Assuming a vegan diet, where the food is not altered or produced by the exploitation of animals. To be sustainable nutrients that are removed from the soil by growing plants need to be replaced. Organic fertilizers are fertilizers derived from animal matter.
Can a modern diet be vegan, organic and sustainable?
Short version: Yes.
Vegan organic farms are actually surprisingly rare due to the fact that usually animal fertilizers are used. But it is possible and they are sustainable. I personally know this example in Germany although it is still a smaller scale farm where only 3 to 5 people work. Different techniques are used to rest the soil, to avoid chemical weeding by using mechanical methods that remove them or prevent weeds from growing, and to coexist with local fauna (like having some surplus that deers feed on so they don't come close into the farm itself.
It's obviously more costly though, but people that care will buy these products. Cheap mass industrial farming will not go anywhere any time soon. But vegan organic solutions are possible, reduce waste and water pollution, and are scalable - even if more human labor is necessary than industrial farming.
