Root Value or Root Cubed? The Q of Planting Spaces...

Right, you lot. You, there, sit up at the back and tuck your shirts in....
Let's be Sirius for a few seconds, please!

I'm mad-busy stocking up on stuff to go in my polytunnel and one of the items is onion-sets.

It says to plant the sets 10-15cm apart. At maturity the onion gets no bigger than 2.5cm / 1 inch!

Why do I need so much room, please?

(Do roots 'compete' and stunt growth even if there's enough water and nutrients?
btw I'm aware of leaves being able to sense competition from other vegetation and negatively influencing growth as a consequence, but roots?)

 

I reckon its just about competition for light and nutrients. Water is easy to supply here in the UK so I'd not be worried about that.

Light is a bit more of a problem. Pack everything close together and they'd shade each other.

Nutrients, probably a bit tricky. Sure you can just add loads of soluble feed and that might be ok, but in nature the abundance of salts are not so readily available to plants, as they are locked up in bulky organic matter like dead vegetation etc. Plants are not actually very good at all at extracting nutrients from soil. In fact many species are pretty rubbish at it. So they enlist some help. They form a symbiotic relationship with soil microorganisms that do all the chomping on the dead animals and veg in the soil, unlocking nutrients as they go. That's the tricky bit. These microorganisms need air just like you or I. If the soil is too wet, they simply drown, and then obviously they are no use to the plant. So, when working out how much room a plant will need, you sort of need to factor in how much soil the roots will need that will hold not only enough water, but also enough nutrients, and still have enough air pockets for the microorganisms to breath.

I reckon you need less surface area if the depth of the loose, nutrient rich soil is greater, so same volume of good soil, just less area.

 

Yes, thanks, clueless.
I think I need to read up on intensive gardening....
(You see, I water with compost tea water, which means loads of beneficial bacteria and nutrients so I should be able to squeeze a few more in, notwithstanding the light issue.)

 

Sadly, yes. I bought the bags before reading them. The sets are about 10mm. I'll try and take a photo of the label later today.