How big is the piece you wish to protect? You might get by much better with a WC-Co or a ceramic coating.
Keep in mind that zinc is often used as a "sacrificial" coating. This means that you expect it to corrode because in doing so, it prevents corrosion of the steel that it is laying on.
You may want a "barrier" coating. This would be a layer which is inherently resistive to corrosive attack. It requires, however, that it be more hermetic than a zinc plate layer.
The zinc-fertilizer reaction may be organic in nature, but it could very well be a simple acid etching reaction (many pesticides are acetic acid based) or an oxidation reaction. For many applications, a sacrificial zinc coating is the way to go. It's cheap and is appropriate for complex (e. g., agricultural) environments. You simply need to optimize the coating thickness and integrity. You also need to periodically inspect the surface. Zinc paints are a cheap, easily replaceable option.
If you want a barrier coating, thermal spray WC-Co or pure ceramics may work well. If the part size, geometry and thermal cycling complicate things, then you might switch to a thermal spray metal. I assume there has been research into which alloys work particularly well with agricultural chemistries. Possibly a high nickel alloy, possibly an MCrAlY or Monel. USDA or
may have some listed.
You also can try other platings such as nickel (both electrolytic and electroless) or chromium.
Another factor may be physical abrasion of the zinc. If this is a factor, then you want a harder coating material. ZnCo is harder than pure Zn.
in diameter and are 100mm (4"