Farmers were already using the cheapest source of energy, sunlight. Oil came late to the party here after plant matter and coal because it was slightly more convenient not because it has more energy.

Back in the day, wood powered tractors beat the fuck out of horses, and horses beat the fuck out of human labor because they could digest cellulose. Oil is just very slightly cheaper. Even today people heat their homes with both wood pellets and oil, meanwhile there’s cheaper alternatives.