Thousands of years ago, agriculture began as a highly site-specific activity. The first farmers were gardeners who grew individual plants, and they looked for the microclimates and patches of soil that favoured those plants. But as farmers acquired scientific knowledge and mechanical expertise, they enlarged their plots using standardized approaches such as ploughing the soil and spreading animal manure as fertilizer to increase crop production. Over the years, they developed better methods of preparing the soil and protecting plants from insects, and eventually, machines to reduce the labor required. Starting in the 19th century, scientists invented chemical pesticides and used newly discovered genetic principles to select for more productive plants. Even though these methods maximized overall productivity, they caused some areas within fields to underperform.