Soil salinity and water stress

You water and the grass is not revitalised, it looks limp and weak. The plant doesn't recover its body. What's going on?

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Sometimes a wet soil supports plants that have a distinct appearance of hydric stress. On these occasions the problem is not water deficit, but the effects of salinity on water uptake.

When the concentration of salts in the water in the cells of the root system is lower than the concentration of salts in the soil water, root-soil flow occurs.

In agronomy, salinity refers to the presence in soil and water of ions from soil erosion in watercourses, from marine origin or from the application of inorganic fertilisers. Depending on the final salt concentration in the soil pore water, the plant will be able to absorb water to carry out its processes or else it will have problems in its productivity and health.

The semi-permeable membranes of the plant kingdom absorb water in search of a salt balance. Soils have lower salt concentrations than the roots, and in order to equalise the concentrations, the membranes allow water to pass through. However, if the situation is the opposite, the plant will find itself in a situation of hydric stress known as plasmolysis.

In the following GIF you can see how not all the cell wall is occupied as it should be for good turgor and even a slow retraction can be seen. This is due to the fact that in the experiment an onion slice was taken and wetted with a saline solution, the resulting effect is the emptying of the liquid volume of the cells to the outside.

Plasmolysis is a much more common effect than we think, it is always seen when some lettuce is left in the salad. The salts from dissolving the dressing leave the leaf looking limp and not very turgid.

The effect of liquid transmission on the equilibrium of solutions is called osmosis.

Andreas Franz Wilhelm Schimper in his collaborative book Plant-geography Upon a Physiological Basis develops the theory of physiological dryness. The concentration of salts in a soil reduces the amount of useful water the plant is able to absorb until, for a particular species, it cannot survive.

Depending on the origin of the salinity the most effective method to remove it is by using irrigation that dissolves the salts and percolates down to deep profiles that will not affect the roots of the plants.

This is not always the reason why salts are harmful to plants, this salinity can have other negative effects that will not be discussed in this entry:

• Toxicity
• Nutrient ratio imbalance
• Energy expenditure due to physiological change in case of stress
• soil de-structuring