Covers the basics of soil biology, what it is “in there” and why it matters. Includes details on the carbon and nitrogen cycles and the soil food web. (Excellent for high school students, good for all audiences)

1. ©2009 Rodale Institute
Soil Biology:
Why is it important? and
Why does it matter to me?

2. ©2009 Rodale Institute
"The soil is not, as many suppose, a dead,
inert substance. It is very much alive and
dynamic. It teems with bacteria,
actinomycetes, fungi, molds, yeasts,
protozoa, algae and other minute
organisms."
~J.I. Rodale
Pay Dirt:
Farming and Gardening
with Composts (1945)
What is soil?

3. ©2009 Rodale Institute
Four basic facts about soil
• Soil is composed of three sizes of mineral particles:
Sand (large), silt (medium), and clay (small)
• The other important type of soil particle is called
organic matter
• Soil is also comprised of a lot of empty spaces,
called pores, which are important for holding air or
water
• Soil is full of living organisms, both large and
microscopic

4. ©2009 Rodale Institute
The Soil Texture Triangle
Drains water very quickly
Not many nutrients to help plants grow
Very dense – nutrient rich, causes water to puddle and not drain
Not much permeability for air or microbes
Holds water and nutrients well
Can get mucky and lose pore space
Erodes easily

5. ©2009 Rodale Institute
What does soil do?
… besides giving us something to walk on
and build on…
Soil supports plants – holds them upright,
provides moisture and nutrients to the roots

6. ©2009 Rodale Institute
How do plants use what the soil
provides?
• Uptake soil water to power
photosynthesis
• Use soil nutrients to build
cell structures and
produce complex sugars,
proteins and other
molecules
• Soil nutrients include
nitrogen (N), phosphorus
(P), potassium (K),
calcium (Ca), magnesium
(Mg), and sulfur (S).

7. ©2009 Rodale Institute
What do plants give back to the soil?
Dead plant material!
What good things do dead plants add to the soil?
Nutrients (plant food, NPK) Carbon (microbe food, organic matter)

8. ©2009 Rodale Institute
Why carbon and nitrogen are vital for plants
Soil Carbon – provides food for
microbes that cycle
nutrients through the soil,
holds moisture in the soil
Nitrogen – one of the primary
nutrients (along with
phosphorus and
potassium) required by
plants to make proteins
and other important
structural molecules
lignin
a simple protein

9. ©2009 Rodale Institute
The Nitrogen Cycle

10. ©2009 Rodale Institute
The Carbon Cycle
How carbon gets into the soil:
How carbon leaves
the soil:
photosynthesis
plant death
plant consumption by
animals and microbes
microbial respiration:
eating larger carbon-
containing molecules, like
sugars, and breaking them
into smaller components,
such as CO2 (carbon
dioxide) and CH4 (methane)
wind and water erosion

11. ©2009 Rodale Institute
The Soil Food Web – powering the cycles
First trophic level:
Photosynthesizers
Second trophic level:
Decomposers, Mutualists,
Pathogens, Parasites, Root-
feeders
Third trophic level:
Shredders, Predators,
Grazers
Fourth and higher trophic levels:
Hither level predators

12. ©2009 Rodale Institute
Plants we rely on, that rely on the cycles…
wheat
soybeans
corn
What about these guys?

13. ©2009 Rodale Institute
Ways to increase soil carbon, in your garden
or on the farm
• Keep your soil covered throughout the year, as much
as possible, with crops, cover crops, and mulches -
retains moisture, prevents erosion, adds organic matter, reduces
weeding
• Grow a diversity of crops, and rotate them into
different locations in your garden each year - prevents
excessive loss of individual soil nutrients and build-up of plant-specific
diseases and pests
• Don’t dig too much! Some digging is good, but too
much breaks down organic matter and soil structure
• If you grow crops on a slope, plant your rows
horizontally across the slope to help stop water run-off
and erosion
• Add compost to increase organic matter, microbial
activity, and nutrient availability

14. ©2009 Rodale Institute
How do you know how much you have carbon in
your soil?
Look for:
Color (darker brown = more carbon)
What’s growing (more plentiful, diverse plants = more
carbon)
Feel for:
Texture (mid- to small-size particles = more carbon)
Moisture (more wet = more carbon)
Weight (fluffier, lighter = more carbon)