Although not so-far described in detail, we have seen that water enters plant roots through the root hairs. Each hair is an extension - up to a length of about 4 mm - of a single epidermal cell (the epidermis is the outermost layer of cells on a herbaceous plant). Because these hairs are so fine, they are able to work their way around individual soil particles and absorb the water surrounding them. This is done by osmosis.
Soil solution is usually HYPOTONIC (contains proportionally more water than solute) to the cell contents of the root hairs, so water moves INTO them by osmosis.
However, problems arise when, for some reason, the soil solution becomes HYPERTONIC (contains proportionally less water), since this causes water to move OUT of the roots hairs and into the soil solution. This is dangerous, since it initially causes wilting, then eventually plasmolysis (when the cell contents pull away from the cell wall) and, eventually, death.
HOW SOIL SOLUTION STRENGTH VARIES
Soil solution strength varies:
- naturally
- induced
As the soil and its solution dries, it loses only water, not solute, and becomes more hypertonic to the root hairs
INDUCED VARIATION BY FEEDING
Liquid feeds are a prime cause. If too strong, they can easily be hypertonic to the roots, especially as the soil dries and loses water. Great care should always be taken when feeding. It is so easy to damage a plant. This is why I do not recommend feeding young plants (plants are more vulnerable when young).
Another cause is positioning strong feeds and composts too close to the roots. I always leave a distance of at least 5 cm (2 inches), and even this is hardly enough. I also try to place feed to the side of the roots, if possible. This way, they don't have to grow through the feed - they can take it, or leave it alone.
There is a lesson to learn from all this: FEEDING IS EXTREMELY DANGEROUS AND SHOULD BE AVOIDED WHENEVER POSSIBLE
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