Greg asked me if its possible to grow field crops without animal input and still build soil.
Regardless of input, I know of no one who has developed a way to grow field/row crops and build soil. The best integrated systems just maintain topsoil.
These pictures illustrate why row crops always damage the soil, whereas a stocked and well-managed pasture builds soil.
Image source: Seaburst.com
When we raise crops, we expose long rows of soil. We struggle to keep these free of "weeds" whose natural job is to secure the soil. We can't stop wind and water (rain), which inevitably course through these furrows carrying soil away from the field. The roots of these crops reach shallowly, so they don't trap rainfall efficiently. If managed very intensively with manure, rotation with leguminous cover crops, and mulch with compost, at best we can replace the soil lost each time we plant with row crops.
In contrast, a pasture looks like this:
Image source: Rebelwoodsranch.com
The soil accumulates year after year because the tight root structure and full cover of grasses and "weeds" protects it from wind and water erosion. The roots themselves draw the water (rain) into the soil. The grass constantly takes carbon dioxide out of the atmosphere, converting it into carbohydrates, stored in the root system. Ruminants are an essential part of perennial grass land ecosystems, providing many services to the grass including fertilization with nitrogen. The grass feeds the soil, the soil feeds the grass, the grass feeds the ruminants and the ruminants feed the grass.
In short, a field of row crops requires a constant battle against natural processes, and its very structure involves us in a largely unsuccessful battle to preserve the soil.
It reminds me of a passage in the Tao Te Ching (Chapter 29):
Does anyone want to take the world and do what he wants with it?
I do not see how he can succeed.
The world is a sacred vessel, which must not be tampered with or grabbed after.
To tamper with it is to spoil it, and to grasp it is to lose it.