A teaspoon of soil generally contains over 100 million bacteria. Most of these bacteria play an essential role in decomposition, breaking down organic matter and storing nutrients in a form that is accessible to other soil life. When these bacteria die, the nutrients are released and absorbed by roots. Other bacteria form partnerships with legumes, trading nitrogen collected from the air for carbon compounds formed during photosynthesis.
A single teaspoon of soil can contain 100 meters of fungi if they were all lined up in a single row. They play a similar role to bacteria, predominantly breaking down organic material, making nutrients accessible to plants. Fungi form symbiotic relationships with plants by extending the reach of root systems, helping to collect nutrients and water, again in return for carbon compounds created by the plants. They also produce useful humic acids which help transport nutrients to the roots.
There are over a million species of worms. They are able to eat their body weight every day and double their population every 40 days. Worms play an essential role cycling soil nutrients, breaking down organic and inorganic matter into compounds that plant roots are able to absorb. They particularly love cardboard, paper, manure, veggie scraps and undisturbed soil.
Humus is dark organic material formed through the decomposition of animal and plant matter- created through composting. Humus contains essential nutrients and slowly crumbles, loosening the soil and allowing air and water to penetrate. Humus is the primary source of food for most soil life.
The three key nutrients found in soil and needed by plants are Nitrogen (N), Phosphorus (P) and Potassium (K). N is a key element in plant growth, P helps covert energy from sunlight into useful forms, K boosts disease resistance and helps form energy storing compounds. Sulfur (S), Calcium (Ca) & Magnesium (Mg) are three additional important macronutrients found in soil. There are 14 micronutrients also needed by plants but in much smaller quantities: Iron, Molybdenum, Boron, Copper, Manganese, Sodium, Zinc, Nicket, Chlorine, Cobalt, Aluminum, Silicon, Vanadium and Selenium.