Why is soil at the bottom of my pot lighter?

Question

I have been planting a lot in pots. Twice recently when I transplanted to bigger pots because plants weren't doing well, I discovered that the soil in the bottom half of the pot was a shade lighter than the top. Why? I always reuse soil. Can I replant in this lighter soil, put it into my own garden composted (not very hot) or into the city compost?

Luby

San Francisco

Answer

The lighter soil at the bottom of the pot is due to a decrease in organic matter, soluble salt buildup (fertilizers), calcium carbonate (hard water and reduced drainage). There is nothing unsafe about it so it can be added to compost piles both at home and in the city yard. If you want to reuse the soil, I recommend mixing it first with fresh soil to disperse the minerals and improve the nutritional content.

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