Must agree about horse muck, nothing better in my opinion. I guess you have heavy clay in your area. But also, really, anything that you can dig in, dig in - if you haven't got a compost heap, start one now and put all your kitchen waste on it to rot down and dig that in. Pile up weeds, let them rot down a bit and dig them in too, as TeeGee says, humus is the answer and you will only build up a good level by adding vegetable matter of some sort.