sealPurdue News

April 12, 2002

Cold soils should cool farmers' desires to plant, agronomist says

WEST LAFAYETTE, Ind. – Warm weather is here, and a farmer's natural tendency is to begin planting. Farmers will need to resist the urge, however, if they want to improve the odds of producing a healthy soybean crop.

Air temperatures can be deceiving. While it may feel warm enough to put seed in the ground, there may not be adequate warmth below ground to help the seed grow properly, said Ellsworth Christmas, Purdue University Cooperative Extension Service soybean specialist.

Soil may not be ready for soybean seed until early May, Christmas said.

"At this point in time, soil temperatures are very low and the soil is full of water – at least the surface soil," Christmas said. "That water has to drain and the soils have to warm up. We're looking not at days, but weeks before that soil temperature gets to 55 degrees at a depth of 4 inches. If you watch soil temperature and use it as a guide, then the chances of planting soybeans early this year, as we did last year, are pretty remote."

Indiana farmers planted the 2001 soybean crop with amazing speed. By May 6, 52 percent of soybean acres in Indiana were planted. A week later, the percentage had jumped to 80 percent.

To put those figures into perspective, from 1996-2000 the percentage of acres planted in Indiana by May 13 averaged 29 percent.

Planting early offers farmers no yield advantage and is more likely to damage soybean crops, Christmas said.

"If you plant into cold soils at 50 degrees or the low 50s, soybeans will start to grow, but it may take them three weeks to emerge," he said. "During that period you've subjected that young plant to a lot of stresses.

"Some of those stresses are root diseases that can either weaken the plant or, in severe cases, actually kill it and reduce the stand. One thing that I think we need to be aware of is that early planting may subject a plant to a more severe infection of Sudden Death Syndrome."

Sudden Death Syndrome (SDS) is a soilborne fungus that attacks a soybean's root system. The fungus Fusarium solani f. sp. glycines enters the soybean plant early in the season but usually manifests itself later as the plant nears maturity. SDS-infected plants develop small yellowish blotches on the leaves. The blotches grow larger in size and number, and the tissue within the infected area becomes brown and dies. As the disease progresses, entire plants may become infected.

"As we move through the season and we get to early pod development, if conditions are just right, we'll see the toxic phase of that disease," Christmas said.

Early-planted soybeans also are susceptible to bean leaf beetles. The quarter-inch insects feed on plant leaves and pods. Since many early-planted soybean fields are slower growing, the pests have more time to chew on the plants.

April and early May cold snaps present another problem.

"Even though soybeans will tolerate lower temperatures after emergence than corn, if planted very early, soybeans can be killed by freezing temperatures, since the growing point is above ground level as soon as the plant emerges," Christmas said.

The perfect soil temperature for soybean germination and emergence is 77 degrees, Christmas said. Indiana soils usually do not reach that temperature at a 2-inch depth until June. But farmers shouldn't wait that long to plant, he said.

"If you look at soil temperatures, then look at them historically, and the physiology of the soybean plant, the ideal window for planting is somewhere from May 5 to 20," Christmas said. "If soil temperatures warm adequately ahead of that time, you can safely begin to plant beans after April 25.

"As you go beyond May 20 to plant, you start to lose yield potential. That loss is not very great until you get around June 5 to 10."

Despite the risks, most Indiana farmers who planted early in 2001 harvested good crops. Indiana produced a state record 273.9 million bushels of soybeans in 2001, at an average yield of 49 bushels per acre.

More information on early-planted soybeans is available in Christmas' article, "Soybean Planting – What is Early/Too Early?" The article appeared in an April issue of the Purdue Pest & Crop Newsletter. The newsletter is available online.

Writer: Steve Leer, (765) 494-8415,

Source: Ellsworth Christmas, (765) 494-6373,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web site
Purdue University Department of Agronomy

Purdue News Service: (765) 494-2096;

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