Plants build humus in soil through a process of feeding the bacteria and fungi that support plant growth, in a mutually beneficial cycle. Grazing can help or hurt that cycle. The good news is that permanent pasture accomplishes one main goal of soil-builders--not plowing. Opening the soil up to wind, rain, and sun leaches carbon out rapidly. So our hay fields and horse pasture are doing that much right.
And I feel pretty good about the hay fields with regard to carbon. Half of the field gets only manure, no chemical fertilizer. Fertilizer damages the carbon cycle--basically, the plants stop feeding their micro-organism buddies because they are getting fuel elsewhere, but as fertilizer is applied as a top-dressing, it encourages shallow roots.
The hay fields get lightly grazed in the spring. Pulses of grazing stress the plants in a good way, allowing even more carbon to build in the soils.
My concern is with my pasture, and whether I can afford to do what I would like to do--build carbon--and still keep my horses in good health. The problem with Morgans and Belgians is that they can't stand too much prosperity. Both breeds are prone to laminitis on rich pasture. Ideally I would be dividing my pasture into paddock and rotationally grazing them. But I don't have quite enough pasture during midsummer for that to work. If I bring up the fertility so the grass grows better, will I create a health problem?
This is something I'm committed to studying. The horses' health must come first, but I'd love a way that the soil could also be improved. The good news, for the horses and the soil, may be the weeds. With their deeper taproots, they access more minerals. If I clip them before they seed, they open up the soil as the roots die, reducing compaction. And horses do graze a number of these weeds, sometimes seasonally, which I believe is beneficial to their health. I just read an account from an English breeder of successful race horses, who never fertilized pasture, and believed that a healthy pasture for horses should support 50 weed species. I'll be out there counting this summer, and meanwhile, I'll go on researching. If you have experience in this area, I'd love to hear about it.