These soups all require a pale vegetable stock. Using stock cubes is quick and convenient but I have found that some of them are too strongly flavoured, particularly with celery. To make a light coloured stock, use this recipe. (based on Rose Eliot's) What you put into it can vary with what's available and each batch of stock will always be slightly different . I store mine in a Kilner jar in the fridge for up to a week.
For 900ml to 1100ml of stock you can use as a basic batch:
1 large whole onion, peeled
1 carrot, scraped
1 celery stick
2 garlic cloves
a bouquet garni (use fresh herbs if you have them (parsley, thyme and a bay leaf tied together) or you can use dried herbs prepacked in a bag.
Cover with cold water (I find that 1½ litres is fine)
Bring the pan to a boil then simmer gently for an hour with the lid partly off. If you are not using it straight away, let it cool before straining and storing. The volume will have reduced down to about 900 - 1000ml.
You can use leek tops (make sure you rinse any soil out of them before adding) and peelings from well washed potatoes. Allspice and juniper berries (a couple of each) and thinly pared lemon zest also add flavour. Use brassicas with care or not at all, the flavours can be quite stong and can overpower delicately flavoured soups.