At its simplest, osmosis in tomato plants is the movement of water into a plant through its roots.

This is done when water molecules move from areas where water concentration is high, to areas where water concentration is low. This happens through a selectively permeable membrane.

Another way of describing it is: the movement of water from a weak solution of nutrient feed – where water molecules are high (outside the roots) to a stronger solution – where water molecules are low (inside the roots).

Osmosis is an example of diffusion, which is when molecules tend to distribute themselves evenly in a space.

Nutrient strength when feeding tomato plants
What happens if the concentration of water molecules outside the roots is the same or less than the concentration inside the roots?

Or in other words … what happens if I give my tomato plants a very high dose of tomato food which reduces the concentration of water molecules outside the roots?

Instead of the movement of water into the roots, it will have the opposite effect and water will be drawn out of the roots and the plant will wilt.

Speed of Movement
This is where the subject of osmosis becomes very relevant to tomato growing.
A weak solution (just water) will move more quickly into the roots than a stronger solution.

In other words, a plant can absorb water faster when nutrient strength is lower rather than higher.

So how might we use this knowledge?

On a hot day when plants need plenty of water, feeding high doses of mineral nutrients will slow their water intake by reducing the water concentration. It may even cause them to wilt.

The problem with soil
We just don’t know what’s in it! Furthermore, there is a tendency for nutrient salts to build up in containers of soil and grow bags and become stronger than we are aware of.

This leads to all sorts of issues including:

Stress, damaged roots, undersized ripe fruit, low yield, nutrient deficiency and toxicity etc. and slow water uptake on a hot day that causes wilting.

Detox to remove nutrient build up
A good flush through with tepid clean water can be helpful half way through the season to remove any nutrient salt build up. Think of it as a “detox” for tomato plants!

Growth can be managed on a more technical level by using various nutrient strengths and the process of osmosis.

More about this in coming newsletters!

These issues are more relevant to those who use standard tomato feeds such as Tomorite than to those who use organic methods. Standard tomato food contains higher levels of nutrient salts than organic feed, and is more likely to affect the osmosis in tomato plants and the uptake of moisture through their roots.