Morocco holds 45% of the world market share in phosphate rock (Australia and China are the other large producers). Global phosphate consumption grew in 2003 and prices rose in 2005. The growing economies of India and China and their increase in agricultural productivity heavily influenced the prices of phosphate. The price rose from US$40 per ton in 2005, to over US$400 per ton in 2008, and then decreased to under US$200 per ton in late 2008.
I gather that a big reason why Morocco wants to hold onto the Western Sahara is because of the phosphorus industry and the possibility for offshore oil drilling. The Moroccan government controls all of Western Sahara's trade and economic activities, which are dependent on agriculture, fishing, and phosphate mining.
Phosphorus factory
Oil containers
References:
PPhilip A. Szczesnia, "The Mineral Industries of Morocco and Western Sahara,"
"Phosphate Market," Nupower Resources Ltd., 2010 http://www.nupowerresources.com.au/INVESTORINFORMATION/PHOSPHATEMARKET/tabid/106/language/en-AU/Default.aspx