Sheet mulching and surface composting. This will not only improve the soil, it will deter weeds, attract friendly soil creatures and fungi and it will hold water and help keep the soil from drying out.
The way you do it is to get a bunch of mulch and spread it as thickly as you can on the ground around your plants with out burying them. That's the basic. You can work with different materials in different layers to achieve different effects and add different nutrients. And it also just depends on what you have to work with. I like to lay down a layer of news paper, several sheets deep, and cover this with wood mulch when I can.
My neighbors use cardboard covered with hay/compost.
If you want a deeper layer for faster improvement, get some bales of hay. Lay down the hay about as deep as you can, leaving an inch or two on top of it. Cover the hay with newspaper (to prevent any sneaking seeds in the hay from getting anywhere) and cover the newspaper with untreated wood mulch. Water the hay and the newspaper as you lay it down.
Here are a couple of articles that talk about different ways to do it:
But they basically boil down to just composting organic matter in place on top of the soil -- simulating the debris rot that happens naturally in the litter layer of a forest.