soil basics - how it works


4. Who are these Microbes, and what are they doing in my Soil

Strong, healthy soil is critical to anyone who has planted anything for any reason. Without it, your chances for success are pretty poor. But what makes one soil support abundant, vigorous plant life while another seems to produce only sickly plants and weeds?

Let’s start with soil structure, which has an important impact on transport of nutrients from soil to plants. There are three main types of soils. Sandy soils are composed of large, loose particles that drain easily. Sandy soils will never compact, but they are also poor repositories for water and nutrients. Unless sufficiently amended and maintained, sandy soils will support only the hardiest of plant life.

At the other end of the spectrum are clay soils. Clay soils are composed of very small particles that are sticky when wet. They have a lot of valuable surface area for adherence of nutrients and retain water very well, but they compact very easily and may be lacking in the air spaces that allow movement of water and nutrients, thus limiting their availability to the plant.

In between sandy and clay soils lie the loamy soils. These soils are actually a combination of sand, clay and organic matter. They resist compaction but retain enough water and nutrients to support plant life when well-managed. The good news is that both clay and sandy soils can be made more like loamy soils with the proper care and amendments.

All plants require 16 essential elements, and many require a few more. They get carbon, hydrogen and oxygen from the air and water, the availability of which is directly related to the structure of the soil they are growing in. Loamy soils with good aggregate structure and adequate porosity allow plenty of air and water to reach plant roots. But no soil, regardless of structure, will support vigorous plant life and deliver enough of the other 13 or 14 essential elements unless it is teeming with the billions of microbes that are necessary for maintaining soil health and supporting vegetative growth. Just what do soil microbes do? The list is impressive:

  • Increase availability of phosphorus, potassium and other nutrients

  • Deliver nitrogen

  • Break down organic residues

  • Increase soil aeration

  • Improve water penetration and retention

  • Increase naturally occurring organic acids that stimulate root growth

  • Improve delivery of multiple nutrients to plant roots

  • Inhibit pathogens

How do microbes do all this? Soil microbes exist in the “rhyzosphere”, the area of soil surrounding the roots of plants. They exist in a symbiotic relationship with the plant roots; microbes deliver nutrients plants need and either directly destroy pathogens or produce compounds that are antagonistic to pathogenic organisms, and plants provide microbes with amino acids and carbohydrate products of photosynthesis. Obviously, the healthier and more extensive the root system of the plant, the deeper and richer will be the rhyzosphere, and vice versa. Many microbes also derive carbon from organic matter that is broken down by bacteria, so the combination of abundant organic matter and a healthy bacterial population is essential to supporting a rich and vibrant rhyzosphere. Indeed, the microbe population in the top six inches of one acre of healthy soil has a metabolic equivalent of 10,000 humans, and each gram of soil may contain 10,000 different species of microorganisms!

Many classes of microbes exist to service the soil and plant life. Bacteria serve to decompose organic matter, leaving in their wake a sticky, mucus-like substance that acts as a glue to hold the soil together in aggregates that provide soil structure with multiple spaces for movement of air and water. Some bacteria also consume many pathogens that might otherwise be able to attack plants or other beneficial microbes. Rhizobial bacteria infect the root nodules of legumes and participate in nitrogen-fixing by these plants.

Fungi are responsible for delivery of a significant amount of the nitrogen that is taken up by plants. While much free solute nitrogen is absorbed directly from the soil water solution, fungi are responsible for delivery of fully one-third of the nitrogen plants need. They do this by linking soil nitrogen to carbon, moving the combined N-C molecule to the root surface, and then, just before delivery, the carbon is released and only the nitrogen is presented to the plant. Other fungi, specifically mycorrhizal fungi, are critical for phosphorus uptake and also aid in improving availability of potassium. These fungi send tiny root-like structures called “hyphae” into the soil and can bring phosphorus to a plant root from four inches away (which is miles in terms of soil nutrient provision!). Mycorrhizae also produce a substance called “glomalin”, which improves soil structure and may form as much as 30% of the organic matter in the soil.

Beneficial nematodes (tiny roundworms) keep pathogenic root-eating nematodes under control. They also consume other nematodes and bacteria, thus releasing nitrogen and phosphorus into the soil, where fungi can make them available to plants. Some nematodes eat pathogenic fungi, thus helping to maintain a healthy balance of microbial life.

One-celled protozoa in the soil serve as soil police squads, preying on bacteria and fungi to keep these populations under control. As with any system made of multiple components, if one portion of the population gets out of control, the system fails to function as it should. The presence of ciliate protozoa, which feed on anaerobic bacteria, indicates that oxygen is in limited supply and some soil aeration is needed.

Arthropods are a class of assorted creatures ranging from the microscopic to beings as large as ants, beetles and centipedes. Their general function is to break up debris and aerate the soil with their foraging. Arthropods will not be present if there is nothing to forage.

Earthworms, though not really microbes, are great contributors to soil richness with their nutrient-dense castings and the aeration provided by their burrowing, and their presence is a good indication that the soil is healthy and abundant in nutrients and multiple forms of microbial life.

Clearly, a soil that is barren of organic matter and microbial activity will have very limited ability to support any kind of plant life. Chemical fertilizers will only be available to plants in the form that is readily soluble in water; much will be wasted and will run off because there are no microbes to transport nutrients. Root systems will be shallow, the rhyzosphere will be thin, and soil structure will suffer. Application of large amounts of chemical fertilizers, as well as herbicides and fungicides, in an effort to feed weakening plants may even decimate what little microbial life exists, resulting in an even more sterile, barren soil. Use of organic fertilizers and amendments such as Bradfield Organics products to build soil structure and support microbial life is critical to the optimal growth of vigorous, healthy plants!


Bago, B., P. E. Pfeffer, and Y . Shachar-Hill. 2000. Carbon metabolism and transport in arbuscular mycorrhizas . Plant Physiol. 124:949-957.

Booker, Karen. 2000. Fertilizers and Soil Amendments: It’s Tricky Business. Erosion Control Feature Article, September/October.

Drinkwater, L. E., P. Wagner, and M. Sarrantonio. 1998. Legume-based cropping systems have reduced carbon and nitrogen losses. Nature. Vol. 396, Nov. 19.

Ellis, J. R. 1995. Mycorrhiza – An essential part of most plant root systems. Better Crops 79(1):10-11.

Marschner, Horst. 1995. Mineral Nutrition of Higher Plants. (2nd ed.)

Wright, S. F. 2003. The importance of soil microorganisms in aggregate stability. Proc. North Central Extension-Industry Soil Fertility Conference. 19:93-98.

Soil Basics - How it Works

1. The Symbiotic Decay-Nutrition Cycle

2. Water Uptake

3. Ions, Nutrition and all that “Scary” Chemistry

4. Who are these Microbes, and what are they doing in my Soil?

Download Soil Basics - How it Works from Bradfield Organics® (Adobe Acrobat Required)


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