Hannah Ritchie and Max Roser published a research paper about how crop yelds changed over the long-term. This article was substantially revisioned in 2019, because in his first version covered aspects of agricultural land use.
In the chart up here we have plotted average corn (maize) yields in the United States from 1866-2014, based on data from the United States Department of Agriculture (USDA) and UN FAO. As we see, average corn yields in the United States remained relatively flat throughout the 1800s until the 1930s. In the period since 1940, yields have increased more than five-fold.
There are a number of factors which are likely to have contributed to sustained yield gains and caused this significant drive in yeld improvements: fertilizer application, irrigation, increased soil tillage, and improved farming practices. A key driver in the initial rise in yield is considered to be the adoption of improved corn varieties from plant breeding developments. The initial period of yield gains in the late 1930s-early 1940s coincides with the transition period of farmers from open-pollinated varieties to hybrids. This process of cross-breeding between open-pollinated varieties, combined with improved breed selection practices is thought to define the key turning point in US corn yields.
Source: Our World in Data