The best "food" for plants are plants themselves! If they grow, they have to have the things they need (of course). Thats how it works in the nature: plants take nutrients from dirt, and then leaves (and sometimes whole trees) containing them fall to the ground and so the nutrients are back in the dirt. But it's a little messy. They have to decompose. To prevent that from destroying the good impression plants make, just put the fertilizer plants in the dirt.
As you can see, the idea is very similar to this http://www.instructables.com/id/Super-easy-natural-fertilizer/ but this one helps all kinds of plants.
But but first things first: when you take weeds (you can also use the fallen leaves) out of the ground, cut away the lowest part (roots and a little part of the plant itself - you don't want it to come back to life even closer to your precious plants). Now you can cut it to pieces (but yo don't have to). Now just dig a hole close to your plant (be careful not to harm its roots). The hole doesn't need to be deep-just so dirt covers the leaves. And my advice: don't put all the weed parts in one hole. They will help your plants faster if you don't. Now just put he dirt over the leaves and you're done! It's the fertilizer that costs nothing and is very easy to "make".
Oh, and by the way, don't forget to vote for me in the gardening and green design contest on Instructables!
Thanks for viewing!