Mycorrhizal fungi develop mutually beneficial symbiotic relationships with plant roots. Photo by Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org.
If you have taken an elementary school science class, you have probably learned the basics of photosynthesis. In case you are a bit rusty, photosynthesis is the process by which plants capture sunlight to manufacture their food. They absorb carbon dioxide from the atmosphere and water from the soil. With these ingredients, they create carbohydrates, or sugars, that supply the energy to grow and develop.
As you can imagine, this energy is vital to the health of the plant. But fascinatingly, plants expel between 20 and 40 percent of these sugars from their roots into the surrounding area around the roots. The sugars that the plant releases, along with amino acids, organic acids, enzymes, and other substances, are called root exudates. The area just inside the root where the sugars are released, and the area just outside the root where the sugars end up, is called the rhizosphere.
But why would a plant waste this energy? This is because they derive benefits from the unique microbial population that inhabits the rhizosphere. Plant roots are limited by the amount of nutrients they can take up in the soil. By feeding microorganisms their sugars, they are essentially recruiting workers to help them scavenge for nutrients in areas that they cannot access on their own.
A microscopic image of mycorrhizal fungi in black walnut. Photo by Robert L. Anderson, USDA Forest Service, Bugwood.org.
Bacterial colonies, which are types of microorganisms, establish themselves within the rhizosphere and feed on the root exudates, allowing the bacteria to multiply. Along with the sugars they take in from the root exudates, they also take in nutrients from the soil. The waste that is produced by the bacteria is rich in bioavailable plant nutrients, which the plant then uses, creating a plant-microbe symbiotic relationship where everyone wins.
Another type of specialized microorganism, mycorrhizal fungi, also develops a symbiotic relationship with plants. Its meaning is within its name, as “myco” literally means fungus and “rhiza” literally means root. There are an estimated 50,000 fungal species that form these beneficial relationships with approximately 95 percent of plant families.
The mycorrhizal fungal hyphae, which are tiny fungal filaments one cell thick, do not have chlorophyll and are therefore not able to photosynthesize. Instead, the fungal networks have a large surface area that allows them to be particularly good at extracting nutrients from the soil. This enables them to access nutrients that plant roots would not be able to access on their own. The fungi drill into the plant root and trade these nutrients, along with water, with the plant in exchange for the sugary root exudates. In this way, both the fungi and the plant benefit from the relationship. Interestingly, these types of relationships will only develop once the plant releases particular root exudates that attract the microorganisms they are seeking. In essence, the mycorrhizal fungal hyphae will not associate with the root until they are invited.
Along with root exudates, root hairs and other plant cells accumulate within the rhizosphere as they grow and die throughout the plant’s life cycle. The combination of the root exudates, dead root hairs, and dead plant cells creates essentially a compost pile within the rhizosphere. This combination of substances establishes an environment where beneficial microorganisms can thrive, and a plant can maximize its nutrient uptake capacity.
Amazingly, there can be up to a billion bacteria and several yards of fungal hyphae living in just one teaspoon of soil! Of course, not all microorganisms are beneficial to the plant. But remarkably, plants have developed many ways in which they benefit, and ultimately thrive, in this diverse soil ecosystem.
As humans, we continue to learn more and more about these complicated and interesting interactions taking place in the soil beneath us. And the more we learn, the more we discover just how important these diverse ecosystems are to the health of the food web, and therefore, to the health of our planet as a whole.
For all my years in the classroom, I never let students say the “d-word” when discussing soil science. In some instances, we had a “d-word” swear collection jar of a quarter when you used the term and even today, I hesitate from spelling the word out in text due to feedback from all those I have corrected. In case you still need a clue on the “d-word”, it ends in irt.
As a horticulturist for 46 years, I have read, heard, and been told many secrets to growing good plants. I still hold firm that without proper knowledge of how soil works, most of what we do is by chance. Soil is a living entity comprised of parent material (sand, silt, and clay), air, water, organic matter (OM), and microorganisms. It is this last item which makes our soils come to life. If you have pets, then you know they need shelter, warmth, air, water, and food. From this point forward think of soil microorganisms as the pets in your soil. If you take care of them, they will take care of your plants.
Sandy soil without any organic matter at the Wakulla County Extension office.
There is a huge difference in habitat from a sandy soil to a healthy soil with a good percentage of OM (5% – 10%). In one gram of healthy soil (the weight of one standard paper clip), you can have bacteria (100,000,000 to 1,000,000,000), actinomycetes (10,000,000 to 100,000,000), fungi (100,000 to 1,000,000), protozoa (10,000 to 100,000), algae (10,000 to 100,000), and nematodes (10 to 100) (1). A teaspoon of healthy soil can contain over four billion organisms (2). These microorganisms are part of the soil food web and they form a relationship between soil and your plants. They help convert nutrients to useable forms and assist with other plant functions.
The question becomes how to take care of your soil pets. For years we have performed practices that compromise these populations. Growing up we put all of our grass clippings in the weekly trash. We know now how valuable those clippings are and to leave them be. Two practices still common today though are tilling and raking leaves.
Master Gardener Volunteer vegetable bed with organic matter added.
Tilling has a limited purpose. If I place a layer of organic matter on top of the ground, then tilling incorporates the OM which feeds my pets. Excess tilling of soil introduces large amounts of oxygen which accelerates the breakdown of OM thus reducing our pet populations over time. Another adverse result from tilling is disturbing the soil structure (how the parent materials are arranged) which can reduce pore spaces thus limiting water percolation and root growth. There is a reason agriculture has adapted no-till practices.
Raking leaves (supposedly the sign of a well-kept yard) is removing large amounts of OM. Do you ever wonder why trees in a forest thrive? All of their leaves fall to the ground and are recycled by the microorganisms. Each of those leaves contains macronutrients (carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, sulfur, and magnesium) and micronutrients (boron, copper, chlorine, iron, manganese, molybdenum, nickel, and zinc) which are necessary for plant growth. You would be hard pressed to find all those nutrients in one fertilizer bag. So recycle (compost) your leaves versus having them removed from the property.
We are in our off season and tasks such as improving soil health should be considered now for soils to be ready in spring. Remember a little organic matter at a time and never work wet soils. As your OM levels build over the years, remember to change your watering and fertilizing schedules as the soil will be better adapted at holding water and nutrients. Soil tests are still recommended before fertilizing.
If you would like more tips on improving your soil, contact me or your local county horticulture extension agents. For a more in depth look at caring for your soils, read The Importance of Soil Health in Residential Landscapes by Sally Scalera MS, Dr. A.J. Reisinger and Dr. Mark Lusk (https://edis.ifas.ufl.edu/ss664).
Chapter 2: Soils, Water, and Plant Nutrients. Texas Master Gardener Training Manual.
The Importance of Soil Health in Residential Landscapes. 2019.
Daniel Leonard, Horticulture Agent at UF/IFAS Extension Calhoun County, answers commonly asked questions about raised bed gardening. In the video he discusses construction materials, the type of soil to use, fertilization, crop rotation, cover crops, and smaller container gardens.
As a compost fanatic, one of my favorite Florida-Friendly Landscaping principles is Recycle Yard Waste – Principle #7. Since the practice of gardening typically leads to various debris, including herbaceous weeds, lawn clippings, woody stems after pruning, and just the leaves that fall from your trees, gardeners must somehow manage all of this material.
Rather than tossing it to the curb, where it goes to the landfill and very slowly breaks down in anoxic conditions (lacking oxygen) leading to increased methane emission, a more sustainable approach is to utilize these materials on site. On-site recycling of garden debris can be done through, my favorite, composting, especially when dealing with herbaceous material.
Grass clippings are best left on the grass, as long as not in excessive clumps. Woody debris can be piled in not-too-conspicuous areas to create brushpiles that will provide wildlife with cover. While many think of snakes and other creepy crawlies in piles of brush, many birds, lizards, mammals, insects, and fungi will call a brushpile home. Fallen leaves can be raked into garden beds to act as mulch, another Florida-Friendly Landscaping principle! Besides not using the energy required and creating the extra methane (a stronger greenhouse gas than carbon dioxide) produced by sending materials to the landfill, recycling materials on-site returns the nutrients in that debris to your soil for future plants to take up.
However, there is a fate of yard debris that is even worse than sending to the landfill (cue scary music) – illegally dumping the materials in a natural area!!
Landscape debris dumped in a natural area can spread invasive plants, insects, and disease. Credit: Mark Tancig.
You may have seen this in a local natural area you visit or heard of folks that try to skip paying solid waste fees by dumping it elsewhere. Unscrupulous landscape companies and homeowners have been known to do this, especially when the work site is near a large natural area, such as a state or national forest. The main problem with this, besides that it is illegal, is that invasive species can be spread with debris directly into a native forest. Invasive plants are documented to be spread in this manner, especially those that are easily rooted from stems. Additionally, illegal dumping of plant material can spread non-native fungi, insects, and animals into these areas and disrupt natural ecosystem functions and the organisms that rely on them.
Not only is illegal dumping bad for our natural areas, but it’s illegal! Credit: Mark Tancig.
Homeowners can make sure they don’t contribute to this problem by recycling as much on-site or properly following local guidelines to dispose of landscape debris. Many municipalities have separate bulk collections or drop-off sites for landscape materials. If hiring a landscape contractor, ask where they take the collected material and/or request that it stay on the property. For large jobs, such as tree removal, the estimate should reflect dump fees. If you notice materials dumped in a local natural area, report it to the land manager so they can investigate and/or clean it up and dispose of properly.
Figure: Increase water holding capacity through home composting. Credit: Tyler Jones, UF/IFAS Communications.
Water is a precious resource for Floridians, even if the last couple of months of rainfall may make you think otherwise. As home gardeners, we should keep water conservation in mind.
Wasting water now may cause restrictions in the future, as basic water needs of a growing population outpace capacity. Of course, we all hope that’ll never happen, but it is possible.
As we start planning for our fall vegetable garden, let’s think about conserving water usage. We can start by putting our “plant biology” cap on. A great way to save water is to plant fast growing, early-maturing vegetables. The strategy, of course, is the sooner a plant matures the less water it will need.
Gardening periods in Florida vary, too. Thus, there are broad choices of planting dates for many vegetables. For Panhandle gardeners, the current trend has shown dry periods in the spring, but adequate rainfall in summer months. Usually with the seasonal change to fall, soil moisture holding capacity is not a great struggle in Panhandle. However, much of our soils consist of coarse sandy particles, which are not ideal for water holding capacity. Amending garden soils with organic materials such as compost, manures, and cover crops, will help the soil hold water better.
Selecting the right irrigation method is also a great way to conserve water. Overhead sprinkling is not ideal for most gardening applications. This method wastefully projects water into areas between rows, outside of root zones and allows for much evaporation loss. Drip irrigation can help solve these issues, by concentrating water directly to the root system. IFAS research has estimated an 80% reduction in water usage when utilizing a drip system.
If drip irrigation isn’t a method you’re interested in, overhead watering the garden thoroughly twice weekly is an acceptable alternative. Remember, there’s a limit to how much water plants can use. Excess water can cause runoff and consistent heavily saturated soils may promote root rot. Mulching also ranks highly among water conservation practices, by allowing the soil to hold more moisture. Examples of mulch types include hay, straw, leaves or plastic.
A common question for gardeners at the end of the season is if one should till the soil or use no till practices. Opinions vary regarding this question, even among Extension Agents. However old crops harbor insects, both good and bad. This phenomenon was noticed on some recently cut back tomato plants. The intention was to cut the leftover spring garden tomatoes back to encourage fall production. Instead, a host plant for mealybugs was provided.
Mealybugs on a tomato plant. Photo Credit: Matt Lollar, University of Florida/IFAS Extension – Santa Rosa County
Mealybugs are soft-bodied insects that possess a covering of flocculent, white, waxy filaments. They are about 1/8 inch in length and usually pinkish or yellowish in color. Mealybugs have piercing-sucking mouthparts which they use to siphon fluids from the leaves, stems, and sometimes roots of many ornamental and vegetable plants. Mealybug damage produces discolored, wilted, and deformed leaves.
One very common example of an insect pest likely to claim residence in your garden’s crop residue, are squash bugs. They like to overwinter on squash, cucumber, and other cucurbit crop residue. If you choose to not till your garden and leave a portion of last seasons crop in your garden, then you should consider applying an insecticide to your spent crop at the end of the season. A product containing a pyrethrin or pyrethroid as an active ingredient would be a good broad spectrum insecticide to control any pest that may reside on plant residue. More information on pyrethrins and pyrehtroids can be found at the EPA webpage: Pyrethrins and Pyrethroids. If you choose to apply an insecticide, it is important that you follow the information on the label regarding pollinator protection. Another option is to plant a trap crop on the edge of your garden to help attract pest insects away from your desired crops. More information on trap crops can be found in the EDIS Publication: Intercropping, Pest Management and Crop Diversity.
An adult squash bug on a zucchini leaf. Photo Credit: Matt Lollar, University of Florida/IFAS Extension – Santa Rosa County
So the answer to the till or no till question is…it depends. It is really up to the gardener. Yes, the residue from crops will add nutrients and organic matter to your soil, but it could also increase pest pressure in your garden. If you don’t plan to remove crop residue and don’t plan to till, then keep an eye out for what could be hiding in your garden.