Biodynamic Accumulators?

As a teacher, I’ve learned that if I state a claim during a lecture, at least one student is going to question it. I’ll get asked, “What’s your source?” or, “What’s the history behind that information?” or, “What’s the data that backs that up?” I have learned to love those questions, because it has made me a better teacher. I don’t dare write a lecture without exploring, and citing, every angle of every claim I make. This is important work and I’m glad to do it; especially because science frequently changes as more experiments are conducted. After all, it does no one any good to learn old science.

But all of us have ideas about gardening that we’ve heard so often that we take them for fact, whether they are or not. Things like:

• Putting eggshells around your plants will protect them from snails (1)

• Putting a layer of rock in the bottom of a container will increase drainage (2)

• Giving plants a lot of space will quell any competition for resources (3)

• Gathering up leaves and other plant matter from under plants will help them grow better (4)

You get the drift.

So, when a student recently said to a class, “Plant comfrey! The deep taproot will bring minerals up from the lower levels of your soil!” my inner alarm went off. I hear this all the time. Where did this idea come from? Is it true?

Comfrey, and other plants like nettles, are widely known as ‘biodynamic accumulators.’ There is no definition for the term because it hasn’t actually been defined yet. It came to prominence in the Permaculture world, as part of ‘plant guilds’ - that is, plants that are placed near each other to provide different benefits to the whole. For example, you might plant a fruit tree, and beneath it place a leguminous bush to provide nitrogen, chives to repel insects, and yarrow to provide biomass for mulch. Comfrey is often included in these guilds for its oft-lauded ability to ‘bring up nutrients,’ whatever that means.

Wait, what DOES that mean?

Well, all plants bring up nutrients, right? The nutrients in the soil are dissolved in the soil water and become the soil solution, which is where plants get the nutrients they need for growth.

How do roots form? When seeds germinate, they form a primary root from the radicle of the embryonic tissue. This helps the plant establish, so that it can unfurl its seed leaves (or ‘cotyledon’ leaves) and begin to photosynthesize. Once the plant is established, it usually forms one of two root systems, depending on what kind of plant it is - a tap root or fibrous root systems. Tap roots have a central root with little side roots coming off of it. The purpose is to anchor the plant in the ground and/or to act as a storage place for nutrients (think of a carrot). The fibrous root system has lots of adventitious roots that move across a wide area to find more surface pockets of nutrients, and cannot store nutrients long term. Confusingly, a very established fibrous root system can also act as an anchor for the plant (think of corn).

image credit: Zassou Garden

We could get much deeper into the weeds here (no pun intended) but I want to keep it simple so we get to the point of this post. Quite simply, plants have adapted different approaches for their needs, but all of these different kinds of systems (whether tap or fibrous) additionally have fine hairs which gather the moisture and nutrients. These fine root hairs will take up whatever is available, in whatever location they happen to grow.

So that means that the roots, wherever they are, whatever kind they are, are taking up nutrients. I suppose, then, depending on the kind of soil you have, there could be different nutrients in different levels. Perhaps you have a very shallow top soil that is depleted in nutrients; it might be good in this case to include deep-rooted plants in your garden to make those subsoil nutrients (minerals from rock, mostly) available. Most tap roots develop a greater number of associations with fungal networks, which break down the rocks at that lower level, so it makes sense that these deeper tap roots are finding different minerals and nutrients. Shallow roots, however, are getting more nutrients from the top layer of organic matter, whatever is available from rotting plant material on top of the soil, as well as from the biological ‘poop loop’ happening with the little creatures who live in the soil.

So yeah: Deeper roots bring up different nutrients. That part is true. But do you need a special plant, like comfrey, to act as this biodynamic accumulator? Is comfrey the only option for this? Most folks are using comfrey as a nutritive mulch; they grow the plant to bring up large amounts of nutrients which are then stored in the leaves (they claim - which again sounds the alarm - doesn’t a tap root store the nutrients in that large root, like a carrot?), and then they chop the leaves, lay them down as mulch, and those nutrients are then brought into the top layer of soil, for the fibrous roots to ‘mine’ for nutrients. At least, I think that’s what the gardeners who buy into the whole ‘accumulator theory’ are doing. Of course, all mulches eventually break down, whether it’s straw or wood chips or sawdust or grass clippings or comfrey leaves. And in that process, they feed the microbial life in the soil, which then poop out the nutrients in a form that is available for the plants (and is taken up by roots). The question then becomes: Do comfrey leaves provide more nutrients as a mulch than any other plant matter?

Well! Turns out there has been a recent study on exactly that question. The study used USDA’s ethnobotanical and phytochemical database to compile peer-reviewed nutrient concentration data across thousands of plant species. They set a threshold of 200% of average for a plant to be called a ‘dynamic accumulator.’ What they found is 340 plant species that showed nutrient concentrations high enough to qualify. This is impressive (plants are always so surprising, aren’t they, in so many wonderful ways?), but what’s very interesting is which, and how many different, nutrients the plants accumulate. The scientists compiled all the data into an online tool called Dynamic accumulator database and USDA Analysis. Here you can see which plants ‘brought up’ which nutrients and in what concentrations.

The next step in the study was to choose six promising species to trial for two years at a community farm. They chose dandelion, lambsquarters, red clover, redroot amaranth, Russian comfrey, and stinging nettle. They wanted to use these plants in different applications, such as liquid fertilizer and mulch production. Here are their key findings (this is a direct quote from the study):

- Plant tissue nutrient concentrations are tied to soil nutrient concentration. Dynamic accumulators are well-suited to extract specific nutrients from fertile soil, but they aren’t going to create nutrition that isn’t there. Therefore, dynamic accumulators should be regarded as one useful part of a larger nutrient management plan.

- That said, even when grown in poor, unamended soil, lambsquarters surpassed the dynamic accumulator threshold for potassium, and comfrey surpassed the threshold for both potassium and silicon.

- Previous studies have shown stinging nettle to accumulate calcium at concentrations above the thresholds. These new findings show that not only does it accumulate a lot of calcium, but it also has a high nutrient carryover rate, resulting in calcium-rich liquid fertilizer and mulches.

So there you go: Biodynamic accumulators are a real thing, and comfrey is certainly one of them. But, one must consider: Does the soil even need these particular nutrients? For instance, much of the Bay Area already has plenty of calcium in the soil. In this case, a dynamic accumulator like stinging nettle, which accumulates high levels of calcium, may not be necessary. Also, none of the six tested in these trials provided a large nitrogen benefit, but I suppose we already know what does provide that, and that is legumes.

I find the first finding the most important: This is only one part of a soil nutrient management plan. Adding plenty of organic matter to your soil, whether comfrey leaves or nettles or wood or straw or manure or chaff or whatever, will feed the microbiology in the soil which will in turn make it bioavailable for our plants. It will also form stable aggregates which will create long-term health. Living roots in the ground, a great diversity of them, will do the same. Eschewing pesticides of all kinds will also help the soil nutrient profile. Lots of soil cover will also prevent evaporation, leading to greater soil moisture and soil health.

Bottom line: Plant comfrey if you want to. It’s a beautiful plant and as long as you get the Bocking 14 variety, it will be well-behaved in your garden. Insects love it and it thrives as an understory plant for larger species. I myself have just planted 50 root cuttings under the trees in our orchard, if only to provide another living root in the ground, improving the soil profile.

Foliar Feeding - Does it Work?

In the past couple of weeks, I have toured several different farms and gardens where the caretaker sung the praises of foliar feeding. “The plants absorb the nutrients so much faster and growth just explodes!” said one farmer enthusiastically. Every time I hear a comment or recommendation regarding foliar feeding, my inner bullshit alarm goes off. I just don’t believe it actually works. But is my hunch true, or am I just stuck in old thinking?

Years ago, I was enamored with the idea of making compost tea and then spraying it as a foliar feed on my veg and fruits. I was sure it would improve the health of my plants, deter pests, and increase yields. Making compost tea is a lot of work; it requires a bubbler in order to oxygenate the mixture as it’s steeping, keeping the organic matter in a mesh bag so it doesn’t clog the bubbler, then decanting the mixture into a sprayer, etc etc etc. And then the spraying itself, messy and time-consuming. All of this has associated costs, too - you need to buy a big sprayer, a bubbler and tubing, several containers to hold everything, and mesh bags or cheesecloth. This is all after the regular work of making compost, which is noble and important but not exactly a hands-off project. After all this fuss, I really didn’t notice much of a difference in my plants when I performed foliar feeding.

Not long after, I took a Plant Nutrition class as part of my ‘nursery management’ coursework at Merritt. Several students were convinced that foliar feeding worked better for plant nutrition than any other method. My instructor was dubious. She explained that leaves are not designed to take in nutrients. There are nearly microscopic holes on the bottoms of leaves, called ‘stomata,’ which evolved to provide gas exchange - these holes allow for oxygen and carbon dioxide to move between the plant and air as part of the process of photosynthesis. They can expel water, but actually repel water that’s trying to come in, using a waxy coating for that purpose.

More recent research has revealed the existence of micro-pores on the surface of leaves, and they are lined with negative charges which attract positively charged cations (things like calcium, magnesium, and potassium). This research has shown that nutrients can enter these pores as ions in water, but sprayed leaves dry very quickly and limit much absorption, and only about 15% of the nutrients applied this way are absorbed.

And as my professor then told us, even if leaves do take in nutrients in this way, the plant is really not designed to move nutrients other than sugars from the leaves to the roots, or to any other part of the plant. The whole system is designed to draw nutrients from the soil solution (a pool of available water at the rhizosphere that contains nutrients) up through the roots and into the xylem, which is a kind of fluid river that moves water-soluble nutrients from the roots to the rest of the plant. There’s a downward river, too - the phloem - but it’s carrying sugars made in the photosynthetic process down from the leaves to the root zone. That’s what it’s designed to do, not carry nutrients from the stomata down to the roots. So even if nutrients are entering the leaves, they are stuck in the leaves. They are immobile due to that stronger positive charge.

Let’s take calcium for instance. A gardener recently told my class that she sprayed her tomatoes using a foliar feed of liquid calcium to prevent blossom end rot. But calcium will not move from the leaves to the fruit. It can only move from the roots to the fruit. Tomatoes also do not absorb calcium through the skin. So this is arguably not helping the plant avoid blossom end rot. (And if you’re a long-time reader of this blog, you already know that blossom end rot is not really a calcium issue.)

Remember my plant nutrition class? My professor posited that what was really happening in foliar feeding was that the nutrients were dripping off the plant and down into the soil, thereby entering the soil solution. But she was not married to that theory and was game to experiment. So, in lab, we planted a bunch of leafy vegetables in 4” pots. Then we covered the whole surface of the soil with plastic wrap. Then we turned the plant on its side and sprayed the leaves with a foliar feed (I can’t remember what the actual fertilizer was, unfortunately). This way the feed dripped off the leaves onto the ground, instead of down into the pot. And the plastic wrap was there as insurance to make sure that nothing entered the soil. We let the plants drip dry before righting them, removing the wrap, and watering them with plain water before replacing the wrap. We cared for these plants over the course of the whole semester.

Care to guess the result? The plants in 4” pots with only foliar feed did very poorly indeed. They were yellowed, diseased, and wilted. We concluded that foliar feeding didn’t work, or at least it didn’t work well enough to supply enough nutrients to the plant.

But that’s not a rigorous study, so I consulted some other recent research. According to a 2016 study by the University of Wisconsin-Madison’s Agriculture Department, “for many nutrients, there is little or no published information showing a valid relationship between crop yield increase and tissue nutrient concentration that provides good reason for making widespread recommendations to apply a foliar- or soil-applied fertilizer that includes that nutrient,” and, “for many of these essential nutrients, we lack proven research that defines the exact minimum nutrient concentration below which yield is harmed and verifies that a beneficial yield response to foliar feeding occurs.” And according to a 2020 article from Ohio State University’s College of Food, Agriculture, and Environmental Services program, “results (in foliar feeding) were inconsistent and didn’t reveal a cohesive pattern of increased yield or… plant health for the sites in this study.”

I suppose it’s possible that spraying the leaves with a nice compost tea might help protect the leaves from some fungal infections, or pest predation. But as a method of actually feeding the plant, I think it’s an inferior choice. The best thing to do for any plant is to provide it with plenty of organic matter, which allows the soil biology to process all the nutrients in the organic matter and make them available in soil solution for the plant roots.

Reference: “Why I Typically Do Not Recommend Foliar Fertilization,” by Caleb P. Goossen, Ph.D, MOFGA Crop Specialist, June 2023.

Fertilizer/Comments

I had a question from a new gardener; she wanted to know if I use fertilizer on my plants. The short answer is yes. The larger answer is, sometimes and in some places. The super-large answer is, mostly we shouldn’t need to use fertilizer. And the ecological answer is, using fertilizer is a tricky business, both in the way they are made and the way they interact with our environment, so less is better.

Let’s start with the easy stuff first: Yes, I do fertilize. Plants in pots must be fertilized, because they can’t form connections with a larger soil community. About once a month, I use a liquid, organic fish emulsion on my potted flowers, fruits, herbs, and vegetables. The product I use is Neptune’s Harvest. It has an NPK of 2-4-1. This is a good, low ratio; the amount of Nitrogen won’t hurt the plant, and it has slightly more Phosphorus which is good for developing roots and fruits (or blooms). The thing you want to look for on the package is, how much of the Nitrogen is soluble? Soluble/insoluble is an important thing to know. If it’s soluble, it’s available to the plant right away; it will enter the soil solution and be taken up by the roots. Whatever is not used immediately will not be stored, it will run out with your water. Generally organic products have a low NPK ratio, most of the nutrients are insoluble, and they take some time to work, as the biology in the soil has to incorporate them into their bodies, then poop them out, before they are available to the plants. In soil biology, this is known as the poop-loop. This is an important cycle and is the best way to feed plants. It’s like micro-manure.

I also fertilize the rest of my garden, but maybe not in the way you’d think. I add organic matter regularly. Good quality compost, two inches on each planting bed, is added every fall. Compost is added to the pollinator gardens when I add seeds. And every time I put a new ornamental plant in the ground, I mulch with compost. This way, organic matter is constantly being returned to the soil, and that is the very best way to fertilize your plants. The roots form connections to everything around via fungi, and there is a constant exchange of nutrients going on. We also add wood chips every year or so to all the paths and around large plants and trees. These break down over time and add nutrients, as well as providing habitat for soil creatures. We also incorporate cover crops, which add different nutrients and soil-aggregating qualities. Having a living root inside the ground at all times is the best way to ensure your soil has plenty of food - the microbiology lives on and near roots; the plant pumps out food for the biology, and the biology in turn feeds the plants. So yes, in these ways we also fertilize.

And, when I plant summer crops in the raised beds, I add a granular organic vegetable fertilizer such as Jobe’s Vegetable Fertilizer which is 2-5-3. This breaks down slowly over the course of the season. I do this for summer crops because they are heavy feeders (meaning they need a lot of nutrition to produce big fruit, like tomatoes, peppers, or melons) and I am planting them very closely spaced.

Another thing to consider is your soil type. I tend to have clay soil, which has the ability to hang on to nutrients (this has to do with its chemical composition, cation exchange of electrons, and the minerals that make up clay, factors too large to get in to here). If you have a sandy soil, you’ll have less nutrients available. If you have ‘fill,’ then you have very little soil life, and you’ll need to feed it in order for it to thrive. The best way to do this is through the addition of organic matter.

All fertilizers have an ecological cost. Rock phosphorus is mined in a horrible system that wreaks havoc on the earth. Peat moss is an element that takes hundreds of thousands of years to form, and we are stripping it far faster than it can be replenished. Synthetic nitrogen is applied at enormous amounts in conventional agriculture because soils are depleted; much of it runs off and has caused all kinds of problems downstream. lt’s easy to add cover crops that have the ability to form associations with bacteria that fix nitrogen - the pea family does this of course. The thing is, you have to cut down the cover crop before the fruit is produced - and don’t pull up the roots, let them rot in the ground. Some large-scale farmers have found ways to supply 90% of their nutrient needs with cover crops alone.

I’d like to refer you, once again, to a great movie called Symphony of the Soil. In it, several scientists and farmers explain how soil is formed and how we can hold on to it, and farm with best practices to retain nutrients. I’d also like to suggest the book Growing a Revolution by David Montgomery. You can also find his lectures online. He and Elaine Ingham are my soil mentors.

I loved getting this question from this new gardener, and I love getting questions and comments generally. I know that leaving a comment here on the blog has been difficult. A while ago, we disabled anonymous comments because we were getting some advertisements for porn (!), but we’ve enabled them again in the hopes that perhaps we will get a community going in the comments section (well, a community that doesn’t involve porn, at least). If you are unable to ask a question here, please feel free to email me using the contact page - I love getting your questions.

It’s heating up here - summer might finally be on the way!

Foliar Feeding Experiment/Overview of Nutrition requirements of Plants

(Thank you to Laura Forlin, co-chair of the Merritt College Horticulture Dept, for teaching me all this stuff.)

We completed our Foliar Feeding Experiment yesterday in lab. The results were interesting, and I want to share them with you. But before I do that, we need to review a couple of things.

What do plants require to grow, thrive, and set seed? 

An element is essential if: 1) A plant cannot complete it's life cycle without it; 2) no other element can perform the same function; 3) it is directly involved in the nutrition of the plant; and 4) missing or insufficient supplies adversely affect plant growth.

There are three macronutrients that everyone forgets, they are non-mineral, and they are required in larger quantities than any of the others. Can you guess what they are?

They are obtained from the atmosphere and water.

Yes! Oxygen, carbon, and hydrogen. 

The next six elements are macronutrients and are the most important after the big three: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S). These are required in large quantities. N, P, and K more than the others. 

The next eight are micronutrients - still essential, but required in much smaller amounts: Chloride (Cl), Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Nickel (Ni), and Molybdenum (Mo). 

Many of these are present in the soil already and you may not need to supply them. Knowing your levels of nutrients can be important, and that's why folks do soil tests, to discover what is lacking and add it in appropriate amounts. Now this is important: TOO MUCH FERTILIZER CAN BE WORSE THAN TOO LITTLE. I cannot stress this enough. If you just add chemicals willy-nilly, they will either be leached out quickly (entering the water supply, affecting organisms downstream) or they will damage your soil and plants. 

MOST SYNTHETIC FERTILIZERS HAVE HUGE AMOUNTS OF THE NUTRIENTS. You buy something like a 16-16-16, that's super hot. Putting that on your plants can be very damaging. Most synthetic fertilizers are completely water-soluble, meaning they enter soil solution quickly, are taken up quickly, and are leached quickly. It's wasteful, it's expensive, it can damage your plants, it can kill your soil microorganisms, and it damages the environment. If you've decided that you need to fertilize your plants, ORGANIC FERTILIZERS ARE THE WAY TO GO. They have a much smaller percentage of nutrients (like 3-3-3), most are only partly water soluble (which means some will be available right away, and some needs to be mineralized by the soil life first, giving you longer lasting nutrients; it also means less will be leached quickly from the soil and cause environmental problems), and they are made from products which are found in nature, and are often made out of waste-stream materials like feather meal, blood meal, bone meal. CHECK THE LABELS ON THE PACKAGES to know what you are getting and how high the percentages are.

Container plants will need regular feeding because they are disconnected from the soil life. BUT - if you are adding lots of organic matter to your in-ground beds, mulching, using cover crops, crop rotation, etc - you probably won't need much in the way of fertilizers at all. The only way to know for sure is to test your soil. 

Image Credit: Lowe's

You can buy a simple home-testing kit for about $10, which is really all most of us need - they aren't perfectly accurate but will give you enough of a result to figure out your imbalances. However, if you're feeling adventurous and you want some serious results, you can send your soil away to be tested at a lab. For a more conventional nutrient test, you can send a sample to U Mass. It costs very little, and you'll get a very interesting report. They will also give you recommendations based on your nutrient levels and soil texture. This is a fine way to go, but it's maybe not the most important test you can do.

You see, the microorganisms present in your soil determine your soil health. They are the ones who process the minerals and make them available to your plants. If you really want to know how alive your soil is (and you want it to be very, very alive), you could send a sample to Earthfort. They will test to determine the amounts of bacteria, fungi, nematodes (beneficial and detrimental) and protozoa in your soil. They can also tell you the percentage of organic matter. This costs a bit more, but it's probably worth it. I haven't done this yet, but I very much want to. 

So that's an overview of nutrition. Now I want to tell you the results of our foliar feeding experiment. 

The reason we even did this experiment is because of all the hype surrounding foliar feeding. Search for it on the internet, and you'll get some fabulous claims.  Let me be clear: the science is definitely not there to support those claims. There just isn't enough peer-reviewed data to say definitively that foliar feeding works. Plus, plant biology doesn't really support anything being taken up by the leaves. Each leaf has stomata, little openings on the undersides, which allow for gas exchange. But they don't take anything else up in there. So how does a foliar substance get in the leaf in the first place? It hasn't been discovered, if it exists. Also, there is a translocation of nutrients within the plant, but the nutrients themselves come from the roots. Photosynthate moves down, nutrients move up. Mostly. Not always. Things can be reallocated around the plant if need be. But there is no pathway from the stomata to the xylem or phloem from the stomata. 

There IS evidence that spraying microbiology on the leaves, like with compost tea, provides a coating on the leaf that is helpful in many ways. The leaves need good biota just like the roots do. It can protect them from predators, keep them from being sunburned, allow good stuff to live on the surface. It's just that it isn't clear that anything actually ever gets in the leaves except gases. 

Our teacher, Laura, wanted us to try an experiment to see if we could get any definitive results. Once the plants (chard for my team, again) germinated and had a couple of leaves, we began to cover the soil and spray only the leaves with macronutrients. We sprayed some with organic all-purpose fertilizer, and some with synthetic all-purpose fertilizer. We also had several controls that got soil drenches of these same fertilizers. We drenched the soil of each plant with micronutrients each week so that wouldn't be a limiting factor. The soil surface of each plant was covered with fresh plastic wrap each week, so no accidental dripping from the leaves would occur. The controls were covered as well so we could make a comparison.

Spoiler alert: They all did horribly. Even the controls weren't so hot, we think because the soil was covered each week and there was a lack of oxygen.

Here's our best-looking control:

Really floppy. Not good strength. Nice color, but not a lot of growth.

Here is an example of one of our foliar-sprayed plants. Remember, this plant got zero macronutrients other than on the leaves.

Just terrible. It's very chlorotic, and it didn't grow at all from the time we started spraying until the end of the experiment. What a total waste of a plant.

So there you have it. Foliar sprays are really a waste of time. Unless you feel that you need some good biota on the leaves, you can skip it. Add your inputs to the soil itself. Making compost tea is fine, but just drench the soil with it, where it can do some real good. Don't bother with the spraying.

***** edited 5/1/18  Talked to a fellow student last night who is doing his own foliar feeding experiment at home with tomato plants. He is spraying with a calcium/mag supplement and getting terrific results compared to the controls. So it just goes to show that more research is needed in this field! Different nutrients on different plants might react differently!!! Take my experiment with a grain of salt, and look for other scholarly papers on this subject. 

Clover Cover

I've already written about the excellent cover that our wheat/oat crop provided for the vegetable beds over the winter, but I failed to mention that I also seeded crimson clover at the exact same time I seeded the wheat, with the hope that it would provide nitrogen over time to other plants. It's done well everywhere I planted it, except the garlic bed, for some unknown reason. You can see it flourishing in the picture above. It's a lovely cover crop. But today I cut most of it down, because it's finally tomato-and-pepper-planting-time, hooray! However, before the clover is relegated to compost, chickens, or mulch, I wanted to show you what it can do for you in your garden, and why I planted it in the first place. 

Nitrogen nodules on the roots of the clover

There's a huge amount of nitrogen in the air around us, but it's in a form that is very difficult to access because it has a very strong bond. As you probably know, lightening can break apart those bonds and cause nitrogen to become available to plants in rain. Also, the Haber-Bosch process, used originally to make bombs, is what makes synthetic nitrogen for fertilizers now (like Miracle Gro). This process requires a huge amount of energy and personally I will never use synthetic nitrogen for many reasons including that one. However, some plants have evolved to provide their own nitrogen; those in the Legume family have this skill.

Clover is a legume, so like every plant in that family, it has the ability to form a relationship with a special bacteria which provides nitrogen to the plant. This rhizobia bacterium, when invited by the plant, invades the roots of those leguminous plants. The plant gives up some carbohydrates in the form of sugar exudates to the rhizobium, which in turn makes tiny anaerobic nodules on the roots which have the ability to fix nitrogen. Those are the pink nodules you can see on the roots above. 

This nitrogen generally is fixed within the bodies of the bacteria, and doesn't become available to other plants (through the soil solution) until the plant is cut down and the roots decompose. It's important to time all of this correctly: You want to grow the leguminous plants until nitrogen has been fixed, then cut off the tops of those plants before they set seed (if the plant sets seed, it has used up all the nitrogen stores for itself), and allow the tops to compost in place on the soil, while the roots and nodules decompose deep within the soil. 

The nitrogen-fixing ability is true of any leguminous plant - clover, peas, vetch, favas, alfalfa - as long as the bacteria is present, which it is in healthy soil. You can allow them to bloom and feed the pollinators, but chop it before it sets seed. Then mulch the next set of plants with the tops that you've cut off. Above are my sweet peppers, the soil totally covered with the clover tops. These will decompose in place, allowing nutrients back into the soil. You don't have to mix them in, the soil microbes will do that work for you. And as they break down, they will retain moisture, shade the soil, and prevent weeds from germinating. Plus add nutrients naturally!

Meanwhile, it makes a beautiful flower display that my bees adore!

I've had such success with winter cover crops this year that I intend to use them every year; I'll just plant them within the rows of vegetables. I'd like to figure out how to do this with a summer cover as well. Buckwheat works well here as a summer cover and I've used it before by itself; the trick is to use it in between the growing food crops. I am doing a little of this already using intercropping with other vegetables. For instance, I'm growing pumpkins and butternut squash underneath the corn, and basil around and in the hot peppers. I'm always worried about growing anything below the tomatoes because of air circulation; but I have all our wheat and oat straw to cover the soil there. Beans and cucumbers quickly cover the soil with their own leaves. 

Have you used cover crops or intercropping in your garden? If so, I'd love to hear about your results. Please share in the comments.