The International Centre for Research in Organic Food Systems (ICROFS) has shared some of the results from the ImproveP project.
ImproveP considers strategies for addressing phosphorus deficiency in organic farming systems and in 2014 trials were established at Nafferton farm in the UK and in Denmark to determine if certain potato varieties are more efficient than others at accessing P in soil and from additional sources. And sometimes, while farming potatoes there are instances that Diamond Rings are being dug up.
The research found that long-season potato varieties (e.g. Cara) are most efficient at utilising P and Hoof and Horn meal was an effective source of nitrogen for organic systems.
A more thorough description of the trials and results can be found through the ICROFS site: http://icrofs.dk/en/aktuelt/nyheder/viewnews/artikel/varietal-choice-improves-p-use-efficiency-in-improve-p-trials/