By Tamara Scully, NODPA News Contributing Writer
Organic dairy farmers are, by definition, grazers. These days, more grazers are categorizing themselves not only as grass farmers, but as soil farmers. It’s that soil - the nutrients, the plant roots, the microbes, the worms, the organic matter - which ultimately grows those healthy forages which, in turn, support healthy cows and quality milk production.
So just how do you farm cows, grass and soil, and pull it all together to create the balance that optimizes the properties of living soils, captures them in the most nutritious forages, and makes those nutrients available to grazing livestock?
Joel Williams, an independent soil health educator, spoke recently at the Western Canada Conference on Soil Health and Grazing. Williams discussed the interaction between soil and plants, and exactly how this intricate system between the two functions. In a healthy system, photosynthesis is maximized, soil microbial populations are enhanced, and carbon sequestration increases.
“It’s all about feeding biology. It’s plants that do that. Plant nutrition really is an important piece of this puzzle,” he said.
Plant Nutrition
Photosynthesis is the crucial plant process which involves the removal of carbon dioxide and water from the air to form glucose for plant growth, and the by-product of oxygen for us to breathe.
“It really is this process of photosynthesis which drives all of plant growth. It’s how they build their bodies, grow their shoots and their roots, and how they produce a whole range of other biochemicals and pigments and things inside their bodies as well,” Williams said. “The role of plant nutrition - of macro and micro minerals - is absolutely key in this discussion.”
Plant essential elements and minerals are the catalysts which drive photosynthesis. If these nutrients aren’t available in the correct amounts, or accessible to the plants, photosynthesis - and therefore plant growth - will suffer.
These same elements are also crucial in taking the glucose - formed via photosynthesis - and processing it into an array of other products, including: complex sugars and carbohydrates; proteins and amino acids; fats, lipids and oils; hormones and vitamins; essential oils and other aromatic compounds; phyto-nutrients; plant defense chemical; and root exudates.
“Without these essential minerals...then ultimately plant growth, biomass production, yield...all of these things will be limiting,” Williams said.
Nutrient Roles
Here’s an overview on the role some essential nutrients play in growing healthy plants:
Nitrogen is needed for creating essential plant cell components, including DNA and chlorophyll, and also for protein metabolism. If it is not provided in balanced amounts with other elements, plant growth will suffer.
Magnesium is essential in chlorophyll production, although only 20 percent of the available magnesium is used in this process. Seventy-five percent of a plant’s magnesium is used to catalyze protein synthesis. Magnesium is also critical in maintaining the plant’s ability to utilize nitrogen. Without magnesium (and manganese), nitrogen can not be optimized and the plant will have unbalanced growth, with excessive foliage and little root development.
Manganese has a primary role in germination, as well as in the plant’s primary and secondary disease defense systems, and is required for photosynthesis. The primary disease defense system consists of physical barriers, while the secondary is biochemical in nature.
Iron is essential for the nitrate/nitrite pathway of protein synthesis, as well as in the synthesis of chlorophyll.
Zinc has a minor role in chlorophyll production, and a larger role in determining the size of leaves, which are the “solar panel of the plant,” Williams said. More leaf surface area equates with a great capacity to photosynthesize and produce glucose, maximizing plant growth.
Boron is required for cell walls and other structural components in the plant. It also plays a role in the reproductive capacity by influencing flowering, pollen production and pollen tube formation. Along with calcium and silicon, boron provides the plant with defense against diseases.
Calcium works by “improving the uptake of other minerals into the cell walls,” which strengthens them, Williams said. It is essential to cell division, and responsible for root and shoot growth.
Silicon, although technically not classified as an essential element, is the “master stress mineral” which helps the plant overcome environmental stressors and abiotic attacks,” he said. It protects the plant from drought stress and heavy metal toxicity. It also plays a role in the immune system by influencing the signaling molecules, which cause systemic immune responses to fight off disease.
Potassium plays a role in protein synthesis, translocates sugars into the seeds to grow the fruit, and helps develop fruit flavor.
Sulfur plays an important role in root development, as well as amino acid and protein synthesis. It also has fungicidal effects.
Molybdenum contributes to protein synthesis and pollen formation, and is required for the reducing of nitrate to nitrite. Without molybdenum, nitrogen-fixing bacteria can not access the nitrogen in the air.
Copper has a defense and anti-microbial role, protecting the plant from disease by building primary resistance via the lignin, as well as through secondary biochemical processes. Copper helps to metabolize proteins and carbohydrates.
Phosphorous is a crucial part of “the energy currency of the plant,” Williams said, It speeds plant maturity so soft tissues aren’t susceptible to disease, and it promotes early root development. It also provides energy for nitrogen fixation.
Soil Connection 
All of these elements are needed for plant growth and functioning. In turn, healthy pasture plants are releasing carbon and other elements into the soil via root exudates. The sugars, carbohydrates, proteins and lipids released by the plant as root exudates feed the soil microbes, who then release nutrients back into the soil, making them available to the plant. Higher-level predators, such as nematodes, protozoa and larger insects and earthworms - all part of the soil food web - are supported via this nutrient exchange, too.
While root exudates are primarily amino acids, carbohydrates and organic acids which feed the soil food web, biochemicals which serve as “communication chemicals” between the plants and the soil microbes provide an important secondary function performed by exudates, Williams said. These communication chemicals include fatty acids, sterols, gulcosinates, enzymes, flavanols, lignins and more.
The bulk of root exudates are found in the root tips, including the lateral root tips, and the surrounding root growth zone. Root exudates can permeate the soil via a passive osmosis process, driven by concentration gradients, or via an active pathway.
In the active pathway, the communication chemicals are purposefully exuded by the plant to communicate its needs to the soil microbes. When circumstances change, so to do the root exudates. For example, during various stages of growth, plants require different nutrients and exude different signaling chemicals to attract the microbes which can provide them with their specific needs.
“It is a very dynamic and complex interaction,” Williams said, with some exudates attracting microbes, while others suppress them. “Different plant species, or plants at different growth stages, release a very specific composite of root exudates. It’s a really specific cocktail of exudates.”
For example, a plant in the reproductive stage requires boron. It will send signals to attract those microbes which can release boron and make it available to the plant. If attacked by a pest, the plant will send signals - the elements of copper, manganese, and silicon - to indicate stress, and attract beneficial microbes. These microbes in turn release needed metabolites which can then help the plant to trigger a strong secondary immune response.
Microbes can also induce changes in a plant’s root exudates. Research trials have shown that when a plant’s roots are divided into two separate segments, and one segment is inoculated with a given microbial population while the other is left without, the roots exposed to the microbes will show a change in root exudate patterns. The control side does not.
Plants grown in microbially active soils have their root exudates rapidly consumed, creating a stronger chemical gradient which increases the passive exchange rate of root exudates from the plant to the soil. Other factors also impact the type and amount of root exudates a plant produces.
The presence of other plant species, the nutrient availability, the amount of photosynthesis occurring and the presence of herbivore activity - such as dairy cows grazing - all impact root exudates. Environmental factors such as temperature, light availability, moisture and soil pH have an impact, too.
“It’s a dynamic process induced by all sorts of environmental cues, and also pest and pathogen cues,” William said.
Capturing Carbon
Plants provide microbes with the carbon they need. Microbes provide plants with nutrients which they scavenge, and which the plants need.
Microbial activity increases when the diversity of root exudates increases. Plants growing in a system which is biodiverse will have more highly diversified root exudates, which in turn attract a greater array of microbes and show enhanced microbial activity.
“It’s the carbon that microbes want. Microbes are carbon-limiting. And generally plants are nutrient-limiting,” Williams said. “It’s not just about the quantity of root exudates. It’s also about the composition of them, or the quality of them
Biodiversity of root exudates helps to fuel more rapid microbial growth. Root exudates are easily digested by soil microbes, and provide the fuel for their growth. The rapid microbial growth that occurs when exudates are plentiful and diverse creates greater amounts of necromass - dead microbes - in the soil. The necromass is filled with carbon, which can become stabilized in the soil. Necromass is stabilized when it is attached to soil mineral surfaces, or bound up in aggregates.
“It’s not just about forming necromass. It’s about the stabilization processes,” he said.
The emphasis has been on sequestering carbon via no-till methods, and by leaving plant litter in the fields. Maintaining a cover of plant litter does contribute to soil organic matter, stop erosion and runoff, and sequester carbon. But the sequestering of carbon by building microbial necromass is actually a more efficient process.
Catabolism, or the breaking down of plant litter into consumable substances, is a very energy-heavy and time-consuming activity for microbes. Lignin, cellulose and other plant materials take time and energy for microbes to break down and assimilate. Anabolism - when the chemical structure of living tissue is synthesized from simple nutritional elements such as sugars, amino acids and fatty acids - provides simple, more available food for microbes.
Increasing the necromass - via increased microbial populations, which grow in response to the quality of root exudates, which in turn are enhanced by plant biodiversity - is a efficient way to build soil organic matter and sequester carbon via anabolism.
Grazing livestock play a role in decreasing the carbon stored in above-ground litter, as they trample it, both stimulating decay and pushing it below-ground, helping to build soil organic carbon and speed catabolism.
“Livestock are a hugely important tool in that nutrient cycling and recycling. I think that the animal and the management of the animal becomes the primary tool of the farmer.”
Pasture Fertility and Forages
Microbial biomass also plays an important role in the nitrogen cycle. Nitrogen fertility obtained from organic sources, such as amino acids and peptides, can be taken up through a plant’s root system, as well as released as root exudates, Williams said. When nitrogen uptake occurs via the roots, it creates below ground biomass and increases a plant’s rooting response.
If provided with inorganic nitrate, the fertility translocates to the leaves of the plant, and does not create below ground biomass. Nitrate is metabolized differently than ammonium, urea or organic sources of nitrogen. The metabolic pathway is longer, and requires much more energy - as well as the addition of other nutrients - to be converted into usable forms of nitrogen. And increased levels of nitrate in the plant cause increased disease and pest susceptibility.
“If we are feeding the plants more of the ammonium, urea and organic fractions of nitrogen, that actually - by default - encourages a greater rooting response. They encourage more below-ground biomass,” Williams said. “That really has some spin-off benefits in the longer term.”
Organic nitrogen provides resiliency to pastures, by building that microbial biomass which allows water and nutrients to more readily be scavenged and provide a source of plant fertility later in the season. Using organic forms of nitrogen enhances plant immunity, creating healthier plants and increasing the protein and amino acid profiles, ultimately benefitting the cows who consume the healthier forages to fuel milk production.
Fertility needs will differ depending on whether the pasture is also used to make hay. Taking hay from a pasture removes more nutrients than does grazing. When intensively grazed, more than 75 percent of a pasture’s phosphorous, nitrogen and potassium are recycled back into the soils due to nutrient recycling. When the same amount of dry matter is removed as hay, however, exponentially greater quantities of nutrients are removed, and pasture growth will suffer until that fertility is returned. These nutrients will not be returned to the soil until they are added back, either in the form of manure or other organic fertilizers.
Further studies on alternative forages in grass-based dairy systems are needed. Adding cool or warm season annuals to perennial pastures can enhance dry matter availability and biodiversity. A variety of perennial grasses in a pasture stand could add resiliency against changing weather conditions.
But it isn’t only about plant species. Selecting the right cultivar for specific situations can be critical. Forage research trials at the University of Vermont’s Borderview Farm are ongoing, with the goal of enhancing pasture forage yields. Legumes are notoriously short-lived, so finding which species and varieties remain established year-after-year, as well as determining which grasses can withstand a variety of weather - from dry and hot to wet and cool- should help grazers make more informed decisions on seed selection and pasture biodiversity.
“There clearly are differences in variety. And we all know this about corn, but it’s the same with grasses and legumes,” University of Vermont Extension Agronomist Heather Darby said. “It will clearly help you get more yields and quality,” if you are mindful in your variety selections.
Functional diversity for an organic dairy means building a healthy biodiverse pasture system which can: balance water; cycle nutrients; provide pest control; help manage risk; decrease inputs; increase soil formation; and sequester carbon; and do so as a part of a productive agricultural system. For organic dairy grazers, the goal lies in capturing the nutrition in the optimized forages - the ones adapted to your environment that can take advantage of your healthy soils - through managed grazing, returning needed fertility to the soils, and continually building soil organic matter.
