Natural air drying is best for soybeans because it’s gentlest on the beans. But it may not work as well this year if harvest is late and the soybeans are wet, according to Ken Hellevang, North Dakota State University Extension grain drying specialist, and Charlie Hurburgh, Iowa State University ag engineering progressor.
A typical airflow rate of 1 cubic foot per minute per bushel (cfm/bu) can usually be expected to dry 18% moisture soybeans to 13% moisture in about 60 days in a normal fall (47 degrees F). With an airflow rate of 1.5 cfm/bu, the drying time is reduced to about 40 days. The drying time for 16% moisture soybeans is slightly less — about 50 days.
Adding supplemental heat to raise the air temperature by 3 to 5 degrees will permit drying the soybeans to about 11% moisture in about 40 to 45 days.
But as the weather cools it gets more difficult to dry soybeans with natural air. Adding heat can help, Hellevang says. The heat can cause the beans on the bottom of the bin to be dried to a lower moisture content than the beans at the top of the bin, though, which increases drying speed only slightly.
Increasing the airflow rate will increase the drying speed in the fall. However, the fan horsepower required to achieve the higher airflow rate becomes excessive unless the grain depth is very shallow. For a soybean depth of 22 feet, each 1,000 bushels of soybeans will require about 1.0 horsepower of fan. Achieving an airflow rate of 1.25 cfm/bu will require about 1.6 horsepower per 1,000 bushels. An airflow rate of 1.5 cfm/bu will need about 2.5 horsepower per 1,000 bushels.
When the outside air temperature falls to about 35 degrees, it isn’t economical to keep trying to dry wet soybeans in the bin. A better option would be to cool the soybeans to between 20 to 30 degrees for winter storage and complete drying in the spring. Start drying in the spring when outdoor temperatures are averaging about 40 degrees.
High temperature drying
If you use a high temperature dryer, don’t turn the plenum temperature up too high, Hellevang and Hurburgh advise. Typically, the maximum drying temperature for nonfood-grade soybeans is about 130 degrees. Even at that temperature, some skins and beans will be cracked.
One study found that with a dryer temperature of 130 degrees, 50% to 90% of the skins were cracked and 20% to 70% of the beans were cracked. Another study found that 30% of the seed coats were cracked if the drying air relative humidity was 30%.
Refer to the manufacturer’s recommendations for maximum drying temperatures but monitor the soybean seeds coming from the dryer and manage the dryer temperature based on the amount of damage occurring.
Be careful not to start a fire when drying soybeans with a high temperature dryer. Soybean pods and other trash can accumulate in the dryer and become combustible. Clean the dryer frequently to ensure trash does not build up, and to reduce the potential for debris becoming combustible. Also, make sure that soybeans continue to flow in all sections of the dryer. Monitor the dryer continuously to limit fire potential.