And in the fall you could try some green manures, that is, cover crops that you let grow over winter and till under in the spring like clover, vetch, rye. If the horse manure isn't fresh, you can use it now. Petty much, if it doesn't smell, it's composted enough to use, although use it sparingly if you don't know how long it has been composting.
We are able to get free horse manure from a local stable and she doesn't use wood shavings mixed in like most of the other stables do. When we get it (about every 6 months), it's been cooking in her pile and could be used right away but unless it's fall and we're just top dressing for winter, we let it compost in a huge pile to use in the spring.
I have mulched with grass clippings when I didn't have straw. When the garden is done, you can work it right into the soil.
