How Cover Crops Affect Your Fertilizer Strategy

Published Jun 29, 2018 

When it comes to cover crops and nutrient management, a lot of the focus is on what cover crops can do for soil fertility. Depending on the species being used, they can prevent leftover nutrients from running off into waterways or even produce some fertilizer for future crop use.

But how should farmers adjust their fertilizer practices for their cash crops following cover crops?

John Pike, a contract researcher and cover crop specialist for the Zea Maize Foundation and IL Sustainable Agriculture Partnership, and a former Research Agronomist at the University of Illinois at Dixon Springs Research station, says that while there are some nuances that need to be planned for, the basic principles of soil fertility and crop management are much the same.

“It’s a matter of fine-tuning the cover crop system to fit the soils, climate and equipment that’s available in the operation that we’re talking about on any given day,” he says. “We’re talking about soil fertility and growing corn and beans and wheat or whatever the crop is, so approach it from a logical standpoint, one step at a time, to figure out the best system for the crops, soils and equipment you’re dealing with. Don’t be overwhelmed right out of the gate.”

Nitrogen timing is critical

Farmers first need to consider the benefits of the cover crops they’re seeding and how they may affect their fertilizer plans.

For example, if a grower is using cereal rye and there is residual nitrogen from the previous cropping season in the soil, a good establishment of rye can hold that nitrogen, which will eventually be available for the next crop. But when that nitrogen is available to the following crop is highly dependant on the timing of the cover crop termination.

Generally speaking, Pike says the longer the cover crop — especially cereal rye — is allowed to develop before termination, the greater the lignin formation in the biomass. This means it’ll take longer to break down and allow for nutrient release than vegetation at earlier stages of development.

Pike points out that cover crops holding onto residual nitrogen is a good thing as far as environmental stewardship is concerned, but that it also creates a challenge in some situations, because it may not be available for the future crop at the time it needs it.

Because of that, Pike says the timing of a nitrogen application to the following crop is critical.

“We need to make sure that corn crop has as much nitrogen as it needs on any given day of growth,” he says. “Even though those needs are going to be relatively low when the crops are little and just emerging, if there’s a deficiency at any stage, that can be negative to our yields.”

The application method of nitrogen is also important — more so than the type of nitrogen source used. In most cases, an injected or banded form of nitrogen is going to be better than a broadcast application.

How Cover Crops Affect Your Fertilizer Strategy

Injecting or banding nitrogen in the soil is preferred over broadcasting it in high-residue cover crop fields to avoid nitrogen tie-up. Photo: Nutri-Placer 930, by Case IH.

Pike explains that by broadcasting nitrogen — whether it’s in the form of urea treated with a stabilizer or a liquid — the soil microbes can tie up a portion of that applied nitrogen before the growing crop can use it, because they need it to break the down the cover crop residue, particularly those that have a high carbon-to-nitrogen ratio.

“Nitrogen is an essential food source for the biology that breaks down cover crop biomass and other crop residue, which in turn makes those nutrients available for uptake in this season’s crop,” Pike says.

Although broadcast applications of stabilized nitrogen fertilizers can work very well in standard no-till management, the additional residue/biomass in a cover crop system can make those applications alone less efficient than other placement options, he explains.

It’s not to say that cover crop users can’t broadcast a portion of their nitrogen, Pike adds — just that it will need to be fine-tuned a little more than if they were just doing a broadcast application in a conventional or no-till system.

Because of this, Pike says a lot of growers are no longer relying on a one-shot nitrogen application. Instead they’re updating their planters or trading them in for ones that have the ability to apply starter fertilizer.

“It’s almost a must for many people, just to make sure nitrogen is readily available,” he says. “Either a small amount applied in-furrow with a starter application or a 2-by-2 treatment that’s going to be available soon after the corn emerges, to where it’s got that quick source of nitrogen.”

Pike adds that it’s also preferred that subsequent nitrogen applications be injected or placed near the roots — such as knifing it in or using something like a Y-Drop application — over broadcasting.

How Cover Crops Affect Your Fertilizer Strategy

Additional nitrogen applications should also be placed near crop roots to ensure the crop has quick access to the nutrient. Photo: 360 Y-Drops by 360 Yield Center.

“I see a lot of growers setting up planters to apply 30 to 50 pounds of nitrogen at planting, then applying the balance of nitrogen through sidedressing at V3 to V6, and having excellent results,” Pike says. “We hear a lot of talk about later nitrogen applications (V10 to VT), and while we’ve found that 50 pounds of N at planting tends to sustain the crop to that point well, there really isn’t a yield advantage to those later application timings in most cases. But it’s good to have those options available in seasons when weather delays the earlier applications.”

Check soil fertility for P and K

As for phosphorus and potassium, Pike says they’re not as critical of an issue as nitrogen in terms of timing and placement, but are certainly key considerations in the overall soil fertility program.

“I think the biggest thing with P and K, as well as soil pH, is to make sure that we’re on a regular soil testing schedule,” he says. “And make sure that our soil test levels are showing what they’re supposed to be for our areas and the particular soils we’re dealing with.”

For growers who can apply starter fertilizer, it’s common to use something like 10-34-0 where some type of phosphorus carrier can be placed on or near the seed to provide the plants an early boost. There are some potassium fertilizers that can also be added to a starter mix, but Pike says broadcast applications of dry 0-0-60 is probably the most widely utilized source in most cases.

Strip-tillers have even more flexibility, as they have the option of banding their phosphorus and potassium — and even some nitrogen — into their strips before planting.

When it comes to choosing a fertilizer source for phosphorus and potassium, Pike says it’s not going to make much difference in most cases as long as it is a quality source and applied uniformly.

“Getting back to the basics, if soil test levels of P and K are especially low, there will be likely advantages to banding near the row, whether applied through the planter or some type of strip-banding applicator,” he says.

Conduct your own trials

Pike says there are a lot of starter fertilizer products on the market, and while they all probably have their place, farmers need to make sure they know what they’re getting and that they contain what their fields and crops actually need.

“While it’s great to be able to add a variety of nutrients to a starter fertilizer program, make sure you actually need what you are applying,” he says. “Every nutrient in the mix adds to the cost of the product. Make sure you are investing in the right product to fit your crop nutrition needs and not simply adding up production costs that don’t generate return to the bottom line.

“So make sure that anything that’s out of the ordinary or new to your operation is proven, or try the product or practice on a small scale before we make a big adoption or change in our fertility plan.”

In the case of starter fertilizers, he says a lot of research has been done for the conventional tillage market over the years and the results have been underwhelming, as long as soil fertility is up to par.

But he’s seeing more cover crop users push their termination to late March or early April, with some farmers even preferring to plant green into a living cover crop. He believes growers who have 8-10 tons or more of cover crop residue on the field may see some advantages to some of the starter fertilizer programs and other blends with micronutrients, because that cover crop may be holding even more nutrients that won’t be immediately available for the following cash crop.

“I think as we look at the nutritional needs of our cover crop, that cover crop is taking up more things than just nitrogen and we probably need to be managing for that as well,” he says. “Just because we hear that something hasn’t worked in the past in a conventional situation, we might find those products have a very good fit in a late-termination cover crop system. But in some cases, we’ll need to do additional testing to be certain just exactly what the best fit is.”

He also points out that it’s important for farmers to test products and application methods on their own farms instead of relying on outside research alone, as your personal situation may yield different results.

“If you hear about something working on someone else’s farm, especially if it’s not in your specific region, try to determine how your conditions might be different and how the same practice might work on your soils and in your production systems,” Pike explains. “Great innovations on a farming operation in Iowa might not work so well in southern Illinois and vice versa. Those clarifications are usually not discussed in magazine articles or research reports, so local and regional information, where cover crop related information is concerned, is usually the most relevant.”

Although discussions about fertilizer programs and options will continue, it seems a sure thing that cover crop utilization overall will continue to expand, Pike says.

“Like any new technology, there is a lot to learn, and while cover cropping is not a new concept at the basic levels, maximizing the benefits of better cover crop systems is leading many farming operations to understand their production systems at a higher level, resulting in profitability and improved resource conservation.”

Cover Crops