While manure is used as fertilizer it is not, technically, fertilizer - it is poop. So to use it effectively you need to spread it out - which is where the Manure Spreader comes into play!
The following are all towed equipment requiring a tractor to utilize...
The following pretty awesome new additions to this section were added to the base game as paid DLC..
Notes: While this would be a lot less efficient and effective than commercial or chemical fertilizers, it has a couple of things going for it - like it is a free byproduct of your cows, and at least in theory (assuming you are not pumping your cows full of chemicals) it means you can claim to be organic in your fertilization... Right?