We use a series of electrical heater bands attached to an auger tube as the oven. This efficiently heats and roasts the grain as it travels up the auger. The grain enters through the auger in the bottom of the hopper and roasted grain and steam exit out the top end of the unit. The grain is in the auger oven for about 5 minutes from bottom to top... 

It's a very simple, effective & reliable design.

As far as temperatures for soybeans...  The "oven" temperature runs between 525°-575°F to reach an ideal soybean roast temperature of 280°-300°F. The roasters are highly adjustable and can be used for other grains as well. Corn for instance needs to be set at a higher temperature and that is easily achieved with the digital thermostats.