I sent you a PM ages ago Piers re split carrots. Just wanted to inform others now of my opinion on the matter to provide a more balanced thread.
By almost all accounts googled on carrot splits, they almost invariably say it is due to moisture irregularity in the soil, for example:
"Summer thunderstorms after long periods of drought can cause the root to take up a sudden increased volume of water. The root then splits open as the skin cannot cope with the sudden surge in growth."
http://www.rhs.org.uk/advice/profiles0804/vegetable_splitting.asp
"Carrots are very sensitive to soil moisture levels. When the soil isn’t moist enough, the main root develops many small feeder roots so it can soak up any available moisture more efficiently. At the same time, it forms extra tissue to carry the water, so it becomes tough. On the other hand, if there’s too much soil moisture, carrots sometimes split along their whole length. This happens more often when they are close to maturity."
http://www.yardener.com/ProblemsofCarrots.html
The only site I came across that mentions Boron is a site that sells liquid fertilisers , but even they admit that the most common cause is moisture stress: "Calcium and boron are critical nutrients for carrot quality and a deficiency of these nutrients causes splitting in carrots and poor skin quality.
Water management is critical in calcium nutrition as the most common cause of calcium deficiency is moisture stress."
http://www.sjbagnutri.com.au/crops/carrots.htmRachan
Split carrots advocate.