I’m in Alberta, Canada just north of the Montana border. Using yields and expected nutrients in the grain I calculated this representative rotation as an example for an upcoming podcast episode:

Consider a dryland four year cycle of peas, wheat, canola, and barley. Nitrogen and phosphorus are the most limiting nutrients in this area so for simplicity I’m only going to illustrate using them.

The peas will make their own nitrogen if properly inoculated and so the only nutrient export per acre in the form of the grain (and assuming the straw is left on the ground) on a 50bu crop will be 35lb of P. A 40bu wheat crop will export 60lbc N and 25lb P. A 35bu canola crop will export 65lb N and 35lb P. Finally, a 60bu barley crop will export 60lb N and 35lb P. Over the four years you will have exported 185lb of nitrogen and 130lb of phosphorus. On average, this means every year you must replace 46lb of nitrogen and 32lb of phosphorus.

In order to be completely regenerative a farmer in my area will need to have a legume that could green manure in place after it fixed that amount of nitrogen. The phosphorus is nearly impossible in my area. I am working on getting some numbers but in only 20-30 years of wheat/fallow the phosphorus supply that had built up over millennia of buffalo grazing was depleted so I find it hard to see how cover crops alone will be able to make up the deficit.

Our area is semi-arid with about 12″ of annual precipitation (6-8″ in the growing season). Organic matter is around 2%. I haven’t been able to find numbers for native prairie because there’s none really left that hasn’t been over-grazed by cattle but I guess it was around 3%.