I can’t comment on any real solution, but might have some thoughts… We are in a similar situation, organic as well. We have abundant dairy manure. We had a significant yield response when we brought in some poultry manure to go alongside or rotate with the dairy depending on the crop/field. Our other main source of N besides manure is feather meal, but it is expensive. Some have used fish products, but I have not tried those yet.

I have heard Klaas Martin from New York State talk about rotating sources of manure as well since each will have slightly different ratios and properties.

Growing legumes would be the only other option, but I understand the difficulty in doing that in shorter growing seasons. Maybe terminating young grasses while the C:N ratio is still favorable.

I’ve heard good things about Fixation Balansa clover (a more winter hearty variety). And hairy vetch is supposed to produce a lot of N. Both can be roller crimped at anthesis, but that could be mid June if your zone 5b is similar to mine, and they need to be established a bit earlier as well.

Curious what others have to say.