When we apply these fertilizer salts they are dissolved and hence no longer a nano-particle, instead it is an ion like any other ion absorbed from elsewhere like roots. In fact the concept beyond the nano-fertilizerz remains questioned and unless any particular difference with regular fertilizers is proved, it could be considered not more than a trade propaganda in my view.
As you know any salt becomes ionized after dissolving in water- this is a central dogma in physics/chemistry (http://en.wikipedia.org/wiki/Ion).
I think that the claim that such so called nano-fertilizers act different while they are essentially salts which ionize upon entering water should be proved and described by valid methods.