Was the horse manure that you added well-aged and fully composted - or was it new?

You should never add and new horse manure on a veggie garden because it decomposes and raises the temperature of the soil. This can burn the roots of the plants and kill them off.

In the way of organic fertilizer - you certainly can add organic fertilizer. Either just add some light grass clippings or leaves - or you could also get a bucket and put some dead leaves in it, add some water, and the water should turn a brown color - like tea. This is perfect liquid fertilizer.

Here is a link that might be useful: BsnTech Gardening Blog

If the leaves are turning yellow, it may be because of overwatering. As far as your beds are concerned, when it rains, bang them into the ground with a hammer lightly each time it rains and it will compact itself, not beating it, just tapping it until it sinks in just enough there is no gap above ground. If your bed is made with concrete or something brittle, don't use a hammer on it, maybe a rubber mallot or put a square made from a few layers of cardboard down where you want to bang on it, so it won't chip your concrete. You may just want to stand on them a little instead of banging anything. Don't do any of this if you think it will bust your bed walls.

I would take a guess to say you have already added as much manure as you need and the horse manure at the bottom is soaking in through the roots of the plants. If you add more, it may kill them. All fertilizers, even Miracle grow, will burn plants if too much is added, so more horse manure on top may be too much.

If you want to loosen up the dirt, don't do it too much. The plants leaves will go limp. Maybe keep a short distance from your plants and then loosen it.