Whitman uses personification to show that all his surroundings are inherently full of life and serve as ‘beautiful’ spiritual markers that embody one’s soul. Whitman personifies his surroundings by using ‘minister’ not in its traditional religious sense, but in the connotation that a minister is a spiritual guide or marker.

Also, dumb isn’t an insult. Here it means unable to speak. By adding novices, he makes this refer to both the people and the environments. As with the rest of the world we can learn from others, so each person is a minister: someone who teaches something, but each person is also a novice: someone who has much to learn.

Whitman deleted ‘you novices’ from the poem for publication of Leaves of Grass 6th edition 1881.

Solids and fluids refer to two things. First, the body and the things that go into making the body (http://rapgenius.com/Walt-whitman-crossing-brooklyn-ferry-lyrics#note-1262103, again, ew). Second the things around people, like the river and the hills he mentions. Whitman was big on learning through experience and making it on your own (ironic since he wrote Leaves of Grass while living on a friend’s estate), so he’s saying that these things people interact with help them learn and better themselves through their experiences with them. The last part means these experiences make people who they are.

A significant alteration: Whitman deleted this entire stanza from his final version of “Crossing Brooklyn Ferry.” The poem reaches its final state in Leaves of Grass 6th edition 1881.