Manure....muck........compost..........orgainc material, by the ton if need be.
Fertiliser only feeds the plants, organic material feeds the soil. Plants will not continue to thrive in poorly fed soil no matter what fertiliser you add.

For veg growing, as well as digging in as much organic material as possible (grow a green manure over winter and dig it in in spring... grazing rye or winter tares are good), try covering the soil with a good mulch of half rotten straw or anything you can get hold off, like half rotted grass cuttings. This will both keep moisture in the soil and also break down over time to add humus into the soil. To sow or plant, you just drag the mulch back, and push it back into place around the growing plants or once the seedlings have come up.
Don't dig it in as this can cause nitrogen robbery but on the surface it will cause no problems and eventually as it rots down it will add nitrogen to the soil and the worms will take it down in winter.
If you are in a rural area, most farmers seem to have a big pile of rotting spoiled straw somewhere which they are happy to give away, if you have or can borrow a trailer to transport it.