A peak through the high tunnel door that shows the lush plants with nice fertile soil.

The Best Organic Garden Soil Amendments

Throughout my years of avid gardening, I have come to learn that the trick to a bountiful garden is nutrient rich soil. Soil health is a complex and intricate science that a person could spend years studying. We have many ways that we help maintain fertile soil in our gardens. Keep reading to learn some of the best organic garden soil amendments. 

Before we jump into amending the soil, I recommend doing some soil testing for at least the first year. This way you will have a general idea of what your existing soil is lacking based on your soil test results.

The History

For years, we would rototill our garden every spring prior to planting. As the years passed and my gardening experience broadened, I determined that the garden might thrive better with a no-till gardening method. Consequently, we slowly switched to a no-till gardening method with the use of mulch and organic fertilizers. The result has been fewer weeds, more worms aerating the soil, and much healthier bacteria being allowed to break down organic material in the soil.

A handful of rich dark garden soil.

How to Organically Feed the Soil

There are many ways to feed our soil: commercial fertilizers, composted vegetable matter, aged manure, and liquid plant-based fertilizers like seaweed and kelp. I have tried all the above, excluding inorganic amendments and commercial fertilizers.

Using organic materials will help improve soil structure, enhance water retention, and increase microbial activity, creating a healthier plant ecosystem. Unlike synthetic fertilizers, organic amendments release nutrients slowly, providing long-term benefits and reducing the risk of nutrient runoff that can harm the environment.

When most of us think of fertilizing or adding compost to our gardens, we think about feeding the plants, and we are, but we’re also feeding all the microbes in the soil, which increases plant health. The happier the plant, the more nutrients the vegetable or fruit has, which makes it more beneficial for our bodies when we consume it.

Types of Organic Fertilizers used for Soil Amendments


Compost is often considered the best soil amendment of organic vegetable gardens. Plant-based compost is made up of various organic matter, including, but not limited to, kitchen scraps, animal manure, leaves, and grass clippings. Organic compost is a slow and steady way to release nutrients into the soil while increasing microbial activity.

Composting is a simple process and can be done in a very small area. When composting your kitchen scraps you will not want to avoid meat, poultry or cooked food.

Just dump all your scraps into a pile, bin, or old garbage can drilled full of holes, and turn it often. Continue to add scraps and clippings as the season goes on. By fall, you will have rich, loamy compost that smells fresh and clean.

  • Improves soil structure by increasing soil aeration
  • Enhances water retention
  • Provides essential nutrients and micronutrients for the plants or worms in the soil
  • Encourages beneficial biological activity
How to Use:
  • Mix compost into the top few inches of soil.
  • Apply as a mulch around plants to retain moisture and suppress weeds.
  • Use in potting mixes for container gardening.


Manure from your farm animals or local farm animals, such as cows, horses, pigs, and chickens, is an excellent organic soil conditioner. It is rich in nitrogen, phosphorus, and potassium, essential for plant growth. However, it cant not be fresh manure. It must be well-aged or composted to avoid burning plants with its high acid content.

  • Rich in essential nutrients – if not aged, it is so acidic it can kill the plant roots
  • Improves soil structure by adding aeration to the soil and microbial nutrition
  • Increases organic matter content, which increases the worms in the soil
How to Use:
  • The best time to Incorporate well-aged manure into the top six inches of the soil surface in the fall to allow it to break down over winter. You can also use manure that is aged for a year and add it to the hole that you are placing your transplant in, covering the manure with a few inches of garden soil before planting.
  • Use composted manure as a top dressing or mulch. By doing this, you will ensure that your plants will be supplied with fresh nutrients throughout the whole growing season.

Worm Castings 

Worm castings are produced by earthworms breaking down the organic material they eat and then excreting it. They are a highly concentrated source of plant nutrients and beneficial microorganisms, making them an excellent addition to any garden. Adding aged manure or compost into your garden in the fall will naturally increase the number of worms in your soil. You may not need to add more worm castings. Worms love the fresh composted manure.

  • Rich in nutrients, particularly nitrogen
  • Enhances soil structure and aeration along for strong root growth
    Increases microbial activity
  • Improves seed germination and plant growth
How to Use:
  • Mix into potting soil or garden beds.
  • Use as a top dressing

There are many other soil amendments I could go into here. Bone meal, blood meal, rock phosphate, kelp and fish fertilizer. But the truth is, these treat very specific poor soil issues that can usually be completely avoided by using manure and compost.

A brown bag of garden fertilizer that has a green and white label.

How to Choose the Right Organic Soil Amendments

Choosing the right organic soil amendment depends on your soil type and your plants’ specific needs. For best results, conduct a soil test in your vegetable garden to determine what your soil lacks and what amendments will be most beneficial to create a healthy soil.

We garden outdoors and in a high tunnel and use the same fertilizing methods. I start with a 6-inch layer of aged horse and cow manure early in the spring, weeks before I start planting. Then cover the manure with old hay. Due to its lack of seeds, I prefer to cover my garden with sedge grass hay or lawn clippings.

We live in the interior of British Columbia, Canada where the weather is scorched in the summer. The extra mulch helps retain moisture and keep the soil thriving in the topmost layer. However, if you live in a wet area and deal with slugs, thick mulch may not suit you.

I pull back the mulch and plant right into the manure. once the manure is aged for at least six months, preferably a year, there is little risk of burning the roots of your plants.

Once the plants are established, I continue to apply mulch as needed. I use partially rotted manure (less than six months) because I’m top-dressing, which has minimal risk of burning the roots. For plants that like extra nitrogen, such as garlic and onions, I will reapply manure around the base of the plant’s mid-season. I do the same method as in the main garden in my high tunnel. The only difference is that I add a tomato-specific organic fertilizer at the beginning and middle of the season. I take extra care to add compost and mulch in the fall once all the plants have been removed. High tunnels dry out during the winter, and the added mulch and manure help keep the ground moist and soil thrive

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