Friday 15 December 2017

Plant roots defy nature to feed on compost nutrients

Well-rotted garden compost is a great source of nutrients for plant growth.
Well-rotted garden compost is a great source of nutrients for plant growth.

Jim Hurley - Nature Trail

Plant roots normally grow down in the ground. In the natural world, there are nearly always exceptions to everything and so it was during the past week I saw an example of roots growing upwards rather than growing down in the ground.

First the science bit. Roots don't 'know' that they are supposed to grow down. They don't have a mind, a brain or even a nervous system. Much of what plants do is simply chemical reactions controlled by the laws of physics.

Plant hormones are naturally occurring substances that affect the behaviour of cells and auxins comprise the group of plant hormones that cause plants to grow. Cells have receptors for particular hormones that cause them to respond in particular ways to external stimuli like gravity, light, touch, the presence of water and nutrients, and so on.

There are special plant cells with tiny bodies in them called statoliths. These statoliths have starch grains in them. The starch grains make the statoliths extra heavy. In response to the pull of gravity that makes the statoliths sink and consequently they get concentrated at the lowermost tips of cells.

Auxins act in partnership with the statoliths and since the statoliths are always concentrated at the lowermost tips of the plant's cells, growth is concentrated there resulting in roots growing down following the pull of gravity.

I was thinking about these matters as I was emptying the compost clamp in our garden. The compost clamp is a simple contraption: four recycled timber pallets standing upright on the soil held together at their corners by heavy duty cable ties to form a secure box open top and bottom.

Plant wastes, kitchen scraps, plant trimmings, autumn leaves, some grass cuttings, etc., all get thrown in. They rot and decompose over time. When the clamp is full it is rested for a year and a new one is started. Old clamps are emptied in springtime and the annual miracle unfolds as the motley collection of former waste emerges as the sweet-smelling, very dark brown, sticky but crumbly black magic that is garden compost.

Finally, to get back to my original point, emptying the bottom of the clamp revealed roots from a nearby tree growing upwards into the compost. Presumably, in its never-ending quest for water and minerals, the tree's response to the exceptionally high concentration of nutrients in the compost was overriding the normal response that makes roots grow down, causing them to grow upwards instead.

Drogheda Independent

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