In order to provide larger amounts of nutritious food to increasing populations, farmers improved existing techniques and tools. Farmers improved fertilizers that generated bigger and healthier crops for lower costs than before. In the 19th century, American John Deere designed the first steel plow. Later, farmers plowed their fields using tractors with revolving disk blades. Other 20th century advancements allowed surface irrigation, sprinkler irrigation, and subirrigation to irrigate more than 320 million acres of land worldwide.