What we're talking about when we're talking about organic fertilizer is a source of food for the soil microbes. Those microbes eat the same food we do. In fact the microbes that live in the soil are cousins to the microbes that live in our saliva, stomach, and intestines. When you think about fertilizer, think about what is already in the soil and what you can add to improve it.
Plants use photosynthesis to create sugar. Plants send that sugar to the roots to deal with things going on in the soil. They reserve the protein they create for seeds. When a seed falls to the ground, it will use that protein to create a new plant. In any case, the soil microbes do not get protein from plants. They normally get protein from dead animals and grasses that get stomped into the soil by other animals. The idea behind organic fertilizer is to control the application of protein. Since plants themselves supply sugar to the soil, we help with the protein component. If you look at the ingredients of a bag of organic fertilizer you'll find ground up seeds (corn, wheat, soy, cotton, alfalfa, and even coffee), animal byproducts (like feather meal, blood meal, bone meal, or meat meal) and other sources of protein.
What does not contain protein? Molasses, corn cobs, tree leaves, compost tea (and I'd argue compost has very little), greensand, and straw. These materials have other benefits to the soil but feeding protein to microbes is not one of the.
Of the popular grains, here is a list in order or protein content from highest to lowest.
corn gluten meal
soy bean meal
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