Soybean Expert: Drought Impacts Soil Microbials

Severe drought conditions limited soybean production (6.2% per-acre yield decline from 2011) across much of the U.S. in 2012. The implications of the 2012 drought may progress well into the 2013 growing season. One area of concern among growers is the impact of the drought on soil microbial populations, most importantly Bradyrhizobium japonicum. Soybeans live in a symbiotic (mutually beneficial) relationship with B. japonicum. The soybean plant provides nutrients and a protective growing environment for the rhizobia, says UW-Madison Soybean Specialist Shawn Conley.

In turn, the rhizobia “fix” atmospheric nitrogen into ammonia (NH3), which can then be used by the soybean plant. For this relationship to exist and benefit the soybean plant, effective nitrogen-fixing bacteria must be present in the soil in relatively high numbers at planting time.

To date, little research has been published that quantifies the impact of soil desiccation (i.e., drought) on endemic B. japonicum populations. Researchers back in 1979 reported a biphasic decline in Rhizobium japonicum numbers in soils undergoing drying. The initial rapid phase of R. japonicum loss coincided with major water loss (simulated drought) whereas the secondary and subsequently slower decline in numbers was governed by the soil water content of specific soils. The same researchers also noted that organic matter content provided little protection against R. japonicum desiccation and loss.

In a parallel study in 1982, similar results were reported when comparing strains of R. japonicum and cowpea rhizobia under desiccation. “As one would expect, variation exists among Rhizobium species as well as individual isolates within species in their response to soil desiccation, though significant losses were evident regardless of species or isolate,” reports Conley.

To further complicate the situation, most soybean acres will be planted into corn ground that was subjected to severe drought stress. This suggests that excess nitrogen may be present for the 2013 soybean crop, Conley continues.

In excess situations soybean will generally utilize the background nitrogen prior to initiating maximum N fixation. This may lead to luxurious early season growth, which in fields with a history of white mold, may cause problems if weather conditions are conducive. High soil N reserves may also lead to increased lodging.

“In either case, manage your soybean crop accordingly to minimize risk of white mold or lodging. This can be accomplished through variety selection (i.e. white mold tolerance, short statured soybean cultivars or good lodging tolerance), decreasing seeding rates, and proper scouting to time fungicide applications if needed,” he says.

“I am not advocating that every soybean acre automatically receive an inoculant treatment, however, the small amount of research that is available on the topic does suggest that growers may expect lowered B. japonicum populations going into the 2013 growing season,” he concludes.

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