Question about nitrogen fixing cover crops

Howdy~
Thanks to y'all, I have a grasp on how legumes use nodules on their roots to work with soil life to convert atmospheric nitrogen into nitrogen available in the soil that other plants can use.

Read something intriguing, though that leads to my question. What I read, in a description of fieldpeas, stated that harvesting the peas, for eating or planting later, removes much of the nitrogen produced by the plant.

Are they differentiating between the nitrogen fixed in the soil by the roots, and nitrogen carried in the above ground plant?

In other words, where is the greatest benefit in available nitrogen from legumes coming from? The roots, or the above ground biomass? Both?

Jeff Peter wrote:Howdy~
Thanks to y'all, I have a grasp on how legumes use nodules on their roots to work with soil life to convert atmospheric nitrogen into nitrogen available in the soil that other plants can use.

Read something intriguing, though that leads to my question. What I read, in a description of fieldpeas, stated that harvesting the peas, for eating or planting later, removes much of the nitrogen produced by the plant.

Are they differentiating between the nitrogen fixed in the soil by the roots, and nitrogen carried in the above ground plant?

In other words, where is the greatest benefit in available nitrogen from legumes coming from? The roots, or the above ground biomass? Both?

Great question. The benefit actually comes from both. The nodules on the roots fix nitrogen and add it to the plant, but a lot of that nitrogen ends up stored in the leaves, stems, and seeds. If you harvest the peas, you are removing some of that nitrogen with the crop.

The roots and leftover plant material return nitrogen to the soil as they break down, so leaving the whole plant in place gives the biggest soil benefit. If you eat the peas, you still get some benefit from the roots and remaining biomass, just less than if the entire plant was composted back into the soil.

Another added benefit is that the nitrogen fixing bacteria present in the nodules get released into the soil after the plant dies. Where they keep producing nitrogen so I heard except the lack the safety of the nodule and the plant feeding it sugars. They have to fend for themselves, until another plant snatches them up. Which is an advantage especially in soils that have been depleted of these kind of bacteria when the farmer fed artificial nitrogen.

Makes sense that harvesting the peas would take nitrogen out of the system. The plant builds protein into the seeds using the nitrogen it fixed, so if you remove those seeds you're removing a chunk of what was produced. The roots and nodules still leave nitrogen behind in the soil, but less than if you'd chopped the whole plant and let it decompose in place. That's why a lot of people grow field peas as a green manure and just mow them down rather than harvesting. You get the full benefit that way.

Just to clarify the piece about making nitrogen available to other plants... it is my understanding that the nitrogen (either nodules on roots or in the plant itself) is not available to other plants until the nitrogen fixing plant dies. So the nitrogen is not available while the field peas are growing... they must be killed either by harvest or chop and drop or be at the end of their life cycle before the nitrogen can help other plants.

This is a very misunderstood subject and Matt's answer is close.

Jeff Peter wrote:... What I read, in a description of fieldpeas, stated that harvesting the peas, for eating or planting later, removes much of the nitrogen produced by the plant.

I've been told the same thing. Producing "seeds" which is what the peas are, takes a lot of strength from the plant, so it makes intuitive sense that chopping and dropping at the flowering stage had different benefits than taking a crop of seeds.

However, many modern soils that have been tilled frequently, are very short on carbon rich organic matter, and that is left over when pea plants die. If you're talking a small area, fertilizer "P" can easily up the nitrogen if watered down and used on the next crop after the peas.

Also consider that chopping and dropping the top part of a plant early on, then letting it regrow and produce some seeds, could affect the outcome. After all, humans chop and (hopefully) drop grass all the time and it will carry on and seed if the season is still right for flowering. Most people "chop" their grass weekly, and if they don't chop it too short, it just keeps on growing.

Jeff Peter wrote:Howdy~
Thanks to y'all, I have a grasp on how legumes use nodules on their roots to work with soil life to convert atmospheric nitrogen into nitrogen available in the soil that other plants can use.

Read something intriguing, though that leads to my question. What I read, in a description of fieldpeas, stated that harvesting the peas, for eating or planting later, removes much of the nitrogen produced by the plant.

Are they differentiating between the nitrogen fixed in the soil by the roots, and nitrogen carried in the above ground plant?

In other words, where is the greatest benefit in available nitrogen from legumes coming from? The roots, or the above ground biomass? Both?

Both contribute, but most of the nitrogen benefit comes from the whole plant, not just the roots.

The nodules fix nitrogen, which the plant then stores in roots, stems, leaves, and seeds. If you harvest the peas, you remove much of that stored nitrogen. If you leave the biomass to decompose, the nitrogen in the roots and above ground material is returned to the soil for future plants to use.

Appreciate the replies. I will leave the entire plants including the peas right in my fallow bed.

I had intended to harvest the peas to use as dry beans, and as seed for next season, but seed is cheap.

I also have fava beans and vetch in that plot, which is intended for corn and pole beans next year.  I left it fallow due to having a total knee replacement which forced me to cut back.