The Soil Food Web

I always knew that plants need good soil to thrive, I didn't know, however, how important the soil food web is in maintaining the health of plants until recently.  After I took a class on soil, and started reading more about this fascinating subject, I became convinced of the importance of the complex network of organisms found in soil.

Do you know how many critters live in a teaspoon of soil...take a wild guess.  There are over one billion bacteria, few yards of fungal hyphae, thousands protozoa, and few dozen nematodes in each teaspoon of healthy garden soil.  They all contribute to the breakdown of organic material so that it becomes available to plants to absorb through their roots.  Fungi eat bacteria, tiny mites eat fungi, earthworms eat mites, etc., they all produce waste, then die and return to dust, so the cycle continues.  But what was discovered relatively recently makes it even more interesting; certain fungi create associations with plant roots and enhance their capability to absorb nutrients through their filaments, or hyphae.  Moreover, plants produce chemicals called exudates, i.d. food, to keep the good microbes near their roots happy and thriving.  Fungi also produce substances that inhibit other organisms from growing near roots, therefore defending plants from pathogen attacks.

The soil food web is really important in controlling plant diseases.  Certain fungi will predate on nematodes that would enter the plant roots and eventually kill the plant.  Competition for nutrients in a healthy soil will also contribute to keep pathogens under control.   When soil microorganisms are killed by non sustainable practices some pathogens will proliferate so the use of more pesticides is seen as necessary.  Moreover the breakdown of organic material won't happen, so more synthetic fertilizers are applied and a vicious cycle will ensue.  The good news is that soil has the amazing capability for recovery and it can happen within few years, depending on the amount of abuse a soil has taken. Synthetic fertilizers leach from the soil really quickly so it is necessary to apply more of them compared to organic fertilizers or compost, which break down more slowly.  Not to mention that synthetic fertilizers are derived from petroleum, which there is less of it so their cost has been increasing in the last decade.  Organic fertilizers' price has not increased at the same rate on the other hand.

Another positive effect the soil web has is on the soil structure.  The soil microorganisms produce excretions to move around or create aggregates to live in, these secretions function as a soil glue keeping tiny soil particles bound together.  Burrowing animals contribute by creating pathways that help soil oxygenation and water penetration, and they also spread the nutrients around.  Thanks to growing knowledge on the soil microorganisms, and many scientific studies being conducted at various universities, a growing movement of no till methods to cultivate the soil has emerged in the last decade.  The principle behind it is that if soil is mulched and plenty of organic material is added to it, the soil food web will take care of the rest, including keeping the soil nice and soft with the action of the many critter that inhabit it.  Tilling disrupts the complex soil food web, by inverting the soil layers and exposing many microorganisms to the wrong conditions.

Elaine Ingham the SFW Queen

Elaine Ingham is a well known soil scientist and works at the Rodale Institute. She is a big proponent of the no till methods. 

Elaine is also doing great research on areated compost tea, which I find fascinating and it deserves a post on its own.

For more reading about this  subject, you can find a great description of soil microorganisms here, together with more articles on soil, at the Sustainable Agricultural Research and Education (SARE).

To learn more about this amazing system I highly suggest the book Teaming with Microbes, which gives a great overview on the soil food web, soil science, importance of mulching, restoration, and how to increase your soil fertility with home brewing compost tea.

Another book worth reading is The Soul of Soil Building, a soil building guide.
Roots Demystified is another great read on plant root systems.

In other words: only fertilize with organic compost (best your own), don't till, protect the soil by adding mulch (I see another post coming on mulch), and if you are so inclined brew your own compost tea (it is cheap and quick).