This list gives the impact factor for agriculture and forestry journals, so you can see how influential they are. I'm afraid I have no idea how these scores are calculated, sorry! More usefully (for me anyway) it lists almost 500 journals in the field. Unfortunatley the specific focus of each journal is not given, though this can often be inferred from the title. (If you want a rather more conciselist, considerThe Digest's '7 best permaculture-related journals' instead).