China is clamping down on fertilizer exports to protect its domestic market, a number of industry sources said, putting an additional strain on global markets that were already grappling with shortages caused by the ongoing conflict in the Middle East involving Iran.

China remains one of the world’s largest fertilizer exporters, shipping more than $13 billion worth of product last year. Historically, the country has used export controls to stabilize domestic prices and ensure supply for its own agricultural sector. Now, those same policies are intensifying concerns across global supply chains.

Shipments moving through the Strait of Hormuz, a critical global trade route, account for roughly one-third of the world’s seaborne fertilizer supply. With that corridor effectively constrained by conflict, additional export restrictions from China are compounding an already fragile situation. In mid-March, Beijing banned exports of nitrogen-potassium fertilizer blends and certain phosphate varieties, according to sources who spoke with Reuters.

The move has not been formally announced by Chinese officials, but Bloomberg News reported earlier this week that the restrictions are already in effect. Combined with existing bans and quotas on products like urea, only a limited number of fertilizers — with ammonium sulfate among the few products still available for export — remain available for export.