How much K2SO4 to add?

The older leaves of many of my faster growing plants look a brown with some
small holes.  Reading here, this may be a sign of a K problem.  So I found 
got some K2SO4.  I no longer have tables of Molar masses so how much of this 
stuff do I have to add to raise the K level 1 ppm in 1000 liters of water?

The FAQ says I should aim for about 5ppm.  It there a test kit for K?  If so
who sells it?  Is there any data on how fast this stuff is used?


Ed Tomlinson (tomlins at cam_org)
Montreal, Canada

Home of pppdial: http://www.cam.org/~tomlins/pppdial.html