to answer your second question you can start using nitrogen fixing plants... in my part of ohio everywhere is clay... on my particular property I look for what is growing there.... we have white clover... sweet bushclover *melilotus* chicory (taproot, breaks up soil) and a melee of of assorted grasses and forbes...i'm trying a daikon radish strain this year to see if i can break up the soil and add organic material. .... these plants change the structure of the soil over time... depending on how quick you want results you may be thinking about incorporating things into the soil... my own personal view on this, especially with clay soils, is that changing the structure of the soil can be quite detrimental... although i still dig holes to put my trees in etc... i try to use the plants as much as I can to do the work... the earthworms and a bunch of other organisms are doing a bunch of work too....in any case... adding organic material would be very good... biochar, mulches, compost... comfrey and nettles will grow well in clay soil.. as will most alders and willows *as these are coppiced they add organic material to the soil and make it more friable*