Q: My onions rotted last year after pulling, as the dung was too rich. So, I put lime on the bed. Is it okay to sow on the same bed this spring? PG
A: What a shame, there is nothing more frustrating than losing a crop this way after putting so much time, love and effort into growing it. To prevent the same sorry fate befalling your next crop, here are a few useful tips.
First and foremost is the importance of practising crop rotation in your garden or allotment, the time-honoured, most effective way, to prevent a possible build-up of crop-specific pests and diseases.
Onions belong to the same plant family as other onion-related crops such as garlic, leeks and shallots. So, all of these edible crops should ideally be grown together in a different part of the garden or allotment to last year, along with legumes (peas, beans) and root vegetables (carrots, parsnips, beetroot, celery, parsley, celeriac), and then in a different part of the garden or allotment again next year, all as part of a three-year rotation plan. Some gardeners follow a four- or five-year rotation plan but for the purposes of brevity, I’m using the three-year model as an example.
In two other distinct areas of the garden/allotment, grow the other two groups: group two can include brassicas such as cabbage, broccoli, Brussels sprouts, kale, cauliflower, radish, swedes and turnips; group three can comprise potatoes, with each one of these consecutively occupying the space taken up last year by your onions.
Crop rotation aside, you mention that manure may have been a possible cause of your onions rotting. Certainly the recent addition of fresh manure can cause problems with storage and also make the soil less firm (onions like a firm soil), which is why it is recommended that baby onion sets are planted into ground that has not been recently manured.
But you also mention adding lime as a way of counteracting the manure, presumably because you are concerned that the latter has made your soil too acidic. Well-rotted manure does not make soil acidic but instead typically does the opposite. Also, like most vegetables, onions do best in a soil pH of six-seven (very mildly acidic to neutral), so adding agricultural lime is not necessary unless your soil pH is coming in below six. A simple soil test (you can get one in any good garden centre) will confirm if that is the case.
Despite their dislike of fresh manure, onions are still hungry crops that like a well-drained soil that is rich in nutrients and well-rotted organic matter, so working some organic chicken pellets and well-rotted home-made garden compost into the soil a few weeks before planting is a good idea.
This crop is vulnerable to a number of destructive diseases, some caused by wet growing conditions (onion neck rot). It is possible given the very heavy rainfall last autumn that your onions fell victim to this disease, one of the reasons why it is important to cure or air-dry the bulbs very well for a couple of weeks immediately after harvesting and then store them in a cool, dry, well-ventilated space. To further reduce the risk of your onions rotting in storage, concentrate on varieties known to store well such as Stuttgarter, Sturon, Santero and Hercules.