Nitrogen is crucial for growing rice, especially in fertile fields. Researchers studied 13 types of rice in China to find the best amount of nitrogen to use. They found that most rice types grew best with 150-225 kg of nitrogen per hectare. Adding more nitrogen made the grains smaller but increased the number of grains per plant. Some rice types needed more nitrogen than others to grow well. Older rice types did best with 225 kg of nitrogen, while newer types needed different amounts.