Welcome to The STEM Sessions Podcast. I am your host, Cody Colborn
The carbon and nitrogen cycle has long been on my list of potential episode topics, but I could never find a hook that captured my interest
- Even though the transfer of nitrogen from the atmosphere to land and sea and back again is really the basis of life and our food supply, I just couldn’t find something to be really curious about
- And if I can’t be curious about something, the research is boring
- And if the research is boring, I can’t be motivated to learn enough to make an episode of this podcast
That recently changed, when I found myself in a conversation about crop rotation
- At some point in future, I may explain why I was having this discussion, but that’s another episode
- Specifically a rule of thumb about needing to rotate winter wheat into a field of alfalfa every four years to “replenish the soil”
- Crop rotation is a long established and widely used method in agriculture, and this rule of thumb may have been totally accurate, but something seemed off
- The specific time period and the verb replenish just weren’t sitting well with me
As I mentioned in episode about rules of thumb, the longer they stick around and are taken at face value, the question of “why does this work” tends to get asked less often
- And when the initial understanding is lost, rules of thumb can be correct but for the wrong reasons
- I suspected this might be one of those cases
So this episode describes my investigation into this rule of thumb
- The questions I asked myself
- The logic I used
- And ultimately the correct answer, which oddly was known to the people I was talking to, but it’s a piece of information they took as general knowledge and therefore didn’t directly link it to the rule of thumb
This is The STEM Sessions Podcast Episode 26 – Autotoxicity
Every four years you need to rotate alfalfa with winter wheat in order to replenish the soil
- That’s the rule of thumb given to me by a few seasoned farmers
Whenever I use a guideline or rule of thumb, I need to understand why it works and what limitations it might have
- Are there instances where it shouldn’t be applied, for example
- When I asked why winter wheat, why four years, and what did they mean by “replenish the soil”, the answers I was given were a bit too handwavey
It’s not as if I felt they were lying to me
- They likely had just been using this rule of thumb for so long, they just took its application for granted, and were surprised that I wanted to know the science behind it
- So after our conversation, I opened up a friendly neighborhood search engine, and began teaching myself the science
For soil to be considered fertile, it must contain potassium, phosphorus, and nitrogen compounds
- I’m using the term compounds here to clarify we’re not necessarily referring to their elemental forms
- Other minerals and factors contribute to fertile soil, but these three nutrients are perhaps the most important because they are critical for chlorophyll production and for successful photosynthesis
When it comes to nitrogen, plants cannot efficiently use nitrogen from the atmosphere
- It needs to be converted from N2 gas to compounds like ammonia, which can then be utilized by the plants
Atmospheric nitrogen is converted to these compounds naturally by bacteria in the soil
- However, while enough is produced for a stable ecosystem of native plants, agriculture is a different story
- When a field is used for crops, you have more plant mass per area than would be seen in a native ecosystem, so nitrogen is consumed faster than soil bacteria naturally convert it
- Thus, supplemental nitrogen needs to be added
To introduce a usable form of nitrogen into the soil, it is common practice to use a fertilizer high in nitrogen
- Fertilizers will also typically include phosphorus and potassium
- That’s what the three numbers on a bag of fertilizer represent, the nitrogen-phosphorus-potassium (or N-P-K) ratio by weight
- For example, a 15-7-10 fertilizer contains 15% nitrogen, 7% phosphorus, 10% potassium by mass
Drawbacks of adding such potent fertilizer is it can run off the field and into surface water, which can lead to algal blooms and other ecological impacts
- It can also be expensive, to the point where adding it every year may be cost prohibitive
- And this is true of both synthetic fertilizers and organic fertilizers – the only difference being synthetics contain a lot more NPK per weight than organics, so they’re more concentrated
An alternate, or supplement, to fertilizers are nitrogen fixing plants
- Nitrogen fixing plants contain bacteria called Rhizobia in their roots
- Symbiotic relationship, plant draws in atmospheric nitrogen, and bacteria convert gaseous nitrogen to useable nitrogen compounds
- Compounds are stored in nodules along the roots for the plant to use throughout its life cycle
More nitrogen compounds are produced than the plant needs
- When plant dies, the nitrogen is released into the soil where it becomes available to other plants
Common nitrogen fixing plants are found in the legume family – fabaceae.
- This family includes crops and garden plants such as soybeans, clover, peanuts, peas, beans, and alfalfa.
- If alfalfa is a nitrogen fixer, it’s already improving soil nutrients for whatever crop comes after it, So how does rotating in a crop like winter wheat “replenish the soil” when it consumes more nutrients than it creates?
Maybe “replenish the soil” is meant more literally, as in winter wheat builds or creates soil
- From the “Crime Pays, Botony Doesn’t” YouTube channel, I’ve learned that native prairie grasses will send out roots over ten feet deep
- And this extensive organic matter decomposes and adds matter to the soil
- Winter wheat is a grass, originating from a prairie environment, though not the North American prairies, so maybe its roots go that deep and rebuild soil matter
Many factors influence root growth: soil composition and density, nutrient availability, water, and sunlight, so studies of root depth show a range of results
- Research shows winter wheat roots stretch around six feet on the deep end, but three feet seems more typical
- Alfalfa sends roots three or four times more deeply, up to 20 feet, with max depth reached over multiple growing seasons
- So rebuilding soil matter with decomposing roots is likely not the reason for rotating out alfalfa, because alfalfa already produces more root material than winter wheat
After more discussion and research, I determined that “replenish” is the wrong word to use, and the better verb is reset or clean-up the soil, because alfalfa has evolved another property to help it survive in non-optimal soil and drought conditions – a property called autotoxicity
Alfalfa is a perennial plant, meaning the plant survives thru multiple growing seasons
- After a single growth cycle, the plant seemingly dies back, but some part, usually the roots, survives and allows the plant to resprout at the beginning of the next growing season
Alfalfa is also an angiosperm which means it flowers and produces seeds
- Those seeds are scattered by wind and animals, hopefully landing in a location where they will germinate
- Germinating next to an existing alfalfa plant would create competition for nutrients and water, resulting in the first generation plant and second generation plant exhibiting stunted growth
To prevent this competition from occurring, alfalfa produces chemicals that prevent other alfalfa plants from growing around it
- The chemicals in question haven’t been definitely identified, but research points to medicarpin, coumarin, and chlorogenic acid as the likely culprits
- It may likely be a combination of compounds rather than a single one
Regardless of which compound or compounds are responsible, the result is direct failure of germination in most seeds and severe damage to the roots of those seeds that do germinate
- When the root system is damaged, the alfalfa plant may appear to grow more vigorously at first as it compensates, but its longevity and yield are drastically reduced
Concentration of toxins in the soil has many factors
- They are water soluble, thus more rain and irrigation, the less toxin build up
- Heavier soils hold on to the toxins longer than lighter soils
- Soil disturbance also helps dissipate the toxins
- So it can’t be said definitively when toxin build up is too much for new plants
Experiment referenced by nearly all papers I read shows that toxin build up is strongest within a half-foot of the plant
- Drops quickly outside of that range
- This means as the alfalfa crop ages, a field may only support one plant per square foot
- Profitable hay production requires four or five plants per square foot
That single plant will become less and less fruitful as it ages, and as long as it’s alive, the soil is too toxic to reseed
- And that’s where rotating in a crop like winter wheat comes in
- Even a rotation of one year seems to be enough to reduce the concentration of toxins to an acceptable level
- Two years gets rid of them entirely according to some studies
Any crop that isn’t alfalfa will get the job done, because the chemicals are only toxic to alfalfa
- Nothing else is affected by them
- The reason winter wheat is specifically mentioned in the rule of thumb is winter wheat is one of the staple crops from that particular geographic area
- But anything will work
- Even killing the alfalfa with an herbicide and letting the field sit fallow (or empty) for a year will do it
- But you might as well plant something to take advantage of the nitrogen fixed by the alfalfa
How many years a single sowing of alfalfa will remain productive can’t be calculated or predicted
- Too many variables such as rainfall and soil quality
- If year after year you have near optimal growing conditions, those first generation plants will stay healthy and bountiful, and you might get five or six years
- If you have less than optimal conditions, those first generation plants will age quickly, and two years may be all you get
- The rule of thumb specifies four years, because that’s been a traditional balance between alfalfa productivity and the cost of rotating the field
- But not only can the time period can vary field to field, it can vary decade to decade
As I said in my episode on rules of thumb, you should take the time to understand why a given rule of thumb works and when it doesn’t
- Once you know the logic behind it, you can make changes to improve results as conditions change
- And to paraphrase the great G.I. Joe PSAs, knowing is half the battle
Thank you for listening to this episode of The STEM Sessions Podcast; researched, written, and produced by Cody Colborn. Shownotes can be found at thestemsessions.com. Feedback and corrections are always welcome.
If you received value from this episode, and wish to give some back, please visit thestemsessions.com/valueforvalue for ways to support the podcast.
Please remember, STEM belongs to everyone. We should not allow it to be siloed or gate-kept by experts, policy makers, or talking heads. Bias is found in every message, so always verify what you read and what you’re told.
Until the next episode, stay curious.
REFERENCES
https://eos.com/blog/nitrogen-fixation/
https://savvygardening.com/fertilizer-numbers/
https://animalrangeextension.montana.edu/forage/documents/alfalfaguide1.pdf
https://alfalfasymposium.ucdavis.edu/-files/pdf/alfalfaFactSheet.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5669304/
https://soilandhealth.org/wp-content/uploads/01aglibrary/010139fieldcroproots/010139ch5.html