I have created several raised beds last year in a small area beside the chicken coop. I have filled them last year with aged horse manure. The manure has some saw dust in it so I thought it will be good enough to preserve moisture but this was not the case. We had a dry season last summer and even after watering it the water would not penetrate deep but rather it was soaked in the upper 1-2cm layer and this would dry up very fast. Wood chips behave differently; they let the water pass down into the deeper layers and then preserves the moisture for very long time (for weeks) even in a very hot and dry weather. Lesson leaned and now I'm covering all the beds with a 5-10 cm layer of aged wood chips.