Soybean quality is highly dependent on how well they were ventilated and dried. These factors are critical to safeguarding the processing and germination potential of domestically grown soybeans. Soybeans can be dried either at a low temperature or with ambient air. Let’s take a look at both options!
Low-temperature drying
When soybeans are damp (> 16%) and there are few dry days, the air used to aerate the soybeans will need to be heated to reduce their moisture content down to the 12% to 14% range, which is ideal for conservation. However, two conditions must be met: aeration must be effective (with a fan powerful enough for the volume of soybeans, a suitable air outlet, and levelling of the soybeans at the top of the silo) and—above all— the right amount of heat must be applied, as too high a heat will reduce soybean quality. So soybeans can indeed be dried, but at a very low temperature. More specifically, increasing the ambient temperature from 3° to 5°C (to a maximum of 50°F) will prevent the air and soybeans from becoming overly dry, which would cause quality to deteriorate. Every 10°C increase in temperature cuts air humidity by 50%. Air that is too dry and too hot damages soybeans for both human consumption and seed. The final temperature of air leaving the dryer should be no higher than 25° to 35°C (80° to 90°F).
Important: Ventilate with warm, dry air, but make sure it’s not too hot!
Ambient-air drying
Whether heat or ambient air is being used, soybeans in a silo need to be levelled out to ensure smooth airflow. Ambient-air drying can be used only if the relative humidity of outside air is lower than the soybean equilibrium moisture content. A fan should not be run continuously without regard to the air’s relative humidity, otherwise the soybeans will regain their lost moisture as relative air humidity rises (for example, overnight). In other words, the fan should only be started if the outside conditions are right for ambient-air drying. Note that air temperature must be below 30°C and air humidity must be between 50% and 70%. It takes 20 to 30 days of ambient-air drying to achieve the right moisture content. A number of factors will affect the required drying time, such as soybean moisture at the outset, weather conditions, fan power, and the depth of the soybean layer. If your goal is to reduce soybean moisture, the fan should only be started if ambient-air humidity is less than 70%. If the fan is pushing air from the bottom upward and there is moisture at the bottom of the silo, soybean moisture will migrate higher until it is exhausted from the silo. Taking samples throughout the process will let you assess the stage you are at.
Understanding this chart
For example: If your soybeans are drying at an air temperature of 10°C and a relative air humidity of 80% over a long period of time, their moisture content will be roughly 15.7% (hygroscopic equilibrium point). At the same temperature but with 90% relative humidity, your soybeans will have a moisture content of about 19.6%; conversely, the same temperature with 60% relative humidity will produce soybeans that have an 11.2% moisture content or so. Thus, if you want soybeans with a moisture content below 14% but above 12%, aerate them with air whose relative humidity is between 65% and 70%.