To understand what goes into the perfect potting mix, it’s helpful to appreciate how plants thrive. Plants have four basic survival needs: water, air, food, and soil structure that shelters roots.
1. Water: Roots pull water from the space between particles. If the mix is too dry, they can’t pull hard enough; if it’s too wet and waterlogged, they can’t function. To complicate things, container mixes can collapse under the weight of water that doesn’t drain, yet they also dry out more quickly than native soil. Water-holding and water-draining ingredients balance these demands in the potting mix.
3. Food: Organisms and organic matter in soil retain nutrients and feed plants. Plant nutrients in container gardens, on the other hand, are added artificially. In a potting mix, these nutrients are supplied by organic materials, along with organic or chemical fertilizers.
4. Structure: Materials that add structure in potting mixes help to anchor roots and shelter them from harsh conditions. These ingredients also prevent compaction and add weight to keep containers from toppling over.
This article first appeared as a sidebar in the article What’s in the Bag by Elizabeth Murphy from Issue #186 of Fine Gardening.
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