|Tent Exhibits||Credits & Thanks||Sponsors|
Emerson D. Nafziger
Professor of Crop Production Extension
Department of Crop Sciences
Do We Really Need All This “Stuff” on Soybean?
Ever since the discovery of plant hormones, micronutrient needs of plants, and bacterial plant inoculants, people have been eager to use such materials to "help" crops to produce more. No crop has been the subject of more efforts in this regard than the soybean crop. Why soybean? It may be partly because soybean was a relatively new and unfamiliar crop in North America at the time these products were being discovered, so it was easy to imagine that this new crop needed such help. The need to use inoculant was universal when the crop was new, and some commercial products (such as Regimate, a formulation of TIBA designed to change the shape of the plant) found at least a temporary market in soybean.
Whatever the reason, the urge to "help" soybean by providing inoculants, micronutrients, and growthregulating chemicals is alive and well, and is the basis for ongoing marketing attempts aimed at soybean producers. We can divide such inputs into the following classes:
- "Conventional" fungicides and insecticides, applied to the seed or to the foliage.
- Seed-applied inoculants, both those containing Bradyrhizobium bacteria known to infect soybean roots to form nodules in which nitrogen from the air is fixed and made available to the plant, and more recent products that purport to add different microbes said to enhance soybean growth and yield, either by producing compounds beneficial to the plant or helping to ward off disease organisms. The latter are sometimes called "plant growth promoting rhizobacteria" or PGPRs. There are also "biologicals", usually microbes of some sort applied to the seed or soil, promoted as a way to "release" nutrients from the soil or otherwise to benefit plants.
- Plant growth regulators, which are chemical compounds that change the way the plant grows and functions. Many of these mimic hormones already present in plants, so are said to change the hormonal "balances" in plants in ways that produce favorable results.
- Micronutrients are inorganic chemicals such as iron (Fe), zinc (Zn), manganese (Mn), boron (B), magnesium (Mg), copper (Cu), molybdenum (Mo), cobalt (Co), and sulfur (S) that are known to be required in small quantities by plants. While almost all soils can provide such nutrients, adding them is done on the chance that the soils can't supply enough, or enough at the correct time, to meet the needs of the plant. These are usually added as foliar or soil applications.
- Macronutrients are inorganic nutrients, including nitrogen (N), phosphorus (P), and potassium (K), needed in relatively large quantities by plants. While soybean plants usually get adequate N from fixation and uptake from the soil, and of P and K from soil uptake, addition of these nutrients directly to the crop has been promoted as a way to make sure leaves have enough nutrients to function properly.
It is helpful to divide these products into those that are normally effective against identified problems (fungicides for fungal disease, insecticides for insects), those that are known to be essential and that come from the soil (micronutrients), and those that are not known to address identifiable plant problems (hormones). Many of the substances we're talking about fall into the latter categories; the chances they need to be added are small, their purported benefits are vague, or the deficiencies they are supposed to correct are not very well defined.
In recent years our soybean research program at the University of Illinois has included a number of trials designed to test the effects of some of these materials on soybean yield. Most of these trials are done in response to companies who provide product and modest funding to carry out the research. We have not looked at most types of products long enough to know if and how consistently they might increase soybean yield, most of the results to date have shown modest if any response in Illinois. This might be due to our productive soils, which tend to provide most of what the crop seems to need, most of the time. Products like growth regulants also produce consistent benefits under greenhouse conditions (where they are first tried), but to fail to produce similar responses once they are in the varying weather and soil conditions of fields.
While it is impossible to ever "prove" that such inputs do not "work" (defined here as providing a return above the cost of using them), the lack of consistent, positive responses is a strong indication that any response to such inputs in unlikely to be consistent in the field. It may be reasonable to treat some inputs as "insurance" providing their cost is low enough and they do not pose the threat of yield loss. Of course, it hardly makes sense to use six or eight "insurance" inputs, given the cumulative costs and small likelihood of response.