Soil is like the gut of the garden. Healthy soil, healthy plants. Healthy gut, healthy body. So amendments play a pivotal role in enhancing soil quality and plant health. Those seeking to grow the healthiest produce through organic gardening will also want to use organic soil amendments.

Amendments improve soil structure, (called tilth) boost nutrients, and encourage beneficial microbial activity, all of which contribute to healthier plants and more productive gardens. And indeed studies show that the same plants grown in different soil can have different percentages of nutritional compounds, with the best results in small organic farms and gardens. ^[1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8349637/

However…

The topic of organic soil amendments can be confusing so clarification is helpful.

Not All Organic Materials Are Safe for Organic Gardens!

Why? Because non-organic gardening uses soil amendments that are classified as non-organic and organic matter, but which may not qualify as natural organic gardening.

This is due to various reasons related to the sourcing, processing, or content. Organic gardening aims to be sustainable and avoid the use of synthetic chemicals, but not all natural products meet the stringent criteria set by organic certification boards. Here are some examples:

Organic Soil Amendments That May Not Be Organically Toxin Free

1. Non-Organically Produced Manure: Manure is a common organic amendment, but if it comes from animals that were not raised following organic practices (e.g., animals that were fed with non-organic feed or treated with synthetic hormones or antibiotics), it’s not likely to qualify for use in certified organic gardening.
2. Biosolids: Sometimes marketed as organic fertilizers, biosolids are treated sewage sludge. While they are organic in the sense that they contain carbon-based compounds, they may also contain heavy metals, pharmaceuticals, and other contaminants, disqualifying them from organic gardening.
3. Chemically Treated Organic Materials: Some organic materials might be treated with synthetic chemicals to enhance their performance or for other reasons. For instance, some mulches are organic but might be treated with colorants or preservatives. If these treatments are not approved for organic use, the amended soil cannot be considered organic.
4. Peat Moss: While peat moss is a natural, organic material, its use is debated in organic gardening circles due to sustainability concerns. Peat bogs are vital ecosystems and carbon sinks, so some organic gardeners avoid peat moss in favor of what’s perceived to be more sustainable options.
5. Contaminated Organic Matter: Organic matter collected from roadsides, such as grass clippings or leaves gathered from areas adjacent to busy roads or industrial areas. Other potentially contaminated organic materials could come from non-organic farms, or other sources that could contain pesticides, heavy metals, or other pollutants.

Even if you use organic seeds, plants and fertilizer, use of these contaminants can negate the organic soil amendments status.

But not to worry. As one reader pointed out, in today’s world of pervasive pesticides and non-organic fertilizers, etc., it’s nearly impossible to avoid some of the less desirable elements. So again, as with our physical health, we can try to eat the healthiest homegrown foods, but there’s probably no such thing as a perfect diet.

However, we tend to favor the 80/20 rule (aka Pareto’s principle), that so long as 80% of what goes into our garden (and our bodies) is the best, then that will strengthen the microbiomes towards optimal health in both.

Examples of Organic vs. Inorganic Gardening Materials

• Organic Matter: In soil science, organic matter refers to plant and animal materials in their various stages of decomposition. It is essential for soil health, providing nutrients, improving soil structure, and enhancing water retention and aeration. Organic amendments are derived from natural sources, however, they may not be free from synthetic chemicals!
• Inorganic Matter: This refers to mineral-based compounds and other non-living materials in the soil. Inorganic soil amendments include lime, gypsum, and various forms of manufactured fertilizers. While they can improve specific soil deficiencies, they do not offer the same long-term benefits as organic matter for soil structure and fertility.

Types of Organic Soil Amendments

Organic soil amendments are derived from either plant or animal sources, and each type offers unique benefits to the soil:

1. Biochar: A form of charcoal used as a soil amendment, biochar is produced from organic matter through a process called pyrolysis. It improves soil fertility, reduces the need for chemical fertilizers, and helps sequester carbon in the soil.
2. Organic Compost: Made from decomposed organic materials like leaves, grass clippings, and kitchen scraps, compost is rich in nutrients and beneficial microorganisms. It improves soil structure, nutrient content, and microbial activity, making it a versatile amendment for most soil types.
3. Cover Crops: Also known as green manures, cover crops are planted to cover the soil rather than for consumption. When turned into the soil, they provide organic matter and nutrients. Cover crops like legumes also fix nitrogen, enriching the soil.
4. Earthworms: Adding earthworms to your garden soil can be considered a biological soil amendment. These garden friends improve soil structure and foster nutrient cycling, and the decomposition of organic matter, thereby enhancing soil health and fertility.
5. Manure: Animal manure, such as cow, horse, or chicken manure, provides a rich source of nutrients and organic matter. It must be properly composted to kill pathogens and reduce weed seeds. Manure improves soil fertility, structure, and moisture retention.
6. Mulch: Mulch such as wood chips and shredded leaves replicate nature’s process in woodland settings, creating rich humus soil.
7. Peat Moss: Sourced from decomposed sphagnum moss bogs, peat moss is an excellent organic amendment that enhances soil moisture retention. However, its sustainability is a concern, as peat bogs are essential for carbon sequestration.
8. Worm Castings: Produced by earthworms digesting organic materials, worm castings are an excellent soil amendment. They provide a rich source of nutrients and beneficial microorganisms, improving soil fertility and structure.
9. Microbial Inoculants: Microbes are an excellent type of organic soil amendments. See that expanded information below.

Microbial Inoculants for Organic Soil Amendments

Microbial inoculants, also known as biofertilizers or soil probiotics, are preparations containing live or latent cells of efficient strains of microorganisms that enhance soil microbial activity and promote plant health and growth. These beneficial microbes, including bacteria, fungi, and protozoa, can provide various advantages when introduced to the soil:

1. Rhizobacteria: These beneficial bacteria, such as Rhizobium species for legumes and Azospirillum for cereal crops, colonize plant roots. They are typically applied through seed treatments or directly to the soil to enhance nutrient uptake and promote healthier root development.
2. Mycorrhizal Fungi: These fungi form symbiotic associations with plant roots, facilitating water and nutrient absorption, particularly phosphorus. Mycorrhizal fungi can be applied as granular inoculants around the plant root zone or mixed with seeds before planting.
3. Trichoderma: A genus of fungi known for its disease-suppressing capabilities and ability to enhance root growth. It is applied to the soil or seed, often in powdered or granular form, to protect plants against various pathogens and stimulate root development.
4. Plant Growth-Promoting Bacteria (PGPB): These include various species like Bacillus, Pseudomonas, and Streptomyces, known for their ability to promote plant growth through different mechanisms. They are applied to seeds or soil to enhance nutrient availability, suppress disease, and stimulate plant growth.
5. Compost Tea: A liquid solution brewed from composted organic materials, rich in beneficial microorganisms. Compost tea is applied as a soil drench or foliar spray to introduce a diverse community of beneficial microbes, promoting soil health and plant vitality.
   See compost tea recipe.

Application Methods for Microbial Soil Amendments

• Soil Drench: Beneficial microbes in liquid form are applied directly to the soil, particularly targeting the root zone where they can colonize effectively and interact with the plant roots to enhance growth and health.
• Soil Integration: Granular or powdered microbial inoculants can be mixed into the soil or applied during planting. This method ensures that the beneficial microbes are well-distributed in the soil, where they can establish colonies and provide ongoing benefits to plants.

Choosing the Right Organic Amendment

Selecting the right organic amendment depends on your soil’s specific needs and the desired outcomes. For instance, sandy soils benefit from compost or manure to improve moisture retention and nutrient content, while clay soils can be amended with organic matter to enhance aeration and drainage.

Creating healthy soil is the very best way to lay a solid foundation for a healthy garden. And a healthy garden helps to build healthy bodies as well as a healthy yard and garden ecosystem, which is a gift that keeps on giving.