Soybeans at 11% moisture have similar storage characteristics to wheat or corn at 13% moisture, so an allowable storage time chart for cereal grains can be used to estimate allowable storage times for soybeans. For example, soybeans at 18% moisture would be similar to cereal grains at 20% moisture, so soybeans would be expected to have an allowable storage time of about 50 days at 50° F. The allowable storage time is reduced to 25 days at 60° F and extended to about 90 days at 40° F.
The amount of natural air drying that will occur in late October and November is limited. The equilibrium moisture content of soybeans for air at 40° F and 70% relative humidity is 13.5%. An airflow rate of 1 cu. ft./minute/bu. (cfm/bu.) is expected to dry soybeans with 18% moisture in about 70 days. Adding supplemental heat to raise the air temperature by 5° will permit drying the soybeans to about 11% moisture in about 55 days.
Only about half of the beans would be expected to dry by mid-November, when outdoor temperatures become too cold to dry efficiently. Adding heat would cause the beans on the bottom of the bin to be dried to a lower moisture content and would increase drying speed only slightly. Cool the soybeans to between 20° and 30° F for winter storage and complete drying in the spring. Hellevang recommends starting to dry when outdoor temperatures are averaging about 40° F.
Increasing the airflow rate will increase the drying speed. 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 ft., the rule of thumb is that each 1,000 bu. of soybeans will need fan horsepower of about 1. Achieving an airflow rate of 1.5 cfm/bu. will require about 2.5 hp and an airflow rate of 2 cfm/bu. will need about 5 hp.
The type of fan greatly affects the airflow provided per horsepower, so use a fan selection software program such as the one developed by the University of Minnesota. It is available on the NDSU grain drying and storage Web site. You also can use the chart for your fan that shows the amount of airflow delivered at various static pressures.
Soybeans can be dried in a high-temperature dryer, but the plenum temperature needs to be limited to minimize damage to the beans. Refer to the manufacturer's recommendations for maximum drying temperature. Typically the maximum drying temperature for nonfood soybeans is about 130° F. Even at that temperature, some skins and beans will be cracked.
One study found that with a dryer temperature of 130° F, 50-90% of the skins were cracked and 20-70% of the beans were cracked. Another study found that 30% of the seed coats were cracked if the drying air relative humidity was 30%, and 50% of the skins and about 8% of the beans were cracked at 20% relative humidity.
The relative humidity is reduced by half for each 20° the air is warmed. Therefore, if air at 40° F and 80% relative humidity is warmed to 60° F, the relative humidity is reduced to 40%. If it’s heated to 80° F, the relative humidity is reduced to 20%. Monitor the amount of damage occurring during drying and regulate the plenum temperature to obtain the acceptable amount of damage.
Food soybeans and seed beans must not have damage to the seed coat, so natural air or low-temperature drying is the preferred drying method, Hellevang says.