CF Industries’ planned $2.1 billion expansion at its Donaldsonville nitrogen complex is on schedule and will begin producing urea, the most popular nitrogen fertilizer source, in the third quarter.

The Donaldsonville complex was already the world’s largest nitrogen plant. The plant produces urea, urea-ammonium nitrate, or UAN, and anhydrous ammonia urea — the three nitrogen-based products most heavily used by agricultural, industrial and other markets.

The company expects the UAN expansion will start operations in the fourth quarter of this year while the ammonia project will be online in early 2016.

However, CF Industries said the cost of the Donaldsonville projects and new ammonia and urea plants at Port Neal, Iowa, have increased by 10 percent. Originally, the Port Neal project’s cost was estimated at $1.7 billion.

CF Industries said the costs for building the Iowa plants has increased because material and labor costs will be higher than originally thought.