Before any question of yield or technique, a farming system has to keep its soil from wearing out, or it will fail over time no matter how productive it looks today. Long-lived farming cultures solved this problem, and their solution let the same fields be cropped without a break for centuries without losing fertility.
When farmland is scarce, farmers can keep a single field producing all year. They grow several crops in sequence or side by side. They rotate in beans, peas and clover, which add nitrogen to the soil, and they plough in green plant material to build up the soil's organic matter.
Farmers under economic strain may lose the initiative that slow, labour-heavy soil care demands. They then turn to chemicals and labour-saving shortcuts in place of methods that keep the land fertile over the long run.
A farm can stay fertile for thousands of years if everything it grows comes back to the soil as manure, including human waste from both towns and villages. Treating these wastes as precious makes them valuable: the people who collected sewage were paid well for it. Flushing sewage into rivers and the sea throws away the nutrients the next harvest needs.
Careful farming without imported food or machines can feed extraordinary densities of people. One farming region supported over 3,000 people per square mile of farmland, along with their animals. Another fed nearly 47 million people on 20,000 square miles of fields while importing almost no food on balance.