Also, phosphorous is supplied as phosphate, meaning that it is attached to three oxygen atoms and another atome for every atome of phosphorous..... plus they don't like using potassium nitrate cos it's explosive. THe problem with nitrogen isn't that it's a gas, it's the triple bond the gas has between the two atoms of nitrogen in the molecule. It takes a lot of energy or some clever fiddling with reactions to split that bond to get the nitrogen into an accessible state. The bacteria that legumes host are very good at the fiddling, the other main source of natural nitrate apart from animal wastes which isn't a primary source, just a concentration of what plants have already done, is lightning.... it just rips apart the nitrogen (and everything else) and some of it recombines with oxygen....
chrisc