Imagine a wall with some swinging doors in it. On either side of the door are children at a party. On one side of the wall, someone gives out cake to any children who come by. You might imagine that children on the non-cake side would move to the cake side.
Now replace the children with water molecules, the wall with a root outer membrane and the cake with salt. Water moves into the roots because the roots have a higher concentration of salt than the water does (because water is attracted to the higher salt similar to how children are attracted to cake).
If there is more salt on the outside of the root than the inside, water moves from the inside of the root to the outside, drying out the plant. Growers who raise plants in containers need to be careful of this because all fertilizers are salts that will build up in the container unless one waters a little more than the plants need (which allows the water to absorb some of the excess fertilizer and leach it away from the root zone). For this reason, most greenhouse growers using media culture want to see 7-10% of the liquid that they feed their plants run through the growing media and be removed from the root zone.
It is important when growing in any container to have a way for the water to drain out in a way that it does not sit next to the media. So if you are hand watering a plant, make sure that you water until water runs out of the plant root and immediately remove that water from around the plant’s container. This will leach the fertilizer away from the plant.
Osmosis allows water into the roots but it does not have the power to feed the whole plant. For that one must look to transpiration.
Water molecules are sticky, including to each other. If you ever see raindrops on a table, you will notice that smaller drops that touch will quickly become one big drop. This is because water molecules like to attach to other water molecules.
In plant leaves there is an important organ called a stomata. It is created by two cells working together to make a gate. When they open the gate, water molecules are able to evaporate from the leaf. The evaporating molecule pulls the next molecule forward, and because water likes to stick to water, the next molecule is also pulled. Each molecule below this is also pulled all the way down to the root of the plant where the molecules pull water molecules on the outside of the root in. It is like a long train with each car pulling the next car.
Water not only sticks to other water molecules, it also sticks to solids. You can see this after you drink some water. Some drops of water often stay on the inside of your glass, adhering to the side. In the same way water sticks to soil particles and other water sticks to the water sticking to the soil. The water molecules directly stuck to the soil adhere quite hard. The water that sticks to the water molecule that is attached to the dirt is less difficult to remove than the molecule sticking to the dirt. And the molecule sticking to the molecule that is sticking to the dirt is even easier to get. So as the plant drinks it needs to exert more and more pressure in the root area to get water that is closer and closer to the soil particles.
In other words, as the soil gets drier and drier, the plant builds up more and more pressure to drink up the more difficult-to-get-to water in the media particles.
Now we’re getting to why this matters to the gardener. When it becomes too dry, this pressure combined with the pressure created by osmosis, together with the loss of moisture is powerful enough to damage to the tiny root hairs.
But, if you water a plant that is exerting a lot of pressure in the root zone, you will suddenly have a great deal of water rushing into the plant and up toward the stomata. This high-pressure water will burst through the cells in the leaves at the end of the veins and kill the tips or edges of the leaves.
For this reason, it’s advised that you should not let the plant dry out too much. A plant that is too dry will start to experience damage in the root hairs, and when you water this plant the pressure will damage the leaf tips. If you do have a plant that has dried out too much, try to only water it a little bit, then wait a few minutes and water it a little more and then do this again. This allows the plant to only take in a little water while the root pressure is high. Let the plant reduce it’s pressure before you feed it a lot of water. This will reduce the leaf tip burn.
If the plant gets too hot, it will shut all the stomata effectively reducing its ability to drink. This is an important function that allows the plant to not loose so much water through transpiration that it cannot keep up with water uptake in the root zone. This can also be stressful for the plant, so it is something to take into account as a grower. Try to keep the temperature cool where possible so that the plant can continue to transpire.
A plant uses two important tools in order to drink, osmosis and transpiration. Osmosis allows water to move from outside a plant’s roots to the inside of the roots while transpiration is the engine that brings the water to all parts of the plant.
This matters to the indoor grower because we must ensure to keep salts from building up in the root zone, letting the plant dry out too much between times we water it, and from letting the plant get to hot. Doing this will keep the plant healthy.