Hmmmm, well from where I stand it looks like you have a little too much organic matter already. I would not add more. A couple of buckets of sand and turn the soil with a fork to get some air into it and it should be just fine. Throw in some earthworms too.
Just a word about soils. Natural soil consists of clay, silt, sand, organic matter, water, air, chemicals, and a host of small living things. The clay silt and sand vary in percentages depending on where the soil is found and what formations it was derived from. These three things make most of the bulk of the soil. The finer particles hold water well. Clay has really fine particles. silt a little larger particles and sand even larger particles. A good soil is 10% clay, 30% silt, 30% sand, 10% organic matter, and the remainder made up of the other things found in soil. The things most often lacking in soil is nitrogen and water. You know where water comes from. The nitrogen comes from decaying organic matter and from the air. The air is rich in nitrogen, but it is not useable in that form. When there is lightning nitrogen is fixed (converted to a form the plants can use) and comes down with the rain.
OK, anytime any of these things in the soil gets out of balance, like too much or too little of it, then you have problems. In this case it looks like too much organic matter and too little air.
Manure is to be used as a soil amendment. A little goes a long way. An inch deep on an area is plenty.
Artificial soils are usually made from peat, perlite, and vermiculite which are all devoid of plant nutrients. They serve as root anchors. Then usually some organic matter is added like a bit of compost or sawdust then some chemical fertilizers. These fake soils are light weight and have been used a lot for pots and do ok as long as you keep adding fertilizer. The big problem is that they often are way lacking in some of the trace minerals that are found in clay and silt.