“When the grain from a grain legume crop is harvested, little nitrogen is returned for the following crop. Most of the nitrogen fixed during the season is removed from the field as grain. The stalks, leaves, and roots of grain legumes, such as soybeans and beans, contain about the same concentration of nitrogen as found in non-legume crop residue. In fact, the residue from a corn crop contains more nitrogen than the residue from a bean crop simply because the corn crop has more residue left after the harvest of corn.”
that is a reference to the nitrogen stored in the plant being utilized in fruiting (i.e. production of grain) that’s why you chop and drop prior to fruiting as I already stated. therefore if you grow peas and allow them to fruit and harvest the fruit the nitrogen is lost in the production of the fruit and a chop and drop or tilling would be pointless.
vaporization is the conversion of Nitrogen in the form of ammonium (often present in manure and urea) into ammonia gas. nitrogen fixation converts ammonia into organic nitrogen so you don’t have that problem.
The law is enforced at the retail level. you simply can’t buy it. the retailer won’t give it to you no matter what you say it is used for. its not like the police are enforcing it. plus I live close to the bay and personally don’t want to impact it negatively if it can be avoided.
Sadly, on the surface once it goes into gas form it goes literally up in the air, never having a chance to be converted into organic nitrogen. The most effective way is drive it under where it can be processed and sequestered by bacteria. I have sandy soils, so I have a preference for compost; slow release nitrogen source that the water driven leaching generates a trickle downward flow. Being a tick layer of compost followed by a thick layer of mulch helps reduce a lot of the outgassing.
Do still look at the horse manure option. As I mentioned the slow release paired by sensible watering means that you won’t be pumping nitrogen away from your area. It both fertilizes and conditions your soil. If you then want to put a ground cover on top of it well it would grow really well.
I don’t have access to horse manure. Also, my father got a load of horse manure and was never able to grow pumpkins again, it came with a lot of cut worms.
But i do overseed my lawn with clover from time to time. The lawn is greenest where the clover is. And when i mow the lawn a lot of the clover plant is magically turned into mulch and rots into the ground, releasing its stored nitrogen.
I’ve never intentionally planted nitrogen fixers near my fruit trees. (Although i have a lot of vetch, which i like to keep because it has a lot of flowers in a season when the bees don’t have much else to eat.) But i just bought a fresh batch of clover seed, because parts of my lawn need help.
It doesn’t hurt that clover is pleasant to look at and to walk in.