Here are some examples of the knowledge we have lost since the advent of mechanized and petroleum based farming after World War II. This list is by no means exhaustive and does not even begin to include home-based skills that are all but forgotten.
1. How to plant seeds. The instructions on how to plant seeds that are on the back of the seed packets you buy at the store are all but useless and in many cases cause a waste of garden space. Do I really need to plant my lettuce 6" apart in rows 1' apart? The answer is 'yes' if you have depleted your soil with pesticides and herbicides and you need to run a large tiller or tractor over the garden. With a nutrient dense, well drained soil you can plant seeds much closer together and lay out your rows much closer together. For instance, when we plant kale or collards - and even turnips - we just scatter the seeds in the row we have layed out. If two or three or fourteen seeds land in the same space it's okay. Nature does the rest. When the plants are up we thin them to make some room for the roots to grow and use the thinned plants to give our animals a treat or eat them ourselves.
2. Green fertilizing - when I was a kid, my grandfather always used to plant alfalfa in his recently tilled fields. He did this in the fall after his corn or wheat were harvested. I asked him once why he did this. He simply said, it's good for the soil. He didn't know or didn't care to share the scientific basis for this. This was in the early 80s. Thirty years later many large scale farmers have forgotten this kind of natural fertilizer. One rice farmer in south east Arkansas, instead of burning the stubble in his newly harvested field, decided to turn it under. Many of his peers couldn't understand it. They burn the stubble from their fields which is now conventional wisdom. The farmers that burn their fields will spend more in artificial fertilizer next year than the farmer who tilled his field under.
3. Natural pest control - my grandfather always planted marigolds around the border of his garden. When asked why - he did not know. "The plants just do better." The scientist will tell you that the marigold releases a scent or something in its roots repels insects. That may be. Now you know why marigolds can be bought in such quantities at your local nursery.
4. Genetic diversity - many of the farmers I know and several of them in their 70s and 80s think tomatoes only come in 1 color and 1 shape. Red and round. When I brought some heirloom tomatoes to the office one day, one of these self-proclaimed old-timey farmers couldn't believe the stripes and different colors our tomatoes had and assumed they were some kind of strange mutated or genetically engineered vegetable. I had a hard time convincing him that the tomato he had been holding in his hand was from seed first recognized hundreds of years ago and that his perfectly round red tomato was the mutant!
5. Pigs eat grass. Yes. I don't know how many times I've been told that I can't pasture my pigs because "pigs don't eat grass". This comes from several decades of mass produced pork. Pork houses keep the pigs on concrete and crowd many animals into one stall. Rings are put through their nose to keep them from trying to root and their tails are cut off to keep the crowded, bored, and angry pigs from eating each other's tails. Now I ask you, what are the two most prominent features of the pig? The snout and the little curly-q tails. What do we do with these innately piggish things? We lop them off or ring them, essentially removing the pig-ness from the pig. Our pigs eat our pasture - yes they root it up too - but the grass recovers. Three other benefits occur at the same time. The soil gets aerated; it gets fertilized; and the pigs are happy and healthier. I know farmers who believe that the best way to raise hogs is to lock them in a 4'x8' stall and force feed them corn. This is the result of knowledge lost from Old Solar Agriculture. The new knowledge they operate by is assumed to be the best because Industrial Agriculture has permeated our methodologies.
6. Forests are the model for renewable agriculture. In 1943 or thereabouts, Iowa was 98% agriculuture. Small farms (under 100 acres generally) dotted with wood lots, crossfenced, and teeming with biological diversity - chickens, hogs, cows, horses, and the forests all playing a part in the fertility of the land. Today, Iowa is 98% agriculture mostly with farms over 100 acres, cleared of trees, fences removed, planted in one vast monoculture of corn. Many farmers buy all of their groceries from the store and chemicals have replaced (insufficiently) the fertility of the soil that is best modeled in our forests. Leaves dropping, getting wet, manured by wildlife, and composting into rich loamy soil ready for the next generation of trees.