Nitrogen fixers for partially-established grassland

I'm an excited new landowner (as of last fall), with big long-term ideas, but I'm more than a little overwhelmed by where to start.

Lots of people seem to start with earthworks, but having spent the better part of a year on the property mostly in observation mode, I don't believe I have an urgent runoff issue that needs to be fixed before everything else. Rain really doesn't run off at all, as long as it's not dumping an inch per hour (which rarely happens here anyway). So I think the next most obvious thing is to add more nitrogen fixers to the system. Or at least that's something that should be relatively easy and I almost certainly won't regret.

The areas on my property that have the best soil (that is, pretty much anywhere it's more than a couple feet deep before you hit solid rock) were cleared for farmland/pasture decades ago. Farther back than satellite imagery goes, at least. But recently, those fields have only seen occasional light grazing (I think the previous owners graze more for the fact that it changes their tax designation than for directly profiting from beef). It's been at least a decade since they were last plowed, and they're pretty far along recovering to something resembling native grassland. I'm not very good at identifying grasses and forbs, but the handful of species I have identified are majority native. A couple of the most notorious invasive species in the area (cheatgrass and russian thistle) are present but don't really seem to be outcompeting natives so much as they fill in the gaps where natives struggle for some reason.

The density of ground cover in these fields is not as high as what I'd expect for true native grassland, but it's enough that I'd really hesitate to hit the reset button on any large area. So that makes it a bit difficult to follow general recommendations for "cover crops" which generally implies planting something in a field that was just harvested, and/or tarped to kill every plant in the area. I don't have any need or desire to plant cash crops anytime soon, so I'd prefer to try and work with the existing plants rather than start by clearing everything.

From the research I've done, the top contenders for nitrogen fixers appropriate to my climate are clover, vetch, and the native shrub mountain mahogany (cercocarpus montanus). Clover and vetch are widely appreciated for their nitrogen fixing abilities, but it seems like that's specifically in a monocrop context. I have no idea whether they'll be competitive enough to survive if sown into a field with plants already growing. I'm pretty confident that mountain mahogany would do fairly well (especially if I can figure out how to propagate it from cuttings), because it's part of the natural succession in this area from stand-clearing fire -> grassland -> shrubs -> pinyon/juniper forest. But it's a slow-growing plant and almost certainly not as effective in terms of nitrogen fixing efficiency, especially compared to an annual that becomes mulch at the end of the season.

So what would you do in my situation? Am I generally on the right track? Any other plants you'd recommend, or additional info on the ones I've mentioned?

I had a similar vitiate on as you in the fall and planted this mix. The sweet clovers and rye did extremely well. Zero land prep just brocast in the field. The sweet clovers get very large and basically even grew in the road. I noticed red and white clovers but much much less. The sweet clovers look nothing like clover. They got 3-4’ tall and had millions of blooms covered by bees and butterflies etc. I’ll know how much it added nitrogen in the fall I guess. I mowed them 2x and they finally started dying out in the Texas heat about a month ago.

This was a very productive mix though so I’m expecting good results.

~ this mix came from a local seed distributor so they may have a local one to you that pre makes blends suited for your area

Hi Josh, Well done for your patience in waiting and observing! I'm not sure what your goals are - to improve grass biodiversity for grazing, or improving cultural conditions for crops. For building soil, Nitrogen fixers sound like a good thing to encourage, but any biomass producing plant will be of help I would have thought. The advantage of shrubby plants is that they can produce a fair amount of woody materials. Also don't ignore the root and leaf parts of deciduous shrubs like the mountain mahogany. I'd be surprised if they didn't amount to more than the biomass in herbaceous legumes. The established root systems will make the shrubs more tolerant of poorer growing conditions too over time.

Thanks for the info on sweet clover and rye, Joe! I'll check to see if any of my local ag supplier's mixes have those.

And re: goals -- I suppose it would help to mention those, haha. I'm definitely more interested in growing crops than I am in grazing, and specifically I'm very interested in tree crops and/or other perennial crops for the long term. I'm not set on any particular species, so I'm hoping to generally improve growing conditions for whatever I end up deciding on down the line. I'm not sure if this makes a ton of difference in strategy this early, but worth noting that I care much more about climate change resiliency than yield per acre (but of course nobody would say no to higher yields, all else being equal).

Good point on deciduous shrubs and leaf biomass. This particular species of mountain mahogany has extremely small leaves, and now that you mentioned it being deciduous I'm double checking my identification of it, and thinking it might actually be C. intricatus -- which is evergreen. There's little to no leaf litter under them, whatever species or variant they are. So the biomass they produce is not really becoming part of the soil, unless their roots die back for some reason. There's other shrubs around that do create a lot of leaf litter; mostly Utah serviceberry and Gambel oak. I think the oaks are the fastest growing, and they're also coppice-able (useless for timber and not great firewood, but definitely good enough for mulch).

The reason I was going for nitrogen fixers first is that I'm under the impression that any plant that's not fixing nitrogen is using it up. I guess it's a net zero change if the plant gets chopped and dropped at some point. But if I actually want to increase total nitrogen in the system, either something has to pull it from the atmosphere, or I need to add it via compost/manure/something brought in from outside.

I also like the thoughts on waiting and observing.

If there is a need for improving pasture land clover is a really good addition.

Legume plants such as vetch are also helpful.

The way nitrogen fixing works is when the plants are chopped/drop (as when moving) and let to lay on the ground.

Another way to build soil is as simple as adding organic matter this is why adding wood chips is good.  

So even the nitrogen-fixing plants aren't adding nitrogen until they die, huh. I thought that at least some of the nitrogen, or the bacteria that actually do the fixing, would "leak" out of the root nodules while the plant is still alive, but not sure where I got that idea from. So, now that I better understand how it actually works, I guess relying on a slow-growing shrub is probably not my best strategy. It looks like the only legume I can get bulk seed for locally is alfalfa, but I also could do smaller trials with seeds ordered online.

I really think it isn't as simple as the non nitrogen fixing plants using up the soil nitrogen. I think of it more like an electric battery perhaps that leaks all the time.....All plants convert available nitrogen into their bodies and then into bioavailable products as they die (or are eaten perhaps). Nitrogen fixing ones will do this better on poor soils with the help of 'tame' bacteria in their root nodules, but again it is mainly when they die that most of the nitrogen is made available. If there is spare nitrogen it leaches away as gases or nitrogen rich soluble run offs, only if a plant grabs it will the soil hold on to it. I'm not an expert, but this is how I think of it at the moment.

So any good biomass plant that grows for you will be good. Nitrogen fixing ones just tend to do better if the soil is low in available nitrogen.

One suggestion I have heard a few time is fodder radish or tiller radish. This could probably be sown now into the grass and may produce tap roots to 'inject' plant matter into the soil. It tends to be killed off over winter in a hard frost area.
A bulky one that fixes Nitrogen might be fava beans perhaps, if they grow well in your area, they can produce quite a bit of upper growth.

I’ve not had much luck simply broadcasting turnips or radish. Any time I got them to take fairly good I had to at least run the disc over the field 2x. And it really turned up a lot more weed seeds that were dormant.

I think each time I mowed the clover released some nitrogen as root die off was happening but I’m not 100% sure so don’t take that as gospel.

Turnip and radish are cheap. Another option could be like a buckwheat and cowpea but again you might have to scuff it a little in order to get the ideal plants for your goal.

Right, it's definitely a simplification to talk just about "total nitrogen." People talk about "available" nitrogen, which means water-soluble nitrogen-containing compounds just kind of floating around in the soil. These can be taken up by roots and used to build plant tissue. But if they aren't taken up, they'll be gradually lost to wherever the water is flowing, or perhaps evaporate, depending on conditions and which particular compound we're talking about. But the nitrogen that becomes part of plants stays in place, and like you say when the plants die they get broken down, and the nitrogen (along with everything else the plant is made of) becomes "available" again. Where nitrogen-fixing bacteria (and the plants they're symbiotic with) come in is that they can take N2 directly from the atmosphere, and turn it into NH4 which is one of those water-soluble compounds that plants can use. Other types of plants can't do anything at all with atmospheric nitrogen, they need to get 100% of it from the soil.

So yes, more plants of any type should mean that less nitrogen is being lost. But non-nitrogen-fixing plants can't ever put nitrogen into the soil that wasn't there to begin with. There is the additional wrinkle that nitrogen fixers won't bother with pulling from the air if there's already enough available nitrogen in the soil for them. But if that's the case, I guess that's good news for me anyway, it means I don't have to worry about a nitrogen deficiency in the short term.

I had heard about radish, particularly Daikon, being good for pulling nutrients up from deep in the soil, and also breaking up compaction. Good to know that it might not take very well without some help. I've never used one of those disc thingies, but I imagine a rake would do about the same thing? For a smallish trial, anyway; I'm definitely not about to rake an entire field by hand, haha.

Fava beans were on my list of plants to try for other reasons; hadn't considered using them like a cover crop but of course they're legumes so that makes sense. The big monocrop around here is pinto beans, which is the same family but different genus so I don't know if that means anything about how well fava beans would grow. But worth a shot.

Hi Josh. you mention that you want to be able to grow crops and not so much graze animals. What crops are you wanting to grow exactly? You are at a high elevation in a semi-arid environment with a relatively shallow soil.  I'm guessing this area did not support Ponderosa pine or other trees of much stature prior to settlement and could be considered a 'wide open space'.  I am wondering about lupins, desmodiums and possibly lead plant as possible nitrogen fixers for your soil other than the traditional clovers that are utilized in more traditional crop, hayfield and pasture vs. open range scenarios. Maybe I'm picturing where you are incorrectly.  Do these sound like options?

Nitrogen does not stay in the soil permanently. The plant (nitrogen fixer) does not have to die to release the fixed nitrogen in the nodules. A defoliation event, such as grazing, hay harvest or mowing serves to release the nitrogen the same as the death of the plant. The advantage of these is that the plant can regrow and continue to fix additional nitrogen. The question I have is what do you need the nitrogen for? It will be crop dependent. The requirements for your native grasses is going to be quite different for a commodity crop or different yet from subsistence crops that are adapted to the area you are in.  

The 'Native Grassland' was probably much more diverse than what you are imagining (plants other than grasses). Also, depending on how long the area was cropped, (before being left to return to 'Native Grassland', nitrogen might not be the highest priority in amending the soil. It might be organic matter. Increasing organic matter (by growing ROOTS in the soil) will buffer and/or improve many variables in the soil where you are.

What crops are you wanting to grow exactly?

This year I planted a small garden of corn, beans, and squash hoping for those to be the primary ones (I figured I should start with the crops that have the longest history here, and I can try the European imports later). I tried amaranth and sunflowers as companion plants to those, plus a couple of smaller experiments on the side. Squash and amaranth seem to be doing the best out of all those so far. Beans really should do well here (and of course they're nitrogen fixers themselves), because that's what basically all the big commercial farms around here grow. But at my smaller scale it seems I'm gonna need to fence out the rabbits first for beans to have any chance. Corn might have done better, too, if it weren't for the rabbits and a couple bad hail storms.

In terms of broad categories, I'm focused on crops that could be used as a staple, or at least provide significant calories. My understanding is that high-calorie crops tend to also be high nitrogen requirement crops (with the obvious exception of beans), but I don't remember where I read that and maybe I just made it up.

You are at a high elevation in a semi-arid environment with a relatively shallow soil.  I'm guessing this area did not support Ponderosa pine or other trees of much stature prior to settlement and could be considered a 'wide open space'.

Correct, no ponderosas here, just juniper and pinyon pine, 20 or 30 feet tall max. If by wide open space you mean mostly treeless grassland, not quite. The natural succession for these fields would be to sagebrush and then moderate density pinyon-juniper, as long as climate change doesn't throw a big wrench in that. Water availability limits the tree density so the canopy never fully closes, and there's always going to be plenty of sun hitting the understory, at least in patches. So closer to a savanna type ecosystem than to old growth ponderosa or the like.

 I am wondering about lupins, desmodiums and possibly lead plant as possible nitrogen fixers for your soil

Lupines are all over the place here, I just hadn't realized that they were nitrogen fixers until just now. I wanted to plant more of them anyway because they're pretty and attract pollinators, but now I'm even more interested. The other two don't look familiar, and according to iNaturalist they don't grow anywhere near here.

The question I have is what do you need the nitrogen for? It will be crop dependent. The requirements for your native grasses is going to be quite different for a commodity crop or different yet from subsistence crops that are adapted to the area you are in.

Definitely more interested in low-input subsistence crops than in commodity crops, or in native grasses for their own sake.

And this is definitely where my understanding starts to get much fuzzier. I've heard both corn and squash called "heavy feeders" which I understand to mean they "need" lots of NPK synthetic fertilizer in a system where that's taken for granted. My very vague understanding with phosphorous and potassium (and various micronutrients) is that if I don't want to depend on fertilizer then I should focus on soil microbiology, which should extract those elements from minerals in the soil and make them available to plants. That sounds much more complicated to me than just sowing seed for nitrogen fixers, so that's why I'm focused on nitrogen at the moment.

Also, depending on how long the area was cropped, (before being left to return to 'Native Grassland', nitrogen might not be the highest priority in amending the soil. It might be organic matter. Increasing organic matter (by growing ROOTS in the soil) will buffer and/or improve many variables in the soil where you are.

I'm not sure of the exact history of my land, but I don't think it's ever been intensively farmed in the way that like Nebraska or somewhere has. I think that the settlers here have tended towards cattle grazing more so than growing crops, on average. I for sure want to increase soil organic matter as well, which is part of why I don't want to start by clearing the field. There's a decent amount of roots there already, but I'd like to see more, and I figure it's even better if those roots are also fixing nitrogen. Whether or not it's not my single biggest limiting factor, it can't hurt, right?

In terms of your microbiology, think about temperature. Surface temperature of the soil can easily vary where you are by 10 degrees Freiheit. That variation is created by the seemingly insignificant amount of shade created by the 'sparse' vegetation there. The cooler the soil, the more active the microbiology will be in the summer. Conversely in the winter, the root masses, and plant crowns, will keep the soil warmer and more biologically active.

Keep a close eye on the density of the junipers especially. They are 'water collectors' and hog much of the little rainfall you will receive if their density gets heigh enough. There is currently an unprecedented increase in the juniper
(all species) across the united states. When there is a scant amount of rainfall, juniper branches intercept it. If you look at the architecture of a juniper, (one that isn't over 120 years old) all of the branches taper/angle down to the trunk. Precipitation hitting the branches runs down the branch, to the trunk, to the base of the juniper to the soil immediately adjacent to the juniper and is promptly taken up by the juniper roots.

The cheatgrass you have present is a real moisture thief as well. As an annual, it is actually better adapted to the environment there and can -and does- respond much faster to increased rainfall amounts (sporadic) than the perennials you have there. I don't know of a good way to deal with the cheat grass, other than to get more perennials established in wet years which are episodic and you never know when they are going to occur. This makes seeding the perennials you want to be a bit like Russian roulette. You might ger good rains for perennial establishment in one out of every 3-4 years. Good luck guessing which year to seed before the rains come, which is when you have to seed. Check with the local BLM especially as well as the USFS and the NRCS is a good idea as well.

Keep a close eye on the density of the junipers especially. They are 'water collectors' and hog much of the little rainfall you will receive if their density gets heigh enough.

Oh my, the junipers... the only other plants that I've heard so many negative things about are ones that are considered invasive. In fact people in permaculture world seem to prefer some invasive species over the junipers (looking at you, russian olive). Not that I'm super excited about them necessarily, it's just they're the trees that I've got, and I'll never be able to establish anything half as mature even if I had multiple lifetimes to do it. This is easily worth a whole separate thread, but I have been reading as much as I can find on the topic and slowly working up the courage to start thinning them. It's just beyond intimidating to think of cutting down a tree that may have been here since before colonization. I know they're pretty much the opposite of a threatened species, but still...

The cheatgrass, on the other hand, I have no problem pulling out. But that seems like a losing battle. I'm just hoping to avoid anything that would clear a large area and let it move in before other stuff can get established.

Don't cut down the really old ones (juniper). Only take out the seedlings (if you can find them) followed by anything under 10 feet and those will be the easy ones. Any that look like an inverted ice cream cone or a pyramid should be considered fair game. I'm not sure about junipers being worse than Russian and autumn olives. The juniper belong here (sort of) the olives don't. The junipers do make nice fence posts.

Nitrogen available for plants can come from rain water due to lightening. As for biological nitrogen fixation, besides the well-known symbiotic legume-rhizobia, there are other diazotrophs that are free living or associated with plant roots. In recent years, people are finding nitrogen fixing microbes in rice, sugar cane, corn and sorghum. Some live in the rhizosphere or root musalige and some reside inside the roots. My impression is that people found out about the nitrogen fixing in non-legume plants because they are looking closely at these important crops. There are many more that we don't know about. As a general rule, having a greater plant diversity and lots of living roots in the ground will increase soil fertility in the long term.