Unfinished compost in planters

Yes basically fill the bottom layer with the hens straw anything really then a layer of manure then a layer of the old compost forked through the manure then finish with a good top soil compost mix.

You could always add some gravel or similar in the bottom to help with drainage and they won't need as much filling.

I have  10 x 4 ft planters, they are 5 decking planks high,if i use manure as a filler could I use Fresh manure, they are huge, I might grow flowers this year in them and do veg next year. They will cost me a bomb to fill and my neighbour has horses

You could look at doing something like hugelkulture.  I found one thread on here but someone else asked about filling the base of raised beds with cut branches recently, I just can’t find that one 

I have seen it done in raised beds before and it seemed to work well.  You just keep the top fairly level, rather than dome it up.    The soil level did fall over time, but it took a couple of seasons and was rectified by top dressing the bed between crops.

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