It's a fairly common scenario: You notice that the soil of a houseplant is bone dry when you just gave it a big drink of water the day before. What's wrong?
There are several possible causes, one of which almost never occurs to home gardeners. But first, the easy answers:
Did you actually use enough water? You know you have when you see water dripping into the saucer beneath the pot. If there's none there after several minutes, you need to water more.
Second is that the plant is rootbound -- which means the root system is much too large for the amount of soil -- and absorbs all the moisture practically instantly and then needs more. This usually occurs with plants you've grown in the same pot a long time and those you've just bought.
How do you tell if a plant is rootbound? Check the hole in the bottom of the container to see if roots are visible. Also, when the soil is dry, carefully remove the plant from its pot and look to see if the surface of the soil covered with roots growing around and around the root ball. If the plant is rootbound, repot it.
But what if neither of those causes seems to apply to your situation? You water and quickly see that water show up in the saucer, but the next day the soil feels dry again.
A common reason is that the potting mix has dried out and isn't absorbing the water. Most commercial potting mixes contain peat, which holds water well once it has been moistened, but -- as everyone who works with sphagnum peat outdoors knows -- is difficult to wet the first time.
What that means in potting soil is that if the peat is allowed to dry out at some point (you went on vacation, you forgot to water on a regular schedule), it won't absorb water readily from then on; the water just runs down the sides of the pot into the saucer.
If you think this is your problem, there are several ways to moisten the potting mixture again so it will work as it's supposed to.
If the plant is in an 8-inch pot or smaller, fill a bucket or tub with lukewarm water. Then, holding your hand over the top of the soil at the base of the plant (to prevent the soil from washing away), immerse the plant, pot and all, until the water stops bubbling. Then take care not to let it dry out completely again
Obviously that won't work with most houseplants in big pots. There you have to use a surfactant or wetting agent to get the soil to absorb water. Surfactants can be bought at nurseries and home stores, but liquid dishwashing liquid (Joy, Ivory, etc.) works just fine.
The main thing is to not overdo the detergent -- that can harm the plant. The usual recommendation is a few drops per gallon of lukewarm water. I'd say not more than 1/4 teaspoon. Slowly pour this over the soil, trying to wet all portions of the surface. (As much as possible, stay away from the edges of the pot, because that water will run straight through and not be absorbed.)
Sometimes that works right away, but occasionally you may have to repeat the procedure several times over a few hours for all of the soil mixture to finally stop repelling water. (Be sure to remove the excess water from the saucer each time.) But it does do the trick.
Two final notes: If you read the bags of the potting soils you buy, you'll find that some already contain wetting agents to prevent just this type of problem, so that can be another solution if this is a perennial problem with your houseplants.
Or -- no kidding -- you can get your houseplants their own accounts on Twitter and have them tell you when they need watering! Sometimes I think technology has gone a tad too far.