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."
- September 2011