P is potash
K is potassium
n is nitrogen

that is where the vaules of 10-10-10 come from. legumes will produce their own nitrogen and by adding it to the soil you'll promote weed growth. The nitrogen will not benefit the clover at all.

The first # in 10-10-10 represents the amount of Nitrogen(N) in the fertilizer. you'll want one with the lowest first number as possible

http://www.fertilizer.com/university/fertuniv02.htm

this link my help you to get a better understanding of what's in fertilizer.