I found this resource today on the UGA Extension website, while I was looking for a recommendation on converting an inorganic fertilizer specification to use organic substitutions. This is exactly what I was looking for!
We all have our own preferences regarding the fertilizers, soil amendments, and treatments that we use in the garden. At my house, we prefer not to use synthetic chemicals. Sometimes I come across good advice or new ideas from conventional (non-organic) sources, and I want to put them into play in my garden without introducing chemicals. This document from the UGA Extension is a good reference for substituting organic fertilizers.
Most synthetic fertilizers are stronger than I like to use, personally. Common N-P-K values for synthetic fertilizers are 10-10-10, 5-10-15, and 20-20-20. As a general rule of thumb, I don’t use fertilizers with nutrient percentages in the double digits. So, when I see 20-20-20, I think – hot damn that’s some strong hooch! We stick to single digits at my house. Typically we follow the general recommendation for a 4:1:2 N:P:K ratio, and my favorite all-purpose fertilizer is a local brand that’s rated at 8-2-4. We change this up, as appropriate, for different crops that require more or less of a given nutrient.
The document below gives a quick overview of the differences between synthetic and organic fertilizers, and a quick explanation of the way fertilizer labels are formatted. Then it dives into the process for making organic substitutions when synthetic fertilizers are recommended. It takes into consideration the pH balance of the soil and the fertilizer, and nutrient availability in different climates and seasons.
There is a step-by-step example for converting synthetic to organic fertilizers on a per acre basis and on a 1,000 square foot basis. There is even an example to help convert a recommendation for a 100 foot row.
Look at Table 1 (towards the end) for a good reference about the N-P-K content of many common organic fertilizers, including a rough timeline for nutrient availability.
Table 2 has alternatives for common synthetic N-P-K ratios like 5-10-15, 6-12-12, and 10-10-10. Many possible substitutions are given for each nutrient, so you should be able to put together a good recipe using the resources that are available in your area.
Many thanks to Wayne McLaurin, Water Reeves, Julia Gaskin, David Kissel, Glen Harris, and George Boyhan of the University of Georgia and Fort Valley State University. Produced and published by the UGA Extension.