You fertilize your tree just by placing fertilizer next to it. Just make sure that said fertilizer isn't in a trench, or it will fertilize the trench and not the tree. How much fertilizer the tree gets is based on the fertilizer's placement. For example, if the X's are fertilizers, the 0's are the tree, and _ is ground:
The blue X will only give the tree one point of fertilizer, since it's only touching one part of the tree. The purple X will give the tree two points, since it's touching two of the tree spaces. Finally the red X will give three points, since it's touching three spots on the tree.
Now, to get a crop up to five stars, it needs 256 points. Assuming that points on spots stack, a tree arranged like this:
will get 32 points a day. 256 divided by 32 is 8. So, if you arrange it like that, your crops will be at 5-stars in 8 days. 16 fertilizers cost a lot, but once the tree is at 5-stars, it doesn't need any more fertilizer; it'll be good to go. The crops from the trees will surely make a profit at 5-stars.
Finally, you don't need the seed maker to get it to five stars (though if you want more 5-star trees without the need for fertilizer, you will need the seed maker).
Sorry if this was an information overload. I just wanted to cover the all the bases. Hope this helps!
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