Yesterday I potted one citrus in a clay pot, and the pot feels very cold, so I decided to measure the temperature of the soil, and in the clay pot it's 18C, and in the plastic - 23C. Did I make a mistake by potting in a clay pot? Or does this mean that the temperature will be more consistent in a clay pot day and night? Right now the environment is 21C.

I can't speak to whether clay would be colder than plastic but I don't think either is much of an insulator. But, I think what you are seeing is how the "heat capacity" of different materials play a role. I think when you measured the temperature it had been cooling down from the day. So, its possible that it takes longer for the clay pot to react to the temperature changes (and holds heat longer).

Its sort of like if you put a piece of metal on the stove and heated it - it heats very quickly and cools very quickly - that is a conductor - the opposite of an insulator. Water is a very good insulator - it takes far more heat to heat it up and holds heat well and cools down much more slowly.

Now if clay is significantly more of an insulator than plastic then it might make some difference but I suspect that if you measured the temp at different times it could be closer.

Also, black plastic absorbs more heat than white or brown, etc. So, if the direct sun is hitting the black plastic then it will absorb more heat. If the clay pot was white it would reflect more heat.

sorry very long-winded response with possibly too much science . . .

Todd

The reason might be that clay absorbs and evaporates water. Evaporation requires heat, which cools down the pot.

Marc is right. The cooling effect is much desired in mediterranian climate because citrus roots do not like (very) high temeratures. But in zone 5 one would not have the heat-problem very often. More frequently clay pots will cause "cold feet" and will limit growth on cooler summerdays. I do not use raw (red) clay pots in zone 7b. The more I would avoid them in zone 5
/Robert

We all should live in clay houses then... Having inside 5C (9F) cooler than the outside absolutely for free! That would be one green technology. I a bit doubt this explanation but cannot propose any other. When keeping my potted seedlings outside, I know the soil is warmer since black plastic pots absorb radiant heat from the sun. In your case Evaldas I am clueless.

MarkV explanation is correct. Clay pots are porous, therefore they absorb water from the growth medium (potting soil). This water is evaporated off the outside surface of the clay container. For water to change from a liquid to a vapor requires heat. The heat is supplied from the container (heat loss), causing the container to cool. Because clay containers are porous, the absorption is also the reason why clay containers dry much faster than the non porous plastic containers. - Millet (684-)

Would it be a bad mistake to plant citrus in a large plastic pot ? Large plastic is not as fragile so the big ones last a long time for me and plastic is cheap too. I try to start small and move to a little larger pot each year. When pretty large I try to put into ground if possible. I have not made the move to grow pots yet but they are probably the best.

Since citrus appreciate humidity in the dry winter climate of a centrally heated house, wouldn't the evaporation from the clay pot be an advantage since it humidifies the air around the tree? Or does the cooling cause more of a problem for the roots than it's worth?