We never have much luck getting any usefulness out of phosphorus that isn't worked into the soil.  Even good rains don't seem to give us much benefit from top-dressed phosphorus applications.   Maybe things are different in your part of the world.  

We can get good benefit from top-dressed potash... provided we get good rains to move the potash into the soil.   But even with potash, beneficial outcome is still more consistent with potash that is worked into the soil.   But again.... maybe things are different on your farm compared to ours.   

good luck,

gordon