The air and water your soil holds depends on the porosity of the soil. Soil that is healthy has a good physical structure (sometimes called tilth), which determines its ability to drain well, store moisture, and provide for the needs of healthy plants.
Well-structured soil has lots of small aggregates and stays loose and easy to cultivate. Aggregates are groups of particles loosely bound together by the secretions of worms and bacteria. In between and within the aggregates themselves are many small air channels (or pores) like the empty spaces left in a jar of marbles. The channels that aggregates create in the soil allow plant roots and moisture to penetrate easily. The smaller pores within the aggregates loosely hold moisture until a plant needs it. The larger pore spaces between the aggregates allow excess water to drain out and air to circulate and warm the soil.
The organic portion of the soil is the result of the decomposition of grass clippings, bits of leaves, and dead bugs. Humus is the generic term for this substance, whose characteristics can vary widely depending on what the original material was and how it decomposed. Humus is the main component of compost and is generally dark brown, porous, spongy, and pleasantly earthy-smelling. This decaying matter provides nutrients to the living microscopic organisms which have the primary responsibility of maintaining the balance of soil's chemical and ecological properties.
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what is compost?