Molybdenum is a true micronutrient, said Nathan Slaton. “It's found in such small concentrations in soybean plants that it's very difficult to analyze for,” said the agronomist and professor with the University of Arkansas.
“But it's also very important. It's required for plants to be able to take nitrogen and build it into amino acids and proteins. In legume plants, molybdenum also allows atmospheric nitrogen gas to be fixed by rhizobia — that's why legumes don't require nitrogen fertilizers.”
To insure proper molybdenum (or “moly”) levels, “we can do a couple of things: either make sure that soils are limed at the optimum level for soybean production or apply molybdenum,” said Slaton, who spoke at the Tri-State Soybean Forum in Dumas, Ark. University tests show that soybeans respond well to molybdenum.
“Researchers have limed soils at different pH levels and planted soybeans (some with seed treatments of molybdenum). What they found is that where molybdenum wasn't applied, soybean yields increased until pH was in the mid-6s.”
In cases where producers don't want to foot the bill for liming, molybdenum application can make a difference in a crop. In another study, when molybdenum was applied to seed planted in a 5.5 pH there was a 15- or 16-bushel yield increase.
“Certainly apply molybdenum to seed when soil pH is less than 5.8 and, perhaps, when pH is less than 6.2. Soil pH can vary widely in a field, so make sure you're soil sampling properly to get a good handle on what pH you have in the field. You may need to put molybdenum on only part of the field.”
One of the first things you'll see with a molybdenum deficiency is nitrogen troubles, said Slaton. This is because molybdenum is required for the soybean plant to utilize atmospheric nitrogen.
“If you've got pH levels in the high 4s or low 5s, molybdenum may help but it may not be the complete answer. That's because the microbes themselves are sensitive to pH. When pH is that low, survival of the needed microorganisms will be low as well. If you get caught in that situation in-season, nitrogen fertilization may be your only option.”
Also key to a healthy soybean crop is Rhizobium japonicum — a bacterium that inhabits the nodules of legume roots. The bacterium allows soybeans, clovers and alfalfa to gather nitrogen from the atmosphere.
Soybeans aren't native to the United States and neither are the needed rhizobia. Both have been introduced to U.S. soils.
“On really sandy soils, the University of Nebraska recommends an inoculum be put out every year soybeans are planted. Most states agree there's a need to inoculate if it's been more than three to five years since the last soybean crop has been planted in a field.
“Some research said if you're working a double-crop system and are too busy in June to inoculate bean seed, just add it to your wheat seed. As long as you get the inoculum out a few months before the bean crop is planted, you're okay.
“You may need to introduce the microbes more frequently if you're not doing a good job managing your soil pH. Acid soil conditions will reduce the microbes' survival.”
Any precision leveling moves soil and microbes and subsoils that may not be well-suited for microbe survival are exposed. Newly leveled fields may need a re-introduction of rhizobia.
The bacterium has a great impact. Studies conducted in the Midwest have focused on benefits of inoculating soils where soybeans have never been grown.
“At a Nebraska site, there was a 26-bushel yield increase. At other sites, yield responses of 14 and 16 bushels were seen. Those are very high yield responses from a simple management practice.
“Some Midwestern states recommend using plenty of microbes. It's cheap, so don't worry about skimping — use a 2X or 3X rate over what's recommended. Make sure enough is put out.”
After applying the microbes, farmers should check young beans, said Slaton. “Make sure root nodules are forming a couple weeks to a month after emergence.”
While there are exceptions, Slaton said a general rule is not to mix inoculum with seed fungicide treatments.
“Make sure you read the product labels closely before doing anything.”
Boron deficiency isn't fully understood but can cause significant yield losses, said Slaton.
“In severe cases, I've seen plants in fields that don't even produce seed. Conversely, I've seen plants that appear close to dying that, without any applications, recover and make a decent yield.”
While unsure how extensive it is, Slaton said he's observed boron deficiency causing delayed maturity. “Early in the season, boron deficiency can be seen in vegetative growth that looks like herbicide injury. One of the Arkansas plant pathologists was checking such a field with me. He said, ‘I think we've been calling this a virus for years.’
“Many of the farmers we spoke with said, ‘We just thought it was herbicide carryover or a virus. At first, it was only in small areas. We only got worried when it began affecting significant acreage.’”
One of the most responsive fields Slaton has worked is in Poinsett County, Ark., and was planted in mid-July. The producer is now a firm believer in boron fertilization.
“He applied a pound of granular boron — blended with P and K — around the test area. In late September, a county Extension agent called me and said, ‘Something is beginning to show up in the field. There's a color change around your plots.’
“About two weeks later, you could see the difference easily: yellow plants where boron was applied alongside dark green plants with very few pods. The boron beans averaged around 35 bushels, which isn't bad for seeding that time of year and the weather they went through. The dark green beans, meanwhile, had no boron applied and averaged only 14 bushels in plots.
“I'm not saying that if you're seeing delayed maturity in your beans that it's definitely boron deficiency. But boron may be the cause.”
When plants don't recover from boron deficiency, they'll be stunted because terminals die and leaves will be cupped and look like “shoe leather.”
Because there's a lot of seed production in northeast Arkansas, Slaton and colleagues are also looking at the concentration of boron in seed. Some data suggests boron can affect seedling vigor and seed quality.
“We're interested in seeing if we can fertilize some of our seed beans with boron and increase the concentration in the seed that's produced. If so, will that mean the (next generation) of plants has an advantage when grown in low boron soil? It may seem farfetched, but may work.”
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