I am not a vegan. I'm not really a very good vegetarian. Which really means that I'm not a vegetarian. I eat dairy products fairly often. If you're one of the many that think that nothing gets killed by drinking milk, you really need to look into veal farming.
I probably could be a vegan with a little more moral fibre. However, what would happen if the whole world decided only to eat plants and plant derived food?
I don't really think that much would happen. I have had discussions with vegans about the use of animal manures to produce nutrients for vegetables. Vegans do not use animal manures because this helps to support the exploitation of animals.
Now I am very sure that no matter how vegan you are, you will still be using animal excreta as a nutrient source, if you grow your own food. Unless you sterilise your soil, worms will be making a major contribution to recycling of nutrients. Not only through their worm casts but they excrete urine through "kidney" type organs. Urine is rich in nitrogen. Wild birds will be manuring the soil as they fly across or forage on the allotment. There are many more examples where animals will make a contribution to nutrient recycling.
Yet animals can only recycle nutrients. They cannot make them. All their food has to come from plants so they will just be adding back what plants themselves have produced. Animals can concentrate nutrients in their manures and this why farmers and gardeners use them but they cannot manufacture nutrients.
So where do these nutrients come from? I think that the major source of nutrients comes from the weathering of parent rocks. While this is an extremely slow process, relatively high concentrations of plant nutrients can eventually build up and be repeatedly used as it is recycled through plants and animals over the years. Over thousands of years nutrients can be imported through the action of glaciers, streams and rivers and wind depositing eroded material.
I have used rock dust as an additive for improving the fertility of the soil but it will take years before the minerals erode enough to make a significant contribution to the soil fertility. This does not stop me from doing it. Investing in the future is one of the things that gardeners are good at.
Before the Haber - Bosch process was discovered, nitrogen could only be fixed by symbiotic and free living bacteria. Legumes like clover, field beans and peas could be used to enrich the soil with nitrogen - an essential nutrient for the production of proteins and nucleic acid.
For the symbiotic Rhizobium, a major nitrogen fixing bacteria, it is only contact with a legume plant that will trigger the formation of nodules and nitrogen acquisition. In its free living state it is heterotrophic, getting its nitrogen from dead organic material.
A small amount of nitrogen fixation is done by free living bacteria. A little more is produced by lightning which produces nitrogen oxides that dissolve in rainwater.
When designing a completely animal free system of food production, composting must play a very important part. Plant composts do not produce nutrients. The nutrients are already there. Composting just concentrates nutrients and converts them into a form that is easy to add to the soil and available to plant roots to absorb.
In a similar way plant derived mulches, which contain nutrients, will eventually rot down and be incorporated into the soil.
Composts do not concentrate nutrients to the extent that animal manures do. However, the amount of nutrient that plants need to produce good, healthy vegetables is less than most people believe.
Composts do not only add nutrients to the soil. They add an ability for the soil to retain nutrients and avoid leaching; they add compounds that improve the soil's structure by sticking particles together and compost provides food for soil invertebrates that aerate the soil and improve drainage.
Some would say that burning woody material will produce nutrients that can be added to the soil. This is incorrect. The nutrients are there already in the plant material. All that burning does is concentrate it in the ash that is left after driving off all the other nutrients as gaseous oxides. Burning is not an efficient nutrient recycling process.
The reason why gardens can be so productive, and they are much more productive than farms, is because gardeners can utilise many sources of imported organic matter. In towns organic waste in the form of leaves, shreddings, weeds and prunings can be brought in from outside the garden and incorporated into the garden top soil. Vegetables can be grown at a much higher density and diversity encouraging a diet that is richer in variety than one based mainly on animal protein.
The production of food would become much more labour intensive. People would have to become much more responsible for the production of their food and where it comes from. Land once used for the production of animal feeds could be given over to planed forest gardens which would produce many more useful products than a rye grass monoculture.
Planning for the sensible use of water and the prevention of nutrient leaching would become much more of an imperative and where the recycling of materials becomes second nature.
So changing our diet to one that consists completely of plants will not change the world. It may well change us though and our relentless exploitation of the earth's resources.