This article originally appeared in the December 2016 issue of No-Till Farmer magazine.
Corn is usually the main focus of a grower’s fertility program, but no-tillers should pay just as much attention to their soybeans by knowing their soil quality and understanding nutrient availability.
Do you really need to fertilize your no-till soybeans? Can’t you just let them scavenge for what’s left in the soil after corn?
Andrew Hoiberg has been asked those questions and his answer is — no.
“You shouldn’t just let your soybeans scavenge out there. You should be proactive in your fertility management and make sure the soybean has what it needs,” says the vice president of research and development at Calcium Products, based in Ames, Iowa. “If we’re growing it as a crop, as a commodity, you want to make sure it’s got everything it needs.”
In a presentation given at the Illinois Soybean Assn.’s (ISA) 2016 Soybean Summit in Rockford, Ill., Hoiberg shared how soil influences soybean fertility, and some considerations no-tillers should keep in mind to ensure they’re creating a successful fertility program.
Know the Texture.
The first step with any fertility program is to understand the soil texture. The reason it’s important is it can help drive fertility management decisions, especially in terms of application timing.
“We don’t want to use a nutrient that’s more susceptible to leaching in a sandier soil,” Hoiberg explains. “That’s the type of information that can help us drive decisions on the fertility plan and also in our operational planning.”
Calcium-to-Magnesium Ratio: Should You Pay Attention to It?
Some no-tillers may be wondering: Should I be concerned about my calcium-to-magnesium (Ca:Mg) ratio?
Andrew Hoiberg says a lot of research has been done on it, and most universities do not support it.
“But I’m not saying you won’t get a response from trying to balance that ratio,” says the vice president of research and development for Calcium Products.
The Ca:Mg ratio comes from research conducted in the 1940s, where they developed the basic cation saturation ratio (BCSR) concept.
“Basically what they said was, if we have a given soil type, and we have the right ratio of cations to each other, things will just sort of work how they should in the soil, and that will give us the best chance for maximizing yield,” Hoiberg says.
That ratio is 65% calcium, 10% magnesium, 5% potassium and 20% hydrogen. Hoiberg admits he doesn’t understand that percent for the hydrogen number, as that would lead him to believe the soil would be acidic.
“I can tell you fairly conclusively that the Ca:Mg ratio is not going to have a huge influence on your yield,” he says. “But what might be happening is if you’ve got too much magnesium in your soil, and you’ve got things like high clay content, what you might want to look out for are reduced infiltration rates, seedling emergence issues and crusting of the soil surface, which obviously can affect yield.”
He explains the combination of high magnesium with some of those other issues means your chances of success from adding a calcium-based soil amendment might be a little higher than it would if you were just applying it for the sake of an arbitrary magnesium value in your soil test.
He notes that with magnesium, unfortunately there’s no determined level for what’s too high that would impact infiltration or create a negative yield effect.
“I would encourage you to test it yourself, before you make any decisions with a wide-spread application,” he says.
What’s Your pH?
The second piece of information no-tillers should know about is their soil pH. When soil pH is kept in the right range, everything works better, Hoiberg says.
While there isn’t a universal target pH for soybeans, the goal is 6.0-6.5.
“What it boils down to for me is, what type of subsoil are we sitting on?” Hoiberg asks. “Do we have an acidic subsoil where targeting your pH of 6.5 is probably going to produce an economic benefit? Or do we have a calcitic or non-acidic subsoil where we’re probably going to be OK if we shoot for 6.0? Know where you are located and what the local university recommends.”
The peak availability for nutrients is a pH of about 6.5, he says. Macronutrients are generally more affected by a low pH, while the opposite is generally true for micronutrients.
“It’s definitely a balancing act to make sure we make everything as available as possible,” Hoiberg says.
Phosphorus (P) is the nutrient most affected by pH, he adds. In an acidic soil, aluminum and iron will want to fix with P, while in basic soils calcium will fix it.
The availability of nitrogen (N), potassium (K), calcium, ammonium and magnesium are also reduced as acidity increases.
In addition to nutrient availability, proper pH is also important for nodule formation. Hoiberg says a pH below 5.6 will negatively impact nodule formation.
“You may see more N deficiency, and that’s not necessarily because of the nutrient availability, but more so because those nodules aren’t able to fix and deliver it to the plant,” he explains.
Help Nodule Formation.
In addition to keeping the pH above 5.6, no-tillers should be mindful of their fertilizer applications and how those may impact nodule formation. For example, if a grower makes a MAP or DAP application for their P source, they’ll need to make sure they’re not putting out too much N.
“A little bit of N is OK early in the season for soybeans, but you don’t want to continue to put AMS or something else out there, because as your N increases, the ability of those nodules to form and be needed by the plant is going to decrease. So then the plant becomes more dependent on N application,” Hoiberg says.
In addition to the amount of N, the type can negatively affect nodule development. Hoiberg says just about every N fertilizer contains some form of ammonium, and when that ammonium converts to nitrate, it’s leaving behind some acidity.
“If you’re generating acidity and adding N at the same time, that’s kind of a two-headed monster with regard to negative effects on nodule formation.”
He adds that nodules need oxygen, so in poorly drained soils growers may see poor nodule development. If it’s a consistent problem, tile drainage may be necessary.
Beware of Salt.
Fortunately, salty soils are not a big issue in the Midwest, but it’s important for growers to know that soybeans are considered a moderately salt-sensitive crop.
Soybeans are tolerant up to 5 dS/m, so if growers see that number on their soil test — usually reported as ‘soluble salts’ — getting close to 5, they’ll want to consider taking action.
“Really the only thing you can do with soluble salts is flush them out of the profile,” Hoiberg says.
No-tillers should also avoid using gypsum if they have high salt levels. Gypsum should be used for sodic — not saline — soils.
“Calcium sulfate is a salt, so if you try and add gypsum to a saline soil, all you’re going to do is make the problem worse,” Hoiberg explains.
Up Organic Matter.
Hoiberg once read that if your soil organic matter is below 1%, your crop is never going to realize its full yield potential.
If growers can increase their organic matter, it not only provides benefits in terms of soil quality, but it also helps promote soil fertility.
“Soil organic matter is going to be the biggest contributor to cation exchange capacity,” he says. “Which means more nutrients and more water-holding capacity. It’s also a great buffering agent for soils; it helps resist pH change.”
For every 1% of organic matter, growers can expect to have about a half pound of sulfur, 1-2 pounds of P and 10-20 pounds of N available to their crop every year.
It’s also important to consider when soybeans take up nutrients. Hoiberg refers to a 2015 paper published by the University of Illinois that found sulfur and K are the two nutrients with more of a season-long uptake pattern in soybeans.
“All nutrients are taken up throughout the season to some degree,” he says, “but sulfur and K are really taken up pretty consistently throughout the season. So when you’re thinking about your fertilizer sources for each of those, you want to make sure you have something that’s going to be there for the long-term.”
“If you’re generating acidity and adding nitrogen at the same time, that’s kind of a two-headed monster…”
He adds that it’s more important with sulfur than K because sulfur is more susceptible to leaching, noting that the solubility of different sulfur fertilizers also plays a role.
“There’s about a 300-times difference between gypsum and ammonium sulfate, so you’re certainly more susceptible to leaching with AMS than with gypsum,” he explains. “Pay attention to your application timing and how you supply sulfur. How you’re supplying every nutrient, for that matter, to the soybean crop.”
Hoiberg notes that growers may want to avoid using elemental sulfur, citing that some experts have stopped recommending its use because its availability was too unpredictable in colder soils.
Look for Deficiencies.
Hoiberg says for soybeans in the Midwest, K tends to be the most common macronutrient deficiency. The telltale sign is yellowing at the margins that move inward on older leaves.
On soils that have limited drainage or a high pH — particularly in the Upper Midwest — iron deficiency chlorosis (IDC) can occur.
“If you’ve got an area where you know you’ve got bicarbonates and high pH, you definitely want to be on the lookout for IDC,” Hoiberg says, adding that growers can make foliar or in-furrow applications to resolve it.