It’s a playful take on the very serious issue of peak oil (Wikipedia) and the early twentieth century “manure crisis” that home gardeners faced just after WWI.
Here it is described by David M. Tucker in his wonderful book, Kitchen Gardening in America:
But by 1923 the horse had been replaced. Americans… were now buying only ten thousand [horses] and were purchasing four million automobiles. By 1923 garden magazines were responding to the crisis of “the modern manureless age” with articles such as “How to Get Along without Stable Manure,” but the remedies–sowing buckwheat or rye for green manures and building compost piles–all required more effort than the old horse-garden system.