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Soil Inoculants: Keys to Healthy, Productive Garden Soil

Soil Inoculants: Keys to Healthy, Productive Garden Soil

The garden’s soil is rich in microorganisms that can affect plants in either beneficial or detrimental ways.

While many of these bacteria break down organic matter in the soil to release the nutrients that plants need, others cause diseases that could kill the plants.

So how do you keep the good microbes and at the same time eliminate the bad ones? You need soil inoculants.

What are soil inoculants? Soil inoculants are doses of good bacteria, fungi, or algae that promote soil health, increase nitrogen content, and make nutrients in soil more accessible to plant roots. It’s an organic way to amend the soil and make it healthier for the plants without the need for pesticides or fungicides.

Soil inoculants work for certain crops, such as peas and beans, but not for others. Read more to find out about enriching the soil with soil inoculants.

Soil Inoculants Explained

If you use probiotics to improve your gut health, you can think of soil inoculants in the same way. So what are soil inoculants, and how do they work?

What Are Inoculants?

Despite the name, soil inoculants are good bacteria injected into the soil so that they multiply and crowd out the bad microbes.

The good bacteria form a symbiotic relationship with the plants and can promote plant growth and health.

Unlike how many vaccines work, these are not weakened forms of the bad bacteria to promote the plant’s response to diseases.

What Does Soil Inoculant Do?

Depending on their type and purpose, soil inoculants can prevent bad bacteria from growing, help the roots absorb nutrients, add a dose of nitrogen or phosphorus into the soil, or improve the growth and health of the plants.

Some types of inoculants, such as diazotrophic rhizobacteria, also increase the yield of the crops.

How Soil Inoculants Work

Soil inoculants work in the same way you eat yogurt or consume probiotics to increase the good bacteria in the gut.

You seed the soil with good bacteria and let them form a symbiotic relationship with the plant.

As the good bacteria multiply, they crowd out the bad microbes in the soil, keeping the soil healthy and fertile.

Types of Inoculants

Inoculants come in two broad types: bacterial and fungal. Their role in the soil varies widely.

Some obtain nitrogen either from the organic matter in the soil or by preying on small insects. Each type of inoculant benefits a specific group of plants. Here are the main types.

Nitrogen-Fixing Bacteria

Nitrogen-fixing bacteria are a type of rhizobacterial inoculants.

These good bacteria bind with the nodes in the roots of the plants and increase the nitrogen in the soil. They’re added to soybeans, peas, beans, and other legumes.

Phosphate-Solubilising Bacteria

This is another type of rhizobacterial bacteria with the sole job of breaking down the organic materials to release phosphorus content and make it more accessible to the roots of the plants.

Arbuscular Mycorrhiza

This is a fungal inoculant that also makes phosphorus easy for the roots to absorb. When added to plants such as tomatoes, they reduce the amount of fertilizers needed by about one third.


This is another fungal inoculant that works mainly with trees such as pines and eucalyptus. The fungi are resourceful and can kill small insects to obtain nitrogen and pass it on to the host plant.

What Plants Benefit From Microbial Inoculants?

Microbial inoculants benefit different plants in different ways. Nitrogen-fixing bacteria benefit legumes the most, and you should use them with chickpeas, beans, soybeans, and other legumes.

Phosphorus-fixing inoculants will benefit plants that need more phosphorus, such as tomatoes and flowering plants. Ectomycorrhiza benefits trees such as pines and eucalyptus.

How To Know if You Need Soil Inoculants

When the plants that grow in the garden show stunted growth, are more prone to diseases, or produce fewer crops than they used to, that’s a good sign that the soil needs to be amended with soil inoculants.

Some crops need soil inoculants whether the soil is in good condition or not. Legumes, for example, need a good dose of nitrogen-fixing bacterial inoculants to improve growth and increase yield.

Learn more about selecting the right soil for your plants in our ‘Topsoil vs. Garden Soil‘ and ‘Topsoil vs. Potting Soil‘ articles.

How To Use Soil Inoculants

Soil inoculants are introduced into the soil while you’re planting the seeds. When growing large crops on farms, you can always mix the inoculants with the seeds, and then plant them together.

However, in a garden, where you’re more likely to plant seeds individually, you should treat the soil inoculants as a form of fertilizer or compost. Here’s how to do it in easy steps.

  1. Prepare the soil for planting, and keep the seeds and inoculants in their separate bags.
  2. Dig a hole for the seed, and drop the seed in it.
  3. Add a dose of inoculants into the hole, and make sure you cover the seed with it. The more inoculants you add, the better.
  4. Cover the hole with soil.
  5. When you’re done with the rest of the seeds, water the patch to help the inoculants take.

Indigenous Microorganisms

Indigenous microorganisms (IMOs) are abundant on the planet. They exist just about on every surface and even inside of animals, in the soil, and in the water.

They perform important tasks such as composting, fixing nitrogen, leaching, biodegradation, and fertilizing the soil.

While some of those innate microorganisms cause diseases, the majority do more good than harm. Here are the main types of indigenous microorganisms. 


Bacteria, for the most part, are the good guys in the world of indigenous microorganisms. They help with everything from pasteurizing milk to composting and biodegradation of organic waste.

Without bacteria, life on earth would be hard to imagine. Your gut is packed with good bacteria that help with digestion and absorption of nutrients and bolster your immune system. 


Algae are closer to plants than they are to bacteria. Although microorganisms themselves, their cells contain chlorophyll, and just like plants, they rely on photosynthesis to generate nutrients.

Algae are aquatic organisms that need moisture to survive, and they often spread in damp and wet areas. They vary in size from microscopic algae to large organisms about 400 feet long.


Fungi are similar to algae, but they can grow on land and don’t need an aquatic medium to survive.

Some fungi are too small to see with the naked eye, while others have complex structures, such as mushrooms and bracket fungi.

They feed by absorbing nutrients within their vicinity. Some fungi are good for the plants and feed them with phosphorus and nitrogen, while other soil fungus, like mold and mildew, cause infections.


These single-cell microorganisms have different shapes varying from spheres to ovals. Some types act like animal cells lacking walls and ingesting food.

Others have cell walls and get their nutrients through photosynthesis. Amoeba are a type of protozoa.


If the majority of bacteria are good guys, all viruses are bad to the core. They are parasites that bring disease and death whenever they invade a living cell.

Viruses infect all forms of life, including animal cells, plant cells, and even other microorganisms.

Viruses cannot survive on their own and cannot generate the energy they need. They attach themselves to host cells to obtain nutrients.


Soil inoculants are an important concept in organic gardening.

The microorganisms, when introduced into the soil, preferably when planting the seeds, process the organic materials in the soil and release the nitrogen and phosphorus for the roots to absorb.

They also prey on bad microbes in the soil to render them neutral.