The prevailing paradigm today is that agriculture is somehow, by its very nature, inherently extractive. There is a widely held belief that the process of farming degrades soils and landscapes. Unfortunately, it is often the case that farming has a degrading effect, but it doesn’t need to be this way.
I envision a world where the process of growing food regenerates the land, revitalizes rural communities, produces food that improves our health, and leads to farming landscapes that are beautiful. Vibrant. And clean.
The vision (and the reality) that agriculture, the source of so many challenges in our world today, has the capacity to be the solution to each of those challenges is so inspiring.
It is this inspiration that motivated me to found Advancing Eco Agriculture (AEA). Since 2006, we have pioneered the development of regenerative growing systems in all types of challenging environments. We have:
Partnered with more than 10,000 growers across over 4 million acres;
Worked with growers of all methodologies and crop types to implement regenerative agriculture systems that require fewer inputs and increase growers’ yields and profits;
Designed and invested in agricultural technology and analysis tools to support growers in making data-informed decisions;
Established trust, built credibility, and created momentum pioneering the regenerative agriculture industry.
We know we have the tools, but the why behind regeneration is the key.
Regeneration needs to be much deeper than just regenerating soil health. Regeneration is fundamentally about regenerating landscapes at all levels. Relationships between soil biology and plants, between livestock and the landscape, between farmers and eaters. But for us, right in this moment, we desire to develop relationships with our supporters, our customers, and our team. Regenerating relationships means shifting relationships to be deeply collaborative, symbiotic, and synergistic, rather than the transactional and extractive relationships that prevail in the contemporary agribusiness world of today.
Regenerative agriculture is commonly defined as a regeneration of soil health. A set of soil management practices that includes non-disturbance (no-till), keeping soil covered, incorporating livestock, utilizing cover crops, increasing species diversity, and maintaining continuous living roots in the soil are generally agreed upon as the drivers of a regenerative farm management system.
However, these management practices all miss a fundamental driver of soil health, which can supersede the impact of all the practices above. This factor is plant nutritional integrity.
The nutritional integrity of a crop determines its capacity for photosynthesis and carbon sequestration. Photosynthetic activity can vary as much as 3-4x based on a plant’s nutritional status. Manganese, magnesium, phosphorus, nitrogen, iron, and other minerals are directly involved in the photosynthesis process. Inadequate levels of any of these nutrients will directly bottleneck photosynthesis, and limit the quantity of carbon that is fixed and converted into sugars over each 24 hour photoperiod cycle.
The foundational requirements of photosynthesis are adequate water, carbon dioxide, sunlight, and a green leaf containing chlorophyll and balanced mineral nutrition. Farmers intimately understand the critical requirement for water. Sunlight is considered a given. Carbon dioxide supply and mineral nutrition are commonly misunderstood or ignored entirely in outdoor production agriculture. Because of this misunderstanding, most crops being grown in an outdoor agricultural setting are photosynthesizing at only a fraction of their inherent genetic potential.
In our consulting work at Advancing Eco Agriculture (AEA), we understand that plant nutrition and microbiome management are the foundational drivers of plant immunity and crop yields. We collect plant sap analysis data through the entire crop life cycle to manage nutritional integrity, and increase disease and insect resistance. Our team has collected tens of thousands of samples over the last fifteen years on dozens of crop species. Almost universally, crops experience significant nutritional imbalances that limit their capacity for photosynthesis.
Greater than 95% of the sap analysis results we see when we begin working with a farm for the first time show manganese and iron deficiencies. Over 60% show low magnesium. Shortfalls of zinc, copper, boron, cobalt, sulfur, and silicon are so common, we expect to see several of these showing up at inadequate levels in practically all the initial samples when we begin working with a farm to transition to regenerative nutrition management. Once we correct these nutritional imbalances, yields and pest resistance increase immediately as a result of the increased photosynthetic activity.
This misunderstanding of the primal importance of photosynthetic efficiency underscores the misconception around the slogan “healthy soils create healthy plants”. While it is true that healthy soils produce healthy plants, the question is: “What creates healthy soils?” At the most fundamental level, what creates healthy soils is plants photosynthesizing, sequestering carbon, and transferring that carbon through root exudates into the soil profile to feed the symbiotic microbial community in the rhizosphere.
Without photosynthesis and carbon induction, there is no soil. Soil without the contribution of plants is nothing more than decomposed rock particles. The generally accepted ‘regenerative management practices’ all point to the necessity of maintaining living plants constantly photosynthesizing, but miss addressing the fundamentals of photosynthetic effectiveness. Thus, it is healthy plants that create healthy soil. Plant photosynthesis is the engine that drives the generation (and regeneration) of soil health, not the other way around.
It is commonly assumed that growing crops is somehow inherently extractive, that we deplete soil carbon when growing a crop, and to regenerate, we need to grow ‘cover’ crops to place carbon back into the soil. In the agronomic literature, it was historically understood that the fastest way to build soil carbon was to grow corn. Today, growing corn is considered one of the fastest ways to deplete soil carbon. This is a result of the nutritional mismanagement of contemporary agronomy, focusing exclusively on a few nutrients (and applying them in excess), while not maintaining nutritional balance to manage photosynthesis. We can build soil carbon levels while we are growing a crop. Any crop. It only requires managing plant nutrition differently, and optimizing for photosynthesis and immune function.
Not considering photosynthetic variability is also a key oversight in most carbon sequestration literature. It is not safe to assume that the rate of photosynthesis is a constant, and remains consistent across different research settings. For example, researchers report wildly varying percentages of plant photosynthates being transferred to the soil as root exudates, some as low as 5%, and some as high as 95%. This extremely high variability depends on many factors, including plant species, stage of growth, microbiome, and soil environment. But the biggest driver of variability remains the rate of photosynthetic efficiency. Imagine how our agriculture might look different if every crop transferred 95% of its total carbon to the soil, as compared to 5%? Our contemporary agronomy management practices ensure that most crops remain on the bottom end of the spectrum.
The best news is, when you increase photosynthesis, you cannot prevent yields from increasing. Healthy plants with abundant energy levels will produce more fruit, seeds, and vegetative biomass. A model of regenerative agriculture based on sound nutrition management has the capacity to increase the yields of many crops significantly, while simultaneously reducing the need for fertilizers and pesticides.
Managing plant nutritional integrity is a fundamental driver of regenerating soils, and the one driver with an immediate economic impact for farmers.
The Oxford dictionary defines snake oil as “a product of little real worth or value that is promoted as the solution to a problem.” Wikipedia says it is a “term used to describe deceptive marketing, health care fraud, or a scam.”
The term is most commonly used in the agriculture space as a derogatory for biocontrol or biostimulant products that are unfamiliar or not yet in widespread use.
In my conversation with Pam Marrone she made a comment to the effect “If fertilizers and pesticides were held to the same performance standards as biocontrols and biostimulants before being registered, many of them would fail.”
Richard Mulvaney and colleagues have reported the reduced yield and quality from fertilization with potassium chloride, the most widely used commercial source of potash. The industry response has been a deafening silence.
Does a prevailing lack of yield response not make potassium chloride fertilization fit the definition of ‘snake oil’?
Just because an application has become ‘standard practice’ does not mean it brings benefit to the farmer. It may be bringing benefits mostly to those who farm the farmers.
What if you could get seeds to germinate as fast in challenging conditions as they emerge in ideal conditions?
This article was published in AcresUSA a few weeks ago, and I wanted to share with you here.
The spring wheat seedling on the right was treated with a BioCoat Gold and nutritional support at planting. The seed germinated 12 hours after planting in cold soil and challenging weather conditions. The comparison seedling is the grower’s standard program.
Root mass development on cover crop with BioCoat Gold and AEA fall soil primer
Seeds with Speed How to manage germination speed and seed quality in producing high-quality crops
When seeds germinate quickly and a new crop becomes visible above the soil soon after being planted, it brings smiles to our faces. We know instinctively that plants that get off to a vigorous start have the potential for a healthy crop with abundant yields.
When seeds germinate slowly because of challenging soil or weather conditions, we recognize that this early stress on the young seedlings is likely to produce a yield drag, as the plant seems to struggle to catch up for the rest of the growing season.
There are many benefits of rapid seed germination and seedling emergence. When seeds germinate quickly, they provide little opportunity for insect larvae such as corn seed maggot to begin feeding on the seed. When root systems develop quickly and fill large soil volumes while the seedling is still small, the possible damage from wireworm or rootworm larvae is greatly reduced. For plant species that produce an allelopathic effect from the root system, rapid root development can have a pronounced effect on suppressing germination of other seeds, producing a field that is practically weed free. When seedlings grow very rapidly, and contain balanced nutrition from the seed, they are resistant to slugs and flea beetles feeding on them shortly after germination. However, none of these positive effects occur when seeds germinate slowly or when seeds are of poor quality.
These positive effects have always been appreciated by organic crop farmers who wait to plant until the soil is warm and weather conditions are as ideal as possible. The rapid seedling emergence that occurs when planted in good conditions provides an opportunity for much better weed control, as cultivation can be done earlier and the crop shades out weed seedlings more rapidly.
Planting conditions are not always ideal, however. With the pronounced vagaries of the weather we are all experiencing every year, it is probable that conditions will be less than ideal more frequently in the future than they have been in the past.
Many growers have also been recognizing that purchased seed quality is not what it should be, and not what it used to be. Many seeds appear to lack vigor, and may germinate only very slowly, even when planted in ideal conditions. This is especially true of commodity grain crop seeds, but also for many vegetable seeds.
Several years ago, a colleague obtained several hundred seed corn samples from seed suppliers and planted them in seedling trays in a germination chamber to test vigor. While most of the seed samples reached the germination percentage on the label, many germinated quite slowly, emerging only 5-7 days after being planted. Some emerged 10 days after being planted, despite being maintained in perfect moisture and temperature conditions for rapid germination.
This becomes understandable when we consider the objectives and processes of corn seed production. The objective has become small seed size, which is the opposite of what would be produced if the goal was vigorous seeds. When we think about how corn seed is produced, no special consideration is given to plant nutrition. In fact, it is considered that seed corn can be grown on poorer soil, since yields are not expected or needed to be as high. Before pollination, the plants are “detasseled” by cutting off the plant above the ear with a mower, which removes a third to a half of the plant’s photosynthetic capacity. To keep seed size small, the plants are desiccated as soon as the seeds reach maturity, frequently with a sodium/potassium chloride solution as a desiccant. The end result of this process is a corn seed that is small, lightweight, low in stored carbohydrate energy, low in mineral content, and loaded up with chlorides. The icing on the cake is fungicides and insecticides the seed is treated with before being planted. A safe summary is simply that commercially grown seed corn is generally of atrocious quality.
It should be no surprise that these seeds germinate slowly and are particularly susceptible to insects and disease. We have established with the use of plant sap analysis that anytime chloride levels in plant sap exceed the levels of total nitrogen, plants are particularly susceptible to insects. You can almost always observe large populations of insects feeding on a crop where this ratio is present. These corn seedlings have been set up to be dependent on constant life support for the rest of their life.
For many crops, seed production is not quite as badly screwed up as corn seed production, but little or no consideration is given to producing high seed vigor, other than as measured by germination percentage.
Qualities of Superior Seed
There are two key aspects to superior-quality seed that germinates quickly. The best seed contains abundant nutrition — mineral nutrition as well as carbohydrates, proteins and fats. Seed with generous nutrition will be heavy, have fewer seeds per pound, and have a high test weight. In addition to the nutritional component, the best seed also carries a population of symbiotic microorganisms on the seed surface that immediately colonizes the root system and leaf surface as the seed germinates.
The speed of microbial colonization on the root system is very important to produce resistance to root diseases. When this beneficial microbiome is not carried through on the seed, the seedling now needs to recruit microbes from the soil to colonize the root surface and develop a healthy microbiome. This process takes time and may require the contribution of more sugar and energy through the root system as root exudates. The length of time for this recruitment process varies depending on plant species and the soil microbiome, but can take up to two weeks. During this recruitment window, when the seedling root system is not yet fully colonized by beneficial and symbiotic microorganisms, there is a window of opportunity for organisms to develop pathogenic relationships with the plant. Fusarium, rhizoctonia, pythium, anthracnose, phytophthora and many other root-rot diseases gain traction in the initial weeks after seed germination when root systems are not immediately colonized by disease-suppressive microbiomes. Seeds that do not carry a healthy microbiome predispose young seedlings to disease susceptibility. Fungicide seed treatments amplify this susceptibility.
The lack of a healthy microbiome on the seed does not only increase susceptibility to disease — it also changes root system development and size. Some of these beneficial bacteria are referred to as PGPRs — plant growth promoting rhizobacteria. These bacteria produce phytohormones that influence plant growth and development, particularly root branching. Many growers have observed that the use of microbial inoculants as a seed treatment produces a much larger root system on seedlings, with a lot of root branching as compared to untreated seed. This effect is produced by microbial colonization and the phytohormones they contribute to the plant. These robust root systems, established immediately after germination, are a critical foundation to produce large crop yields when plants are expected to obtain the majority of their nutrition from microbial metabolites rather than from soluble fertilizers. Without a large root system, plants are unable to obtain enough nutrients during the fruit-fill/grain-fill period to produce exceptional yields.
In addition to producing large root systems, the phytohormones produced by the PGPRs also contribute to overall stem size and expansion. For plants to carry a heavy fruit load to maturity, they require a large water and nutrient-transport pipeline. Frequently, plants have the genetic capacity to produce a lot more fruit, but the pipeline is not large enough to supply the water and nutritional requirements to support a heavier fruit load. Having the plant growth promoting rhizobacteria present from the moment of germination is foundational to increasing yields above average baselines.
Given the value of seed quality, what management actions can we take to improve our crops’ performance?
If you produce seed, manage nutrition and biology to go above and beyond, and produce the heaviest and largest seed size you can. This will also produce very positive epigenetic results, where the following generation is almost certain to be more vigorous than the parent generation, and may begin expressing itself differently, especially over several generations.
If you market seed, produce superior quality, and market it accordingly. Growers care — a lot. This is an easy opportunity to be a market leader.
If you buy seed, get the heaviest and largest seed you can find for a given variety. Check seed counts per pound. Book seed well in advance so you can get it untreated with -cides. This is the nexus of where you want life to proliferate — not death.
To test seed vigor and the effects of inoculants and nutritional supplements, plant test seeds in clear plastic cups so you can observe how quickly roots reach the wall of the cup, and how many are visible.
Given the quality of seeds generally available, it is important to think about how we can enhance seed microbiomes and nutritional integrity in an effort to make up for what was missed during the production process. Adding microbial inoculants and nutrition that can get inside seeds can produce some remarkable results.
I believe it is important that microbial inoculants contain a combination of beneficial bacteria, mycorrhizal fungi, microbial biostimulants and probably other organisms as well. I refer to these combinations as “synergistic stacks,” where one plus one produces something greater than two — sometimes much greater. Living organisms can produce a compounding effect, rather than an additive one. This is exactly what we need at the critical stage of seedling development.
In our consulting work, we recommend an inoculant almost universally on planted seeds because of the rapid germination and root development responses we observe. Microbial inoculation at planting is consistently the lowest cost and highest ROI of almost any application type that a farmer can make.
In addition to inoculation, I am also very intrigued by the possibilities of nutritional seed treatments, where the nutrients are actually absorbed and utilized inside the seed. It is well established that seeds with abundant levels of trace minerals such as manganese, zinc, copper and boron will germinate much more quickly than those without.
We have worked with growers who have applied a combination of chelated trace minerals in amounts ranging from 25 to 100 ounces each of manganese, iron, zinc, cobalt and copper per ton of seed. These liquid trace minerals are combined with water and mixed with seed. The amount of water used will vary depending on seed type, but we want to use just enough to get good distribution and to allow the seeds to absorb all of it, while still feeling dry to the touch and flowing through planting equipment well. It is possible to use small enough amounts of water that seed can be put back in storage for several weeks before being planted.
Think of seed treatments as colostrum for the developing seedling — nutrition it should have gotten from its parent, but probably didn’t.
Many agricultural soils contain abundant levels of manganese locked up in soil reserves. Yet, for the reasons of selective suppression of biology, oxidation and immobilization discussed in these blog posts most crops are constantly manganese deficient, which limits water hydrolysis for photosynthesis.
The reasons above are why field research seldom shows a positive crop response from manganese. In the short term, until microbial populations are established which can release the unavailable reserves of soil manganese, the only effective solution is to supply manganese with foliar feeding so it does not limit photosynthesis.
The symptoms of subtle manganese deficiency have become so commonplace today we don’t recognize them as anything outside of the ordinary.
When plants have adequate levels of manganese, the leaf veins will be the same shade of green as the area between the veins.
These cotton leaves are showing light colored veins, symptoms of inadequate manganese. Once you begin looking, you can see this almost everywhere, in undomesticated plants as well as crops. There are a very few plants which have light colored midribs genetically, even those will darken when supplied with manganese.
How many plants have you observed that did not have light colored veins?
The success of an intervention depends on the interior condition of the intervenor. ~ Bill O’Brien
Most growers in developed countries only have experience with contemporary farming systems, which relies on constant fertilizer and pesticide inputs and purchased seeds.
If you are reading this post, you are aware to some degree that regenerative agriculture management systems have so much more to offer than contemporary systems: higher yields, improved nutritional integrity, disease resistance, insect resistance, reduced input costs, increased profitability, reduced climactic risk, regenerating ecosystems, improved public health outcomes, and so much more.
As we realize the incredible potential and untapped opportunities we want to share our excitement and the information we are learning with our peers.
Many growers are struggling today, in different ways. With overwork. With difficulty in making ends meet. Not having enough time with family and friends.
Once we observe and experience the possibilities of regenerative agriculture, of course we want to share them others.
If we desire to facilitate change, it is critical that we recognize the receptivity to different/new ideas has nothing to do with the potential benefit of those ideas. The openness to a new approach has nothing to do with how skillful you may be at implementing, or at pitching a new approach.
Anytime we attempt to guide someone in a different direction, we engage in intervention. Even in matters as trivial as what to make for dinner or which socks to wear.
The outcome of an intervention has nothing to do with the skills of the intervenor. It has everything to do with the place within from which the intervenor comes.
You may be the most skillful carpenter in the region, but not be able to guide home builders to consider important improvements in their new home.
You may be the most knowledgeable agronomist in the state, but not be able to get farmers to shift management practices.
You may be a very successful regenerative farmer, but other farmers who observe and listen to you make few changes on their own operations.
Because our ability to facilitate change in others has nothing to do with skills or knowledge. Instead, it has everything to do with our internal state, where we are coming from within.
When we come from an internal place of love, appreciation, care, and respect, others can feel this and are able to respond, to be open, vulnerable, without fear of judgement, and engage with our input on a deep level. The level required to actually make changes.
When we come from an internal place of judgement, this is felt as well, and there is no opportunity for openness and hearing what the other has to say by either party.
If we desire to facilitate change with our friends, neighbors, colleagues, all the people we really care about, it is not useful to focus on a discussion of what we consider ‘bad’. It is much more powerful to be for something than it is to be against something.
Being ‘anti’ can give us a shot of energy from all the drama created, but doesn’t inspire people to actually change. Anti-GMO, anti-glyphosate, anti-tillage, etc., is seldom a productive message.
The farming community needs you to be a leader. To engage in authentic conversations where you are able to come from a place within that can inspire change.
Are you ready to step into this internal space?
John Kempf2022-02-02T07:08:17-05:00February 18th, 2022|
When crops are foliar sprayed regularly through the growing season, managing spray solution electrical conductivity (EC) becomes very important.
If crops are only sprayed two or three times in a growing season, it is possible to apply a very concentrated product solution, and only observe positive crop responses. Growers regularly fly on spray solutions that were as much as 50%-70% high EC products with only 30%-50% water at a rate of 3-4 gallons of total solution per acre, with very good results.
When higher value crops are sprayed every 7-14 days through the growing season, managing the solution EC becomes important. When the electrical conductivity of the spray solution becomes higher than 3800 microsiemens (or 3.8 millisiemens), plants seem to not respond as well and absorb nutrients less readily from later applications. I don’t know why this is the case, but a lot of experience indicates this is something we must pay attention to.
It is critical to only use water that is known and tested to be clean, that does not contain bicarbonates above 70 ppm total mineral content.
John Kempf2022-02-17T06:43:17-05:00February 17th, 2022|
All plants respond dramatically to changes in phytohormone levels.
The use of plant hormone products usually corresponds to a crops value. High value crop growers quickly recognize they cannot afford to not use these products because of the exceptional crop responses they can produce when applied in a timely fashion.
We often observe instances where cultural management practices produce a profound negative effect on a crop, because the impact on phytohormone levels is not appreciated.
As an example, abscisic acid (ABA) is the phytohormone which leads to the development of good fruit coloration. When fruit does not contain enough trace minerals, ABA levels remain low, and apples (or any other fruit) remain green instead of coloring well. Some apple growers use foliar applications of ABA to enhance fruit coloring. In parallel they may also use water deprivation to improve fruit storability. However, water deprivation results in elevated levels of ABA.
The combination one-two punch of water deprivation and a foliar of ABA often causes early fruit drop. After all, abscisic acid gets its name because it triggers fruit abscission. This combination of cultural management practices having misunderstood consequences costs apple producers a lot of yield and profits. It’s like shooting off both your feet at once.
For those growers producing crops other than apples, no need to feel relief to early. The odds are good that similar foot shooting practices have become commonplace for your crops as well.
With this background context in mind, I find it intriguing to learn that low level glyphosate residues in the soil profile result in changed phytohormone profiles in crops.
It seems a logical expectation this would occur, given glyphosate’s disruption of the shikimate pathway, but before this paper, no one looked at how soil residues would affect following crops.
Here are a couple of highlights from a great article:
– Glyphosate disrupts the shikimate pathway which is the basis for several plant metabolites. The central role of phytohormones in regulating plant growth and responses to abiotic and biotic environment has been ignored in studies examining the effects of glyphosate residues on plant performance and trophic interactions.
– Plant hormonal responses to GBH residues were highly species-specific.
– Potato responded to GBH soil treatment with an increase in stress-related phytohormones abscisic acid (ABA), indole-3-acetic acid (IAA), and jasmonic acid (JA) but a decrease in cytokinin (CK) ribosides and cytokinin-O-glycosides. (An expression of reduced root growth. JK)
– Our results demonstrate that ubiquitous herbicide residues have multifaceted consequences by modulating the hormonal equilibrium of plants, which can have cascading effects on trophic interactions.
– Accordingly, the consequences to non-target plants can range from growth stimulating to changes with their biotic environment. Herbicide residues are ubiquitous and it is necessary to unravel their consequences for ecological interactions and their involvement in shaping evolutionary processes (Riedo et al., 2021). In conclusion, to elucidate the full picture of effects of GBH residues, it requires thorough understanding of the “soil legacy” including the study of soil microbiota and how it is affected by persistent herbicide use.
While this research is relevant for all soils and crops that have had glyphosate applied historically that has not yet degraded, it is of particular relevance to tree crops where lots of glyphosate has been applied in the tree row, or any soil with a history of generous glyphosate applications.
It is worth reading the paper, and noting how phosphate applications effect glyphosate solubility and crop performance.
Have you observed any changes in plant expression where soils had higher levels of glyphosate residue?
“We report how a single gene mutation from a functional plant mutant influences the surrounding community of soil organisms, showing that genes are not only important for intrinsic plant physiology but also for the interactions with the surrounding community of organisms as well.”1
Several growers have been reporting that different weed species are dominant the following growing season on soil where GM crops are planted, compared to soil with that same crop that is not GM.
Other growers have observed that disease expression is much higher following a GM crop than a non GM crop.
We understand that both of these changes can be produced by shifts in the soil biome. As biological populations shift, different weed species become dominant, and diseases can be either suppressed or enhanced.
A single gene change in a plant can produce a changed phenolic compound and sugar profile in plant sap and root exudates. This change in root exudates results in a changed microbiome in the rhizosphere.
A single gene change in a plant can produce a changed microbiome in the soil. What might this mean for breeding and GE?
Of course, few GE crops being used today only contain a single altered gene. Usually a number of genes are added or altered, along with a number of non-target changes that are also produced.
How might the soil microbiome differ between GE and non GE crops, and how does this influence the development of the soil microbiome?
How can we consider this characteristic to intentionally breed crops and select cover crops which shift the microbiome in a disease suppressive direction?
Is it possible that when we select for disease resistant crops the disease resistance mechanisms are largely or partially a result of a changed microbiome?
Can we deliberately breed crops to produce a disease suppressive microbiome, as occurred accidentally with the selection of crown-rust resistant oats?
– The microflora of most soils is starved. As a result,there is a fierce battle in the rhizosphere between the microorganisms that compete for plant-derived nutrients.
– Most soil-borne pathogens need to grow saprophytically in the rhizosphere to reach their host.
– The success of a pathogen is influenced by the microbial community of the soil in which the infection takes place.
– Every natural soil has the ability to suppress a pathogen to a certain extent. This phenomenon is known as general disease suppression and is attributed to the total microbial activity.
– Organic amendments can stimulate the activity of microbial populations in a conducive soil, resulting in enhanced general disease suppressiveness.
– ‘Specific suppression’ occurs when specific microorganisms cause soils to be suppressive to a disease. Specific disease suppressiveness is superimposed on the general disease suppressiveness of soils and is more effective.
– some soils retain their disease suppressiveness for prolonged periods and persist even when soils are left bare, whereas other soils develop suppressiveness only after monoculture of a crop for several years.
– Induction of suppressiveness by itself is remarkable, because for most plant species, successive monocultures will lead to a build-up of specialized plant pathogens .
– Nonetheless, development of disease suppressiveness in soils has been reported for various diseases, including potato scab, Fusarium wilt, Rhizoctonia damping-off , and take-all.
– Microorganisms that can confer suppressiveness to otherwise conducive soils have been isolated from many suppressive soils.
– Mechanisms through which rhizosphere microorganisms can affect a soil-borne pathogen have been identified and include production of antibiotic compounds, consumption of pathogen stimulatory compounds, competition for (micro)nutrients and production of lytic enzymes.
– Many beneficial soil-borne microorganisms have been found to boost the defensive capacity in above- ground parts of the plant. This induced systemic resistance (ISR) is a state in which the immune system of the plant is primed for accelerated activation of defense.
– Although locally plant immunity is suppressed, an immune signaling cascade is initiated systemically that confers resistance against a broad spectrum of pathogens and even insects
– In addition to plant growth-promoting rhizobacteria, beneficial fungi such as mycorrhizal fungi, Trichoderma spp. and other fungal biocontrol agents have also been found to induce ISR.
– As well as inducing systemic resistance, mycorrhizal fungi can also form a connecting network between plants that can convey a resistance-inducing signal to neighboring plants
– The microflora of most soils is carbon starved. Because plants secrete up to 40% of their photosynthates into the rhizosphere, the microbial population densities in the rhizosphere are much higher than in the surrounding bulk soil.
– From the reservoir of microbial diversity that the bulk soil comprises, plant roots select for specific microorganisms to prosper in the rhizosphere.
– Some plant species can create similar communities in different soils. Even within species, different genotypes can develop distinct microbial communities in the rhizosphere, suggesting that plants are able to shape the composition of the microbiome in their rhizosphere.
– Plants can determine the composition of the root microbiome by active secretion of compounds that specifically stimulate or repress members of the microbial community
– Furthermore, plant-associated bacteria produce and utilize diffusible N-acyl-homoserine lactones (AHLs) to signal to each other and to regulate their gene expression. Such cell-to-cell communication is known as ‘quorum sensing’
– QS-interfering compounds enable the plant to manipulate gene expression in their bacterial communities
– Recent evidence suggests that differences between plant genotypes in a single gene can have a significant impact on the rhizosphere microbiome. The production of a single exogenous glucosinolate significantly altered the microbial community on the roots of transgenic Arabidopsis.
– These results indicate that the plant genotype can affect the accumulation of microorganisms that help the plant to defend itself against pathogen attack. Indeed, differences have been found in the ability of wheat cultivars to accumulate naturally occurring DAPG-producing Pseudomonas spp., resulting in differences in disease suppressiveness.
– Specific wheat cultivars support specific biological control bacteria differentially, which further establishes that there is a degree of specificity in the interactions between plant genotype and the composition of their microbial community
– White fly feeding also led to significant changes in the rhizosphere microbial community. Although total numbers of bacteria were unaffected, the white fly- induced plants had higher populations of Gram-positive bacteria and fungi in their rhizosphere. The authors hypothesized that plants recruit plant-beneficial microbes to their roots in response to the attack.