When crops are actively growing in the mid- to late part of the season, they demand greater quantities of nutrients on a daily basis. Immobile nutrients applied to the soil surface can face difficulty getting down to the roots where they’ll be taken up. When looking at in-season corrective measures, foliar fertilization has been growing as the go-to option for growers to supply those nutrients.
While foliar applications are a helpful management tool for in-season applications, there is a risk for crop damage if applied incorrectly. Here’s a look at the "how-to" and limitations of foliar fertilization from Dan Kaiser, extension soil scientist with the University of Minnesota/
Tips for Foliar Application
1. Determine if you really need the application
Find a tool to help determine whether or not a nutrient is going to increase yield when applied. Take soil or tissue samples to test for nutrient deficiencies before or at the time of planting. Plant tissue analysis has filled the void left by soil tests in-season, but these results are highly subjective. See our tips for more effective tissue sampling for more information.
Keep in mind the relative mobility of nutrients in the soil. Apply mobile nutrients to the soil surface. With immobile nutrients like P, K, Zn, Fe, Cu and Mn, apply at or near planting to allow for easy uptake by plant roots. For micronutrients, use a starter fertilizer package at planting.
2. Keep rates low to prevent leaf damage
The major risk associated with foliar applications comes from damaging the plant itself. Make sure the rate you’re applying at is not causing more harm than good. Increasing carrier volume will not decrease the risk of foliar damage.
3. Look at the compatibility of your mix.
Pay attention to the mixes you’re putting together to make sure that you’re not causing excessive foliar damage. We see this a lot when we mix high nitrogen sources with fungicides, resulting in leaf burning that can reduce yield. The other thing to look out for is mixing with adjuvants. Typically, we see two classes recommended for foliar application. One is a non-ionic surfactant or an acidifying agent to help distribute that spray over the leaf area and help potentially penetrate the leaf.
The other class is oil concentrates, which are there to try to dissolve the cuticle to try to increase penetration. Adjuvants can increase the risk for damage, which, for products like UAN and ammonium or potassium thiosulfate, can present a risk for damage when applied without adjuvants.
When making decisions about your mix, stop and think, “Is this really my best option?” With some sources of immobile nutrients, you may be better off putting that directly on the soil surface, which would limit the damage potential, because you’re not applying that directly to the plant. Stick with other options like micronutrient packages, which are safer for on-leaf application
One of the barriers we see is getting that nutrient from that spray onto the leaf, into the plant. This limitation comes from the plant itself. Most terrestrial plants have a waxy cuticle, which is there to prevent water loss, and prevent the plant from desiccating. One of the issues with the waxy cuticle is that wax is not charged, so when we use ionic fertilizer sources, which are mostly comprised of salts, those salts can’t actively penetrate that cuticle.
The stoma is another potential avenue for penetration, but it is there for air exchange, so there is debate as to whether any uptake will actually occur or not. When applying, to actually get into the plant, urea is a good source because it’s an uncharged molecule that can penetrate through the leaf.
Keep these tips and limitations in mind when making foliar applications. Talk to your supplier. Look at the documentation for what you’re applying and always read and follow label instructions.