In addition to petroleum and natural gas, the country's main commodities, Kuwait produced caustic soda, chlorine, cement, clays, clay products, fertilizer, lime, salt, and sand and gravel. The cement and fertilizer production plants were damaged by retreating Iraqi troops during the 1991 Gulf War. Cement production rose from 98,000 tons in 1991 to 500,000 in 1996 and two million tons in 2000. Ammonia production (nitrogen content) in 2000 was 409,500 tons; and output of urea (nitrogen content) was 287,600 tons.