I’ve been having a conversation on Twitter with John Harrison (@allotmentorguk) about the truth or otherwise about manure causing forked. He posted advice a couple of days ago reminding gardeners that carrots & parsnips don’t like soil that has been manured the previously autumn as it causes the roots to fork & split. I disagreed with him, at least about parsnips, as I had a vague memory of reading somewhere that research had indicated that this wasnt actually true. John tweeted back that he’d be interested in the link, but could I find it? Nothing! I’d lost the information & felt a bit silly.
However, I’ve been sleuthing & have come up with some interesting results. Firstly, its evident that information gets repeated without much checking back as to its veracity. Every search I did came up with a list of sources, each merely repeating the ‘fact’ of manure causing forked parsnips, but no evidence. I know from our allotment that carrots do fork in manured soil, but our parsnips tend to fork if sown in stony soil – & our chalky soil is full of flinty stones. I also remembered the origin of my questioning of the link between manured soil & forked parsnips; it was Joy Larkcom, in her book, ‘Grow your own Vegetables’, who said: There was a long-held belief that growing on recently manured ground led to fanged roots, but experiments have demonstrated that this is not necessarily true (Larkcom, 2002; 298). Unfortunately, she gives no further information.
This sent me on a trail for evidence & I discovered a note in the Journal of the Royal Agricultural Society of England, which suggested a different tradition of husbandry. J. Le Couteur, writing from Bell-Vue, Jersey, in April 1840, states:
In one case I found the plants to answer well by spreading a portion of the manure on the surface of the ploughed land and then earthing it up into small ridges, a foot apart, with a double-mould board-plough.
The seed was then sown on the top of the ridge and rolled in, which succeeded extremely well. (p.421)
Earlier, Le Couteur mentions the practice of growing parsnips on poor black heath soil no more than 7-8 inches depth, yet raising a good crop through heavy manuring. He does acknowledge that the crop forks away into fingers, but indicates this is the plant’s response to the hard subsoil rather than the manure because, where the soil runs deep, the roots will run down a foot or two, of a good size.
Le Couteur also reports, as a footnote to his recommendation to use new seed, his own experiments with 2 year-old parsnip seed saved from 1838, not a grain of which will vegetate.
The Bulletin of Miscellaneous Information, published by the Royal Botanic Gardens, Kew has an article in its 1918 edition that deals with the incidence of canker in parsnips grown in the Vale of Evesham. The section on parsnip diseases suggests that, rather than causing forking in parsnip roots, heavy manuring of soil before sowing could lead to too rapid growth & leave parsnip roots susceptible to canker if they then split in spring & summer in response to intermittent & alternating heavy rain & sunshine.
My conclusion is that sowing in manured soil does not necessarily lead to forked parsnip roots, although carrots will produce fanged roots; however, growing parsnips in heavily manured soil could increase their susceptibility to canker if the roots grow too big too fast, & crack in response to intermittent rain.
Because carrots have overtaken parsnips in popularity, we tend to lump them together & assume what’s good (or bad) for carrots is also the case for parsnips. Steven Jay Gould wrote an essay in which he explored the use of secondary sources by academics, & how their consistent failure to return to original material had led to inaccuracies creeping in over time, until the quote or example used no longer resembled the original, but was accepted in its place. I think similar things happen with practical growing advice; we need to observe & break the rules in our own growing practices in order to see whether things work or not & be able to comment with knowledge rather than repeat advice we’ve never tested.