A method and system for controlling an air cart to deliver custom blends of fertilizer to a field is disclosed. A farmer purchases base nutrients which are mixed together during delivery to the field. An air cart having multiple compartments is selected according to the number of base nutrients and/or seed to be delivered. Metering units associated with each compartment control the rate at which product is delivered. An operator defines a desired blend of fertilizer and identifies each of the base nutrients loaded into the air cart. The controller generates control signals output to each metering unit to control the rate at which product is delivered according to the desired blend of fertilizer. Thus, a farmer may purchase and store base nutrients rather than various blends of fertilizer and create multiple customized blends as needed according to the area and the type of seed to be planted.
