Factbox: About the crop nutrient potash

TORONTO (Reuters) - Potash was once of little interest outside the agricultural sector, but the crop nutrient has become a hot item, with pricing having outperformed other commodities during the boom and the subsequent downturn.

A small coterie of companies control the vast majority of global potash production.

Here are a few facts about potash:

* Potash is the common name for various compounds containing potassium, which are used mainly as fertilizers.

* Potash pricing has been extremely volatile for years. The price of the nutrient began the last decade below $150 a tonne and soared above $1,000 a tonne during the commodity boom in 2007-08. It has since crashed to about $350 to $375 a tonne levels, but is still well above its historical average.

* Potassium, identified in the fertilizer world by the chemical symbol K, is one of the three main macro nutrients required by plants, along with nitrogen (N) and phosphate (P).

* Potash helps improve a plant's disease resistance, crop quality and increases yields. It is the only potassium fertilizer source; there are no practical substitutes.

* Only 12 countries produce potash. Canada, Russia, Belarus and Germany account for more than 75 percent of global supply.

* Eight companies currently control more than 80 percent of global supply: Potash Corp of Saskatchewan, Mosaic Co, Agrium Inc, K+S, Uralkali, Silvinit, Belaruskali, and Israel Chemicals.