Plant it in oats, wheat, or rye grass a couple of times and plow it under. Its cheap and fast. Get your organics up and hold some moisture and worms will naturaly come. Two things stop them,too wet and too dry. They feed on the microbes and things from rotting or decaying organic matter.Thats why you find them under leaves,in manure piles etc. But they cannot survive way down in the ground,because oxegen and moisture levels are too low. Conversly they cannot live if its too wet because they simply drown, thats why you see them crawling across the ground after a heavy rain.By plowing a green manure crop into the ground you create a area a few inches deep with adequate food,moisture,and oxegen. Basically you create a layer of topsoil or humus which is where they live. If you live in a area with lots of rainfall, probably wheat or rye grass would be best. If you live in a area thats normally dry, try oats ,it holds more moisture. I personaly like oats here because i can go down and buy just regular whole feed oats pretty cheaply and get a good stand. Check on the net for the soil requirements for growing the thistles you have. Compare that to the ones you need for the type of clover you are trying to grow,and you will have a very good understanding of what your soil is lacking. Basicaly its a old timers soil test. Every weed has a ph range,moisture content,nitrogen requirement it likes best to grow, just like clover does. By correctly identifying your weeds,you can very closely find a crop that will grow on the same soil. Take a feild of sandburs for instance,it only grows on the poorest and worst soils,nitrogen kills it. If your looking to farm a place covered with sandburs you can bet you have a long row to hoe building the soil. Conversly if you find a place covered with cockleburs they only grow in good soil and that land will grow an adequate crop easily. Looking at a field full of weeds believe it or not is a pretty good,and 100% true indicator of the health of your soil. To actually physicaly change your soils makeup can take a long time. Most folks want instant gratification and get a soil test ,pour on tons of fertilizer,lime and things. But its best if you take your time and change it over several seasons slowly. A good place to start generally is with one of the cereal type grains,wheat for instance will grow on some fairly poor soils. It wont give a bumper crop by any means,but even on poor soils you can get a good stand. Its one of the few crops that even with no fertilizer or anything will. Corn for instance wont make,but wheat will nearly always produce something.Its sort of amazing in that with no inputs at all output will fall to a certain level and stay there year after year. faster of course to pour on fertilizer and things,but you have to remember your trees also,deep plowing,mowing and haying etc ,compacts the soil and is hard on your trees. A crop that you can cultivate shallowly,let grow,come in and imediatly harvest and get out,would be better in a grove than one you have to cut several times a year followed by raking and baling with heavy equipment. Sorry for getting off your original question,but i hope this helps.Look in the back of field and stream or outdoor life magazines,you can buy worms in bulk there,but you need a habitat suitable for them first or your wasting time.If you dont have any now,the ones you put out wont stay either.They will simply either die,or move off.