Graphic of the plant rhizosphere

The Benefits of Soil Inoculants

Hopefully you don’t think of soil as just dirt.

Soil is actually an incredibly complex ecosystem, with billions of organisms found in a single teaspoonful. These organisms range from fungi and bacteria to earthworms, nematodes, and insects.

Good soil health is critical for plants to survive and thrive, so the survival of humans indirectly rests on the health of the soil.

The process of soil inoculation involves taking specific microbes or nematodes and incorporating them into the soil, or applying them directly on seeds and plant roots.

  The Plant Rhizosphere

Many microbes, both beneficial and destructive, live in an active zone around plant roots known as the rhizosphere. This is the area in the soil where the plants secrete compounds like sugars and amino acids.

Up to 44% of the compounds that plants make from photosynthesis can end up in the rhizosphere. This makes the rhizosphere a relatively lush area for microbes.

In addition to microbes, other organisms like protozoa and nematodes live in the rhizosphere. They feed on the fungi and bacteria that live there and release their remains as waste products. This “waste” is a great source of nutrients for plants.

  Types of Inoculants

  Microbes in the ground help plants in many different ways. Some limit the growth of pathogens. They can do this by out competing the pathogens or even parasitizing them.

  Other microbes have symbiotic associations with plants. That means the microbes and plants have a mutually beneficial relationship. This includes bacteria that fix nitrogen or fungi that live as mycorrhizae and provide the plants with nutrients that may be otherwise unavailable.

  Yet others stimulate a plant’s immune system, through what is known as acquired resistance. The plants may grow more robustly as a result, or they may produce chemicals to fight off pathogens or insects.


Biofertilizer applied to plant

Biofertilizers are living microbes that enhance plant nutrition by either by mobilizing or increasing nutrient availability in soils. Various microbial taxa including beneficial bacteria and fungi are currently used as biofertilizers, as they successfully colonize the rhizosphere, rhizoplane or root interior.

This is an extremely valuable contribution to plant health. Often the soil can be full of plant nutrients, while the plants in that soil suffer from serious nutrient deficiencies.

This is a particular problem with potassium (K). Up to 98% of the K in the soil is tightly bound to minerals like feldspar and mica. In fact, it so tightly bound that it is unavailable to plants. While geological weathering will slowly release the potassium, making it available to plants, they commonly suffer from a deficiency of K.

However, so many types of bacteria release K that there is even a term for them, KSB, potassium solubilizing bacteria.

Biofertilizers such as Rhizobium, Azotobacter, Azospirilium and blue green algae (BGA) have been in use a long time.

  Nitrogen-Fixing Organisms

Graphic of nitrogen-fixing process

Some microbes live closely with a plant and help to make nitrogen available to it. This is how legumes (such as peas, beans, and clover) can live in poor quality soil.

Microbes such as the highly specialized bacterial genus Rhizobium live in an intimate association with their host legumes in root nodules. They are able to take nitrogen from the air and change it into forms that plants can use.

  Nutrient-Solubilizing Microbes

Mycorrhizal fungi are the most well-known of this class. They live in close association with plant roots, and their hyphae extend way out from the roots. This enables them to obtain nutrients from a wide span of the earth.

Different types of mychorrhizae are specialized for different plants. You can either buy a mixture of them to apply to your garden, or buy ones that are specialized for the particular type or types of plants that you are growing.

Other types of fungi and bacteria produce specialized compounds called siderophores that bind with iron and make it unavailable to other microbes. The good thing about it is that plants can use the iron that the siderophores make available. These organisms perform best in soil that hasn’t been heavily fertilized.

  Microbes that Produce Plant Growth Hormones

Some microbes produce chemicals that directly stimulate plant growth. This can be so beneficial to the plants that they can sometimes outgrow the pathogens! One example of this is fungi that produce a hormone called gibberellin that helps seeds germinate and improves plant growth.

Soil microbes can produce the important hormone auxin. Many strains of bacteria in the earth produce auxin, which has an enormous effect on plants by stimulating the elongation of plant shoots, stimulating root branching and promoting the development of fruit.



Nematodes are effective killers of pests but not beneficial insects. They have specialized bacteria live in their guts that can kill insects within 24-48 hours.

  Tips for Using Soil Inoculants

  Remember that while the bag of microbes that you buy may look like any other product, it contains living organisms. Be careful with how you handle them.

  The best place to keep them is in the refrigerator. It is not a good idea to leave them in the sun in a hot car or truck, for instance.

  Be careful when handling them. While these organisms are pathogens of other microbes and generally not humans, people with compromised immune systems could conceivably be infected.

Further Information:

 The Life Cycle of Plants: Fertilization
 The Spruce: How to Make Your Own Fertilizer
 Pollination and Fertilization
 Use Eggshells For Your Plants
 How to Use Banana Peels in Your Garden

Beneficial Species
Fertilize & Mulch
Garden Plans
Garden Pests
Lawn Management
Quick Tips
Soil Management
Specialty Gardens
Bees flying footer graphic