look here:
Config->rf management-> <802.11a><802.11g>Radio profile-><profile name default is the one that most ap's are using>
from the users guide:
The Spectrum Load Balancing feature helps optimize network resources by balancing clients across channels, regardless of whether the AP or the controller is responding to the wireless clients' probe requests.
If enabled, the controller compares whether or not an AP has more clients than its neighboring APs on other channels. If an AP’s client load is at or over a predetermined threshold as compared to its immediate neighbors, or if a neighboring Aruba AP on another channel does not have any clients, load balancing will be enabled on that AP. This feature is disabled by default. For details, see