How many years have they been in the tub? It might be time to take them out and replace some of the soil with fresh. If the manure is well composted it should be ok. However if it is still hot I think it would be best to compost it more over the winter. I just reworked some of my lily beds and dug in steer manure that I bought in bags at the store. In the spring I will toss a couple of inches of my garden compost on top of the soil. That should make them happy!