Beginner Gardening: What Plants are Sensitive to Mineral Buildup on Clay Pots, 0 by tapla

tapla wrote:
Katie - clay pots are permeable, allowing gasses to pass through container walls fairly freely, and liquids to pass to a lesser degree. The deposits you see on the outside of your pot are dissolved solids. That is to say that there are dissolved minerals and fertilizer salts in the soil solution that pass through cell walls in water. When the water with its dissolved solids come into contact with the air surrounding the pot, the water evaporates and leaves the (formerly) dissolved solids behind. The deposits you can SEE, are harmless, even if left on the outside of the pot - simply because if they are clinging to the walls, they are not in the soil solution. They are, however, evidence of OTHER dissolved solids still IN the soil solution. These are the dissolved solids that do give cause for concern.

We want to keep them at a low level because the lower the level of salts in the soil solution, the easier it is for the plant to take up water and the nutrients dissolved in the water. We don't want the level so low though, that there isn't enough fertilizer in the soil to satisfy nutrient requirements. You can see that there is something of a balancing act going on. If you want to know more about how to keep this balance favorable, just ask.

Because the walls of terra cotta pots are permeable, they allow water vapor and water to pass through. This means that clay pots dry out much faster than their impermeable counterparts made of plastic, glazed clay, ceramic, and other impermeable materials. That is also why terra cotta helps to keep roots cooler - by way of the evaporative effect I just mentioned. Since they are permeable, they also allow potentially harmful gasses to pass out of the soil. The excess CO2, as well as the sulfurous gasses and methane that can inhibit root function, pass out of soils in unglazed clay more readily than other pots. Also, the need to water more frequently is a blessing in disguise because when you water, you are assisting mechanically in forcing these gasses from the soil more often.

I grow a large % of my plants in terra cotta. I like the health benefits and enjoy that nice patina that comes with age, worn by my older pots.

Al