How to Make Your Own Slow-Release Fertilizer

Note 6/14/12: Appalachian Feet recommends fertilizers as a way to improve soil in new gardens and planting areas. For established plantings and beds we recommend more sustainable fertility like hugelkultur, plant nutrient accumulators, biochar, cover crops, and livestock manures.

Thoroughly exasperated at the bank-breaking prices of tiny organic, slow-release fertilizer packages, I decided it was surely cheaper to make my own.

Organic slow-release fertilizer is much less expensive when made from scratch.

Photo Caption: Organic slow-release fertilizer is much less expensive when made from scratch. I store mine in large plastic tubs with the recipe written in permanent market on the side.

I began by researching ingredients that would provide the correct NPK ratio (Nitrogen, Phosphorus, Potassium). I also wanted to include micronutrients — elements such as calcium that plants need in smaller doses than NPK.

I don’t fertilize everything I grow but I do like to get most of my vegetables producing at their peak ability.

Though I knew I would be unable to get around some mined and processed ingredients, I at least wanted to avoid livestock factory farming byproducts.

My hope is to eventually find a recipe that is entirely sustainable. For example, I’d love to get a modest dovecote installed over my compost pile so that the doves can fertilize my compost with extra phosphorus.

For now, the “P” answer is really old bird poop — otherwise known as rock phosphate. This was by far the most expensive thing to buy in bulk. However, my 50 lbs bag will last for a long time. Very little is needed in the recipe and once you’ve applied rock phosphate it usually stays in the soil for more than one season. If my soil tests next year indicate that my garden has plenty of phosphate, I will only add the cheaper ingredients of my homemade fertilizer to balance it out.

The “N” ingredient is the least expensive. Cottonseed meal is high in nitrogen and as a textile byproduct is also low in cost. The larger the size you can buy, the cheaper the product is overall. I purchased a 50 lbs bag of this as well.

For “K” I used greensand. I would have preferred kelp meal but I could not find it in bulk near me. If you have a garden center that stocks large bags of kelp meal you can substitute it for the mined greensand.

A closeup of the homemade fertilizer shows a confetti of ingredients.

Photo Caption: A closeup of the homemade fertilizer shows a confetti of ingredients.

For micronutrients I used pelleted limestone for calcium and magnesium. Limestone changes the pH of soil and as my soil is naturally the correct pH I did not want to use too much limestone in the mixture. Therefore, I also used pelleted gypsum, which adds calcium and sulfur but does not alter the pH.

Finally, magnesium sulfate (Epsom salt) was used to add an additional source of magnesium and sulfur.

I was pleased to find all these ingredients already in stock at my locally owned feed & seed store.

The presence of certain nutrients signal plants to behave in specific ways. Heavy nitrogen tells a plant to form leaves while potassium helps roots, improves plant hardiness, and bolsters disease resistance. Phosphorus encourages flowers, fruit, and healthy roots.

Because of that, I created three different blends and labeled the plastic tubs accordingly. Only one of the blends uses the rock phosphate. Depending on soil conditions and plant needs, phosphate may not need to be added each season. I plan to use a soil test to determine if the garden needs it or not.

These are the recipes I used:

Phosphorus: Fruits/Vegetables/Seeds/Flowers/Roots
-4 parts cottonseed meal
-2 parts greensand or kelp meal
-1 part rock phosphate
-1 part gypsum
-1/2 part lime
-1/2 part Epsom salt

Nitrogen: Foliage
-7 parts cottonseed meal
-1 part greensand or kelp meal
-1 part gypsum
-1 part lime
-1/2 part Epsom salt

Potassium: Roots Crops
-4 parts greensand or kelp meal
-3 parts cottonseed meal
-1/2 part gypsum
-1/2 part lime
-1/4 part Epsom salt

Apply at rate of 1/2 cup per transplant


Apply 5 lbs per 100 square feet


Apply 5 lbs per 100 row feet

If you’d like to read more about homemade organic fertilizer I’d recommend this Mother Earth News article by Steve Solomon. His trusted recipe calls for a couple factory farm ingredients like blood meal and bone meal (which you can substitute). However, the article is very informative and also has a thorough list of low, medium, and high demand plants so you can get the most out of your fertilizer applications.

Eliza Lord

I'm a Greenville, SC native (the Appalachian foothills) who wears the hats of Greenville Master Gardener & Upstate Master Naturalist. I love to write about food and sustainability.

18 thoughts on “How to Make Your Own Slow-Release Fertilizer”

  1. silica - September 11, 2009 8:53 am

    What a fantastic site! Vegetable gardening is still well beyond my patience level but reading this site makes me FEEL as if I’ve moved off the grid and eaten a full day’s supply of leafy greens. I’m sure that just looking at this site boosted the micronutrients in my own system. Looking forward to the next meal…er, post.

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    I live in East Tx and would like assistance in growing onions!

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  13. Country Mouse - June 10, 2011 9:31 am

    What a great idea! Very informative post – As a native plant gardener I don’t use much fertilizer at all, but a little bit does help with propagation of sturdy new plants – Now you’ve got me thinking about mixing my own! I already have rock phosphate – which I didn’t know was “old bird poop!”
    Country MouseĀ“s last blog post ..Whats the Madia with These Plants

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  15. Harley - March 18, 2012 9:16 pm

    I really like your recipes this will work for me. But what size container do you use to make up your parts? 1 gal, 1 quart, i just wondered. I was going to use a galllon can.

    1. Sustainahillbilly
      Twitter: appalachianfeet
      - March 19, 2012 12:03 pm

      It really just depends on how much fertilizer you want to make. As long as there are equal parts, it doesn’t matter how large or small your measuring cup is.

      1. Babak - October 22, 2012 5:42 am

        Is there any catalog or book about the methods of slow-release fertilizer making as plate or tablet?

        1. Sustainahillbilly
          Twitter: appalachianfeet
          - January 16, 2013 10:06 am

          I don’t know. I will say that I’ve mostly switched to hugelkultur and am not using fertilizer at all now (other than composted livestock manure).

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