With the recent explosion of foliar applied nitrogen products in the marketplace, I often field questions regarding whether or not these products actually work or are they just another in an endless line of snake-oil products. This is a fair question; especially since the products are not cheap to buy and/or apply, but if they do perform as advertised they have the potential for a nice return on investment.
So how do we know if they work or not? The first step in evaluating these products is to look at the agronomics behind the stated benefits. With many companies claiming nutrient use efficiencies through foliar feeding, as well as a long window of availability through “controlled release” formulations, it’s imperative that we understand just how these products work.
One of the main selling points of foliar applied nutrients is the increased efficiency of foliar application versus soil applied applications. One of the more popular foliar N products on the market even goes so far as to claim that 1 pound of foliar N can replace 5 pounds of soil applied N, for a 5:1 efficiency ratio. But is this true?
Plant leaves, by design, are not very efficient in taking up nutrients. Instead, they were designed for a different purpose. Darren Goebel, Area Agronomist with Pioneer Hybrids addressed this subject in an article last summer. He said;
… leaves are designed to collect sunlight, photosynthesis, transport sugars to other parts of the plant and transpire water vapor and gases. Leaves are covered by a waxy cuticle, making them virtually impervious to water and carbon dioxide. Stomata make up 10% of the leaf surface and account for 90% of the water and gas movement on the leaf.
Goebel goes on to say this;
Because such small amounts of nutrients can enter plant leaves, macronutrients like nitrogen, phosphorus, and potassium should not be considered for a one-or two-shot foliar feeding program. It is highly improbable that foliar feeding can get enough of these nutrients into the plant to make a difference. In fact, several University studies have confirmed little difference in yield when foliar applying macronutrients to corn and soybeans.
Obviously there isn’t much of a gain in efficiency when trying to feed macronutrients through the leaves of the plant; the plant is not capable absorbing nutrients in large enough quantities to do much good. However, if you have seen the advertisements for these products, you know that the inability of the leaf to absorb large amounts of nutrients makes no difference because you only need a small amount of product to see a sizable yield increase. In fact, most of these products have a recommended application rate of 1-3 gallons per acre, which at 2.5 lbs. of nitrogen per gallon, definitely qualifies as a small amount of N being applied.
Most of these products are recommended to be applied with glyphosate as a planned application program. At this time – especially in corn – the plants N usage is beginning to increase rapidly. This also coincides with the particular growth stages when the plant is determining ear size (corn) and setting pods (beans). The theory behind this is that a little extra N at this critical time can pay big dividends. The question is; is there any validity to the theory? We shall see.
Note: From here on, I’m going to focus on corn, but what I am going to say can certainly be applied to beans also.
As I mentioned previously, at the time these products are recommended to be applied, the plants N usage is increasing at a rapid rate. As you can see from the chart below, a corn plant’s N consumption increases from 1.5 lbs. per day for 4″ corn to 6.0 lbs. per day when the corn is waist high. This is the period of time when foliar applications are recommended because you want to make sure the plant has enough N when it needs it the most. In theory, this makes a lot a sense. Although one must remember that this is theory – reality is usually somewhat different.
Lets assume that we have a normal corn crop. The corn was planted in a timely fashion, we have a good stand, and the corn is not showing any N deficiencies. In this example, it is recommended that we apply 1 gallon of a 25% controlled release nitrogen product with our normal glyphosate application. Since it’s a 25% N containing material and it weighs 10 lbs. per gallon, we are applying 2.5 lbs. of nitrogen per acre.
Under normal circumstances, the timing of the glyphosate application would occur around the V5 to V6 growth stage. Looking at the N use chart, we can see that at the V5-V6 stage the corn plant is using somewhat less than 6.0 lbs. of N per day. We know at this point, the corn plant is just beginning a period of extremely rapid development so we can safely assume the plant is using between 3 and 4 lbs. of N per day. It’s important to note that the plant is using this amount regardless of whether it is taken up by the roots or absorbed through the leaves.
Given the above facts, it’s obvious that the 2.5 lbs. of N we are applying with the foliar product is not even supplying enough nitrogen to feed the plant for one day. But in reality, the plant is not receiving the full 2.5 lbs. because not all of the product is being intercepted by the plant. A significant portion of the material is hitting the ground rather than the leaves.
When spraying corn that is approximately 16″ tall (V6), the crop canopy intercepts approximately 50% of the spray (assuming nozzles positioned both directly over the row and between the row), which results in an application of only 1.25 lbs. of N to the crop. This is barely enough N to feed the plant for 1/2 a day. Now we begin to see the futility in foliar feeding macronutrients. But there’s more to the story, so let’s continue.
From the label of the product we are using, 75% of the N is in the urea form with the remainder in a slow or controlled release form (I’ll have more on controlled release later). With urea nitrogen, approximately 70% of the N is absorbed into the leaf within the first 24 hours – the rest of the N is lost. If we plug the 70% figure into our calculations, only 0.875 lbs. of N is being absorbed by the plant. This is only enough N to feed the plant for 6 hours. By this point I’m sure somebody is going to bring up the fact that they use more than 1 gallon of product, but even at the 3 gallon rate, we are supplying enough N to feed the plant for 18 hours instead of 6 hours. This is still a minute amount of N when compared to the plant’s needs.
Slow Release Formulations
Another argument that frequently comes up is the fact that the product is slow or controlled release, thereby spoon-feeding the crop over a 3 to 4 week period. Assuming that all the N being applied is in the slow release form (as implied through the advertisements), 0.875 lbs. of N spread over a 4 week period only amounts to 0.031 lbs. of N available to the plant per day. But we know this is not the case because only a small portion of the N is in the controlled release form.
Remember the label stated that 75% of the N is in the urea form. This means that only 25%, or 0.63 lbs., is in the controlled release formulation. If we take 0.63 lbs. and divide that by a 4 week period, we come up with 0.0225 lbs. of N provided to the plant each day. To you and I, this would be like taking 1 milligram of Human Growth Hormone (HGH) per day when the recommended amount is 165 milligrams, and expecting to look like Arnold Schwarzenegger in a few weeks; it’s not going to happen. And so it goes with slow release nitrogen. The plant is receiving such an infinitesimally small amount of nutrient, it is not possible to affect a positive change in the crop.
Although the theory sounds good and the sales pitch can be very convincing, foliar feeding macronutrients such as nitrogen is just not feasible. Because plants require such large amounts of these nutrients (it’s why they are called macronutrients) over an extended period of time, and because leaves are not designed for the absorption of nutrients, it’s physically impossible (without harming the plant) to introduce enough nutrient into the plant in order to gain a benefit. A person should look upon these types of products with a skeptical eye and weigh the stated claims against known agronomic principles. Oftentimes, one will find that the claims just don’t add up.