A 40 litre container will let you grow between 3-5 seed potatoes in it and produce a large crop as long as they are well fed and watered.
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What do you feed your potatoes?
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There are many suitable fertilizers for this job that are in most good garden centres.(can't name a product on here)
I have read adverts saying that 100lbs of spuds can be achieved in these 40litre grower bags.
You must not let the compost in your grower bag dry out, it should always be moist but not saturated.
The water is the key to a good crop. A regular watering system will work best rather than one big splash of water a week.
Regular watering should give you consistant growth and stop the potatoes cracking.
Make sure they get plenty of good sun light and support the Haulm with a cane if it starts to fall over.
Is there anything else you are unsure about?