I posted in April about the ubiquitous advice on not sowing carrots & parsnips in manured ground, & suggested we should return to older practices for evidence. Some readers took my suggestions & are planning to keep an eye on what happens with their crops, others just reiterated what I would describe as received opinion. I decided to review the evidence further, although I still can’t find any current research on the effects of manure on root crops, especially carrots & parsnips, but will keep searching [links to current research most welcome]. However, I’ve a few points to share.
I’ve talked to a number of experienced growers in Brighton (UK) who have been growing food crops seriously for well over a decade or more, & for whom growing is what they do. Simon (@handsinthesoil) suggested that we’ve forgotten the old practice of double digging, or digging 2 spits (spades) deep, filling the trench with rotted manure, then re-filling with earth. This is very much the practice I reported from Jersey [here] about Parsnips.
Several documents about allotment gardening I’ve found from the early 20th Century also indicate this might be the case. Percy Elford & Samuel Heaton, in The Cultivation of Allotments, 1917, offer this advice on growing carrots:
Carrots. The soil for carrots should be well trenched or dug in during the winter and if any stable manure is applied it should be well rotted and should be placed at the bottom of the trench. (Elford & Heaton, 1917, p.27.)
This guidance in using manure to grow carrots,offered in a guide to growing by Sutton & Sons, Reading, 1921, reinforces the distinction between the application of manure to the surface of your beds & incorporating it during double digging :
For the main crops double digging should be practised, and if the staple is poor a dressing of half-rotten dung may be put in with the bottom spit. But a general manuring as for a surface-rooting crop is not to be thought of, the sure effect being to cause the roots to fork and fang most injuriously.
The other issue is use of fresh rather than well-rotted manure. Could this be the problem, not the presence of manure itself? Another friend stated that we shouldn’t be using fresh manure anyway, as it’s more efficient to use it in the compost heap than on the earth when it’s fresh.
I suggest that perhaps our understanding of gardening practices over the years has been damaged, with a break in the handing on of knowledge & experience due to the fall in popularity of allotment gardening during the 1960s up to the 1990s. As a consequence, we have a broken line of understanding about growing practices, a failure of memory, a void that is now filled by gardening programmes & publications on ‘grow your own’ as a lifestyle choice; also a proliferation of web-based advice & blogs. But what often happens is a repetition of ill-digested information. We would benefit from a return to autodidactism, exploring, sharing & debating issues to do with the land, food & the ecology of growing things. We should create & share knowledge through practice & our own research: a generous dose of citizen science!