Why are we instructed not to use manure before planting carrots?

Question

It's said that we shouldn't use manure when preparing ground intended for carrots. Is it that carrots don't thrive on manure, or is it that they fork and splinter too much?

What about other roots? Radishes, beetroots etc?

Answer 1

The root cause (no pun intended) is that excess nitrogen will make the roots fork. I've heard two different theories as to why:

1. "Hot" fertilizer like fresh manure burns off the fresh roots and makes them fork.
2. A lot of nitrogen in shallow soil fails to encourage the plant to scavenge deep in the soil for nutrients, and they send out roots sideways instead.

Or it could be combination of the two -- but the latter came from a more reliable source and fits more with my experience. (One of my books versus something seen on the internet -- sorry I can't remember exact sources.)

Even in soil where I haven't added extra nitrogen but was very fertile with other nutrients, I have seen carrots develop not so much forked roots as fat sideways roots instead of the usual fine "hairs".

Answer 2

Fresh manure can cause carrots to fork. If you use any fertilizer, make sure it's well-rotted manure instead. (source)

Same goes for beets as well. (source)