This paper proposes an integrated, ontology-based agricultural information system (AIS) to provide all-round and precise information for efficiently guiding farmers and agri-professionals to conduct agricultural processing. Since the existing independent AIS platforms can only offer specific but incomplete agricultural information service, aiming at this issue, the newly proposed AIS system employs ontology techniques, including RDF-based representation and semantic reasoning, to integrate the index information provided by all involved independent agricultural information platforms. As a result, this newly proposed AIS system can provide users with integrated and accurate response information. Through a realistic case study and relevant experiments, it is clear that the accuracy ratio and integrity ratio of response information offered by this ontology-based integrated AIS can be enhanced to a great extent. In summary, not only agri-professionals, but also farmers, who might not understand information retrieval skills, can benefit from this newly proposed AIS to conduct activities in agricultural production life cycle.