential component of Urease. Plants that are deficient in Ni tend to show phytotoxicity due to foliar-applied nitrogen.

- Craig Holland, Product Manager, Northwest division