How to Measure Your Favorite Organic Fertilizers

a-farmer-adding-fertilizer-to-his-soilI 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.

Click Here to View or Download the Original PDF from the UGA Extension

Happy fertilizing!


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.

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Michael Ford


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Michael has been the resident editor at The [Grow] Network since January 2015. Michael grew up in St. Louis, where he became a lover of nature - hiking and fishing his way through the Ozark hills in Missouri. He attended Baylor University in Waco, TX, and he currently lives in Austin. Michael has background experience in small-scale farming, commercial growing, vegetable gardening, landscaping, marketing, and software development. He received his Permaculture Design Certification from the Austin Permaculture Guild in 2013.


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4 Comments
  • geraldc

    Thanks for showing us this paper published by the UGA Extension. In Savannah GA area a man had used chicken litter on his garden for several years and the garden just did not produce like he though it should. His boy, starting a high school agriculture class, asked if the chicken litter could be analyzed, so being a good father he said, “go ahead and do the analysis test.” So to send a sample to UGA they used a ton of chicken litter, soaked in water for 2 months, put the liquid in a clean jar and sent to UGA. This was done 3 times while the boy was in high school. Each test came back as .5 – .5 – .5. Point is you need to know where these samples are coming from and what is being fed to these animals and what is being sprayed in/around these pens where samples are taken. Example of RoudUp spray drift: test on 3 farms 500 miles apart for 3 years – RoudUp ready weeds were found an average of 15 miles away from the fields sprayed. Not saying RoudUp was spray around these test samples but we do not know what is sprayed anywhere, by road crews, by tree farms, or from aircraft.

  • Anna

    Hi,

    Could you please help me with fertilizer for my container growing. I’m at a loss as to how much fertilizer to use and fear burning up the plant. I use earth boxes and some pots. Also, I have four (4) blueberry bushes and am clueless as to how much fertilizer I should use for each plant. I do use organic compost, but sometimes a good fertilizer is needed.

    I’m also having a problem getting my plants to bear the produce (tomatoes, squash). I’m assuming I have not fertilized enough?

    Your instructions are great for someone with a large garden, but unfortunately mine is tiny!

    Thank you.

    • Hi Anna – Sounds like you’re already aware that you need more fertilizer in your containers. A lack of nutrients, or an imbalance of nutrients, would explain why your plants aren’t bearing fruit. Regular fertilization is especially important in containers. I would make a 2-part recommendation for you:

      1 – Start a regular regimen of liquid fertilizer for each container. For most plants, I think fertilizing every 2 weeks is sufficient. During heavy production, I might fertilize a tomato plant once a week, depending. For plants I am really nursing, I usually apply some other non-fertilizer supplement on the off weeks, like seaweed or compost tea. So, a container fertilizing schedule at my house might look like: Week 1 – liquid fertilizer, Week 2 – aerobic compost tea, Week 3 – liquid fertilizer, Week 4 – liquid seaweed, repeat. If you use a well-balanced organic fertilizer, there shouldn’t be any concern about burning the plants – what I use is a fish emulsion based fertilizer that is 3-1.5-2. Burning would be a concern with a stronger formula like 24-8-16.

      2 – Next time you plant, mix a solid granular fertilizer into the potting soil, throughout. Most granular fertilizers will have a time-release component, and that will help keep your potting soil vital over the course of the season. Use an organic fertilizer with single digits for the N-P-K macronutrient values, like 2-2-2 (not 20-20-20). For general use, you should look for something with a 4:1:2 ratio, like 8-2-4. You might use a different ratio for tomatoes and peppers, etc. Here’s a great granular for that purpose, 2-1.5-3 – no worries about burning your plants with this http://maestro-gro.com/tomatoandpepperfood.aspx. A good product will have a recommendation on the bag about how much to use. When I don’t have a documented recommendation, I use 1 big handfull per plant. You don’t need to be super-scientific when you’re working with safe fertilizers.

      Good luck! Hopefully some other people from the community will chime in with their recommendations too…

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