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
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
Break down organic residues
Increase soil aeration
Improve water penetration and retention
Increase naturally occurring organic acids that stimulate
Improve delivery of multiple nutrients to plant roots
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
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. www.forester.net/ec_0009_fertilizer.html
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
Wright, S. F. 2003. The importance of soil microorganisms in aggregate
stability. Proc. North Central Extension-Industry Soil Fertility
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?
Basics - How it Works from Bradfield Organics® (Adobe