Soybeans respond to environmental fertility better than to corrective applications, making it essential to keep soil nutrient levels in balance. If fertility is lacking, it will take longer to build levels back up in soybeans compared with corn.
NDVI mapping and balanced fertility will do the trick
Maximizing the yield of any crop is a challenge, but soybeans are in a class by themselves. There are a couple of reasons for the yield challenge.
Reason one: "Unlike corn, soybeans respond more to the environment than to management," says Farm Journal Field Agronomist Ken Ferrie. "That means you can’t fix a fertility problem in one season, simply by applying extra nitrogen, phosphorus or potas-sium, as you can with corn. For
top yield, you have to maintain a balanced soil fertility profile, from pH to micronutrients."
Reason two: Problem areas within a field, whether caused by fertility or other factors, are difficult to identify. "In corn, if you see stunting and discoloring during the vegetative stages, there’s a good chance it will show up on a yield map," Ferrie says. "You can go to that area after harvest, pull soil samples and track down the cause."
The nature of soybeans makes it difficult to get a good yield map. Soybeans often bounce back from stunting and slow growth, so symptoms go away. Separating out management zones in a yield map is hard because they only fluctuate a few bushels across a field.
As a result, problem areas in soybean fields often lie undiscovered, getting more serious by the year. When you finally do discover them, fixing them may take several more years.
You can’t change soybeans’ cantankerous nature. But you can prevent minor problems from becoming serious. Doing so will set the stage for a bumper bean yield when Mother Nature bestows ideal growing conditions.
Detect deficiencies. Spotting problem areas is the first step in fixing them—but the nature of soybean plants can make that difficult.
Some problems are easy to see. "For example, no-till soybeans sometimes look tough early, especially to first-time no-tillers," Ferrie says. "But the beans grow out of it—there’s really no reason to till for soybeans. Another problem that’s easy to spot is iron chlorosis. There are other problems that are harder to spot."
The way to identify the problem areas is by using NDVI (Normalized Difference Vegetation Index) mapping. NDVI maps are made from aerial photos based on near-infrared and red light reflected by the plants. Maps made during the course of the growing season visually depict changes in plant health. You can spot problems before soybean plants mask them, whether the cause is fertility, nematodes, disease, insects or drainage.
"One place we use NDVI maps is to manage soil pH," Ferrie says. "Sometimes we find fields that have pockets of high-pH soil located within another soil type. The pH reading in the pocket may be as high as 7.3 or 7.4, while the rest of the field is acid.
|This NDVI map shows problem areas in a soybean field that
probably would not have shown
up in a conventional yield map. SOURCE: Crop-Tech Consulting
"In the past, farmers often would avoid liming that field to avoid making the problem area even more alkaline. But using an NDVI map to select soil test locations, along with variable-rate application technology, you can make pH uniform across the entire field."
After using the technology, Ferrie has discovered that NDVI maps are precise enough to identify a 50' to 75' transitional area around a problem spot, where soil is becoming more alkaline but you can’t yet see visual symptoms. So you can fix the problem before it gets serious.
Set the stage for yield. Because soybeans respond to environmental fertility rather than applied fertility, you don’t ever want to let fertility levels become depleted. "If fertility gets run down and you have to build it back up, you’ll find that soybeans respond much slower than corn," Ferrie says.
Soybeans differ from corn in their ability to take up potassium. "With soybeans, it’s more important to have soil potassium [K] levels in the optimum to high range," Ferrie says. "If we find a deficiency of major nutrients in the plants, it usually is potassium."
With K, two problems can arise related to the clay content of your soil.
"If the clay content is high, it can fix potassium to the soil particles, making it unavailable to plants," Ferrie says. "In sands, on the other hand, potassium can leach out of the soil. If you farm these kinds of soils, consider applying potassium every year, rather than putting on a two-year supply ahead of corn."
If you maintain adequate soil levels of phosphorus (P), soybeans will do their part. "We don’t find phosphorus deficiencies often in soybeans if soil levels are adequate," Ferrie says.
Because of their taprooted nature, soybean plants are efficient feeders of P. The plants’ roots release hydrogen, creating an acidic area, which helps them solubilize P from the soil.
Liming is essential. "Balancing pH is more important in soybeans than in other crops," Ferrie says. That’s because acid soil inhibits nitrogen production in the plant.
"A good soybean crop needs 400 lb. to 500 lb. of nitrogen per acre," Ferrie explains. "Part of that nitrogen is released by the soil, and part of it is produced by rhizobium bacteria. Acid soil reduces both types of activity."
With nitrogen, the acidic area around soybeans’ taproots—which helps with P uptake—works against you. "If you let soil pH levels fall to 6.0, you may get by for a while, as long as the soil has plenty of moisture," Ferrie says. "But if it gets hot and dry, the pH level may fall dramatically in the zone around the taproot. Rhizobia bacteria won’t survive in that acid zone, so the plant won’t be able to nodulate and produce nitrogen."
Both acid and alkaline soils tie up nutrients (even when soil test levels are high), creating deficiencies in the plant that slash yield. "If you farm calcareous soils that are naturally alkaline, the problem will be difficult to fix," Ferrie says. "You might want to rotate away from soybeans entirely. If you can’t do that, search for varieties that tolerate alkaline soils."
Keeping pH in the 6.3 to 6.5 range will let microorganisms thrive, ensuring that plants can take up plenty of P, K and micronutrients, he concludes.
Starter nitrogen. When planting corn, starter nitrogen is essential. In soybeans, it all depends on where you are in the growing season.
"In a normal planting window, from the end of April through early May, our studies in central Illinois show it’s hard to get a response to nitrogen," Ferrie says. "In fact, most applications of starter nitrogen have led to a yield decrease—even though the soybean plants were taller and greener."
That said, nitrogen fertilizer might help speed up growth early in the season. If you are planting unusually late and the carbon penalty slows growth for two or three weeks, you are restricted on the length of the vegetative growth period, Ferrie says.
Starter nitrogen is of questionable benefit, except in certain situations. However, NDVI mapping and P, K and pH management are sure ways to lay the groundwork for maximum yield.
- September 2011