Good Soil Structure

Adding humus and worm castings to soil improves its structure.  How?

  • Worm castings are full of beneficial microorganisms which benefit soil and the root zone.
  • Humus/organic matter create soil spaces for air and water. These spaces allow the microorganisms room to grow and reproduce.
  • Air spaces enhance root growth by making it easy for the roots to travel through the soil.
  • Aerated soil increases rain absorption and decreases runoff.
  • Good soil structure creates a perfect environment for earthworms to live which continue to aerate the soil.

Adding humus and worm castings to your soil on a regular basis ensures continued good soil structure and a healthy root zone.

soil airspaceA healthy root system needs plenty of air spaces in the soil to grow and reach the water and nutrients.

soil air 2

Well-structured soil has plenty of space for air, water and nutrients.


Good soil structure has space for roots to grow.


To grow and photosynthesize, plants need the sun, water and nutrients from the soil. simple cycle


Longitudinal cross section of a root and the surrounding rhizosphere.

A plant absorbs these nutrients through the root system.  Some nutrients are in unavailable forms, so the roots cannot absorb them.  Microorganisms present in the soil can change these unavailable forms into nutrients the plant’s roots can absorb.   The greatest concentration of beneficial microorganisms is found close to plant roots.  This area is called the “Rhizosphere”.  The microorganisms are called plant-growth promoting rhizobacteria or PGPRs. The microbes absorb carbon for energy and change it into carbon forms plant roots can use.  This process also ties up the nutrients in the soil, so they are less likely to leach out with rainwater or manual watering.  As the number of beneficial microorganisms increases the number of pathogens, or microbes which harm plants, diminish.  They compete with each other for carbon, oxygen and space.  Continuous introduction of beneficial   microorganisms helps to maintain these protective levels in the soil and improves the nutrient uptake at the rhizosphere.


The left side is an example of a root without PGPB and the right side is an example of what the root would be like with PGPB. Source: “Ganesh Tree and Plant Health Care”

Stable Carbon in Soil


How Do Gardeners Create Stable Carbon in the Soil?

Applications of quality compost/worm castings inoculate the soil with beneficial bacteria.  To live, these micro-organisms remove CO2 from the atmosphere and fix/convert the carbon into organic matter.  This organic matter is humus, the stable form of carbon in the soil.  Regular applications of quality compost/worm castings increase the number of beneficial organisms in the soil biomass and helps diminish unhealthy soil pathogens.  The “good guys” basically choke out the “bad guys”!  The entire plant root zone benefits. Healthy soil yields healthy plants!

carbon cycle

This simple diagram shows micro-organisms taking CO2 and forming humus.  Follow the arrows of this basic carbon cycle to see the movement of carbon in our environment.

Soil Biology Basics


Heather Robertson- horticulturisthumus-550 (1)


Factors which improve our soil:

An increase of stable carbon in the soil improves it immensely.  Humus is manufactured by beneficial bacteria in the soil by breaking down plant and animal residue until it is barely recognizable.  Humus is a stable form of carbon in the soil.  Its presence increases soil fertility, soil moisture retention, aeration, nitrogen fixation, mineral availability, disease suppression, and soil structure.  A farmer might fold these desirable soil characteristics under one term: tilth!humusThe best gardening practices build up a garden’s soil carbon reserves!