Agricultural Bond Improvement Act of 2005 - Amends the Internal Revenue Code to: (1) exempt agricultural bonds from the volume cap applicable to private activity bonds; (2) permit a Federal guarantee of an agricultural bond under the Consolidated Farm and Rural Development Act without a loss of tax exemption; (3) increase the loan limit on agricultural bonds to $450,000 and index such amount for inflation after 2005; and (4) eliminate the dollar limitation in the definition of substantial farmland used for agricultural bonds.