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Organic Gardening
Compost and Mulch

How is soil formed from compost?


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April 23, 2009 10:16PM

Think of soil as a mixture of materials, some of them living (fungi and bacteria) and some of them non-living (minerals, water, dead plant/animal material). Although a geologist could give an extremely detailed answer, I usually think of soil particles as coming in 3 sizes: Sand (the largest marble shaped particles) Silt (much smaller irregularly, jagged shaped particles) and clay (extremely small, flat particles shaped like playing cards).

The ideal growing medium for turf is humus. This is a loose, friable, living soil system which is high in organic matter.

By adding compost (which by definition is material that has been decomposed by bacteria/fungi) to any of the 3 soil types listed above you are increasing the "fertility" or the ability of a plant to get adequate moisture and nutrients.

Some fungi digest rock (also known as minerals), other fungi digest organic matter (commonly thought of as any plant or animal tissue). This process of digestion by microorganisms puts all of the raw materials that plants need to sustain themselves into a form that is "available".

By "available", I mean that the compounds are bound to the soil particles in such a way that they don't just wash away at the first rain (water soluable vs. water insoluable). Another meaning of the word "available", when thinking of nutrients, is that an element (potasium or calcium for instance), is in a form that the plant can readily use. I visualize this concept in this way: I'm in a desert, longing for a drink of water. I come upon a giant ice cube the size of a house. Because i don't have any tools to break off a chunk of ice that will fit into my mouth, I can't consume any water and I continue to go thirsty. The nutrients in compost are "available" to plants (they are in a form that is readily/easily consumed) and consequently add fertility to any soil profile.