The Invisible World of Roots

The Invisible World of Roots

By Robert “Skip” Richter

Brazos County Horticulturalist

Texas A&M AgriLife Extension Service


A plant’s roots are too often “out of sight, out of mind” for gardeners. We see the above-ground stems, leaves, flowers and fruit, but are often unaware of what is going on below the soil surface. As a result, we tend to undervalue the critical roles roots play and miss the direct link between root problems and many of the plant problems we see above ground.

Roots anchor a plant, take up water and nutrients, and store nutrients and carbohydrates — as is evident with root crops in the vegetable garden and perennials that return each spring from the reserves stored in their roots. However, these simple, essential tasks don’t begin to describe the complex interworking of roots and the soil life and chemical processes that result in healthy, productive plants.

Photosynthesis in the leaves is an essential process that drives the growth and health of every part of the plant. The processes occurring underground in and around the roots are equally important. When diagnosing plant problems, I consider what is going on below ground as much or more than what is going on above ground.

Where are a plant’s roots?

Have you seen diagrams of a tree where the roots are basically a mirror image of the above-ground parts of the tree? That is not how tree roots grow. Over 90% of a tree’s roots are in the top 18 inches of soil, with the majority of that mass being in the top foot, depending on the soil type and conditions. Rather than the “mirror image” concept, think of a pancake with a stalk of broccoli in the center and you’ll get a pretty accurate model of a tree’s top compared to its root system! Therefore, “deep root feeding” (which involves pushing a probe 12–18 inches into the soil) is not an efficient way to fertilize trees.

Smaller-statured plants also have much of their root systems near the surface. This is where the oxygen levels in the soil are adequate to support optimum root growth and microbial populations. With all plants, the species, type of soil and soil moisture levels influence root depth.

Roots of woody plants live many years, with old growth branching out to form new growth as the root system gets larger and more extensive. The fibrous roots of grasses generally only live for about a year. New roots originate from the base of the grass clump or from the nodes along the runners (stolons) and underground rhizomes as old ones die off.

How do roots grow?

Have you ever wondered how a root is able to push through the soil? At the tip of a root is the root cap. Think of it as a football helmet protecting the tender new cells forming behind it from being damaged as the root pushes through soil. Cells in the root cap can sense gravity and light to direct their growth. They also secrete a slimy substance that helps roots push through the soil, essentially greasing the way for the growing root.

Behind the cap is a region called the zone of division. In this area, cells are dividing to form new root-cap cells to replace those being worn away. And new cells also form behind the new root-cap cells.

Next, after this zone of division, is the zone of elongation, where cells expand like balloons being inflated. This expansion pushes the root tip and cap forward through the soil with a force of up to 100 pounds per square inch. Good thing there is a protective root cap!

Only the root cap and zone of cell division move through the soil. Once cells start to elongate and mature, that part of the root will be stationary as long as it lives. All of this division and elongation happens in the first centimeter at the tip of the root.

Behind the zone of elongation, the roots are forming root hairs. These tiny projections are each created from a single cell and reach out into the soil. This dramatically increases the root’s ability to take up water and nutrients. One millimeter of root in this zone can contain 100 or more root hairs. The presence of root hairs increases a root’s absorptive surfaces by several hundred to a thousand or more times.

Most water and nutrient absorption are done in the zone where root hairs are present. Iron, for example, is primarily taken up in this zone. If something such as lack of oxygen from overwatering causes roots to stop growing or die, the plant’s ability to take up water and nutrients can be severely reduced.

It is amazing that all these areas of cell division, elongation and maturation (root-hair zone) are found near the last inch or so back from the tip of the root. Farther back, the root serves primarily as a pipeline for moving water and nutrients up into the rest of the plant.

How do roots take in nutrients?

Many plant nutrients are found in the film of water covering the soil particles that make up the soil. I like to call this the “soil soup.” This “soup” enters the roots either by diffusing in from an area of higher nutrient concentration outside the roots toward the area of lower concentration inside the root or by an active process in which the plant expends energy to move the nutrient across its outer membranes into the root. This active process is able to move nutrients into the root even if the concentration inside the plant is higher than the concentration outside the root.

Now this is where it gets really cool! Many of the nutrients a plant needs, including potassium, magnesium and iron for example, exist as positively charged ions attached to negatively charged soil particles and organic matter. As the plant’s roots take up the positively charged nutrients, they release positively charged hydrogen ions into the soil. These hydrogen ions acidify the area around them, helping to release more nutrients into the “soil soup,” where they can then be taken up by the roots.

Over 90% of the nitrogen and sulfur, and up to 50% of the phosphorus in the soil, are found in organic forms. However, plants take up nutrients typically in their ionic forms. Therefore, plants are dependent upon microbes to take the organic form and make it available to plants. I’ll say more about microbes later.

So, whether you fertilize with synthetic or organic fertilizers, the form the plant ends up taking in is primarily the same for both. When soil conditions are not suitable for a proliferation of microbial activity (such as cold soil temperature), nutrient release slows accordingly.

What do roots want?

In order to live, grow and function properly, roots need moisture. Too little and they can die; too much and the saturated soil will exclude oxygen that they need, resulting in death. Keep soil moderately moist to maintain root growth and optimal water- and nutrient-uptake.

Plants need 17 essential nutrients for survival and production: nitrogen, phosphorous, potassium, calcium, magnesium, sulfur, boron, chlorine, cobalt, copper, iron, manganese, molybdenum, silicon, sodium, vanadium and zinc. Plants need these nutrients to be in some sort of balance to prevent adverse interactions and they likewise need the soil pH to be in the range preferred by the particular plant species so that the nutrients are available.

Roots also need soil microbes. While plants can grow in sterile soil if provided the optimum balance and amounts of nutrients and moisture, nature is designed to provide everything roots need through a microbial partnership in the soil.

When we add compost, we achieve three outcomes. We improve soil structure, add a source of nutrients and provide fuel for microbial activity. With compost, we mimic nature’s provision of dead grass roots, fallen leaves and other organic materials that naturally improve the soil.

Roots live in an intimate relationship with soil microbes. Plants do way more than add organic matter to “fuel” microbial growth. Plants “leak” much of the carbohydrates they produce into the soil around their roots, thereby fueling a microbial population explosion that is 100 times greater than that found in the surrounding soil. Why? Some bacteria provide a protective coating around roots. Others produce compounds that fight diseases. Still others release organic acids that dissolve minerals in soil particles, releasing them into the “soil soup” for plants to take up.

There are bacteria in legume (bean and pea) roots that can take nitrogen from the air (in the soil) and convert it to a form roots can use. Root hairs on these plants literally curl around the bacteria and incorporate them into the roots. The roots share their secretions in exchange for the bacteria providing nitrogen to the plant. Think of it as hostage-taking by benevolent captors! Research has also discovered certain bacteria that provide biochemical signals to the root that result in enhancement of the plant’s natural defenses against pests.

Fungi serve numerous functions, especially in releasing nutrients from organic materials that most other microbes cannot break down. Mycorrhizal fungi live in close association with plant roots either around or inside the root. Some reach out from the root, bringing nutrients such as phosphorus back to the plant from distances beyond the reach of the root. Others form a protective shield around the roots, fighting off disease organisms.

Researchers have found that mycorrhizal strands connect underground to link roots from two separate plants, forming a sort of communication system. We know that plants respond to pests by releasing substances to make their leaves more resistant to attack. Recent studies have found that when a plant is attacked by a pest, an adjacent plant that was not attacked can begin developing the same resistance response, because of the underground communication system via mycorrhizal strands.

The complex interactions between roots and microbes in healthy soil are beneficial to both plants and microorganisms. There is definitely a reason plants leak much of the carbohydrates they work hard to produce back into the soil.

Common root problems and solutions for gardeners

Soil compaction, excessive dryness, sogginess, overly wet soil — any of these conditions for extended periods can account for nutrient imbalances and many problems the roots of our plants encounter.

We can reduce the impact of many of these issues by avoiding walking on planting beds in the vegetable, herb and flower gardens, by watering properly, by using raised beds when drainage is less than ideal, and by choosing plants that are suited to the site whether it tends to be soggy or drought-prone.

When beginning a new garden, always start with a soil test and fertilize accordingly. Prior to planting, we can do a lot to make roots feel at home by adding compost to help get the nutrients and soil organisms properly balanced.

Fertilizing without a soil test is flying blind and may well result in you doing more harm than good. There are fertilizers sold for specific plants such as “citrus food,” “vegetable food,” “palm-tree food,” and so on. I contend that there is no such thing as any of these. The best fertilizer for any plant depends on what is, or is not, already in the soil.

In addition to the above issues, root diseases, nematodes and root-feeding insects can become a problem for your plants. Good drainage helps avoid some, but not all, root diseases. Healthy plants are often able to fend off problems, but there are times when a fungicidal drench may be needed to save a very high-value plant.

There isn’t, as of yet, a good control for nematodes in the home garden and landscape. We have ways to suppress them but not eradicate them. Resistant plant species and/or varieties may be the best solution in nematode-infested soil.

Root-feeding insects can occasionally be a problem in turf and with some vegetable crops, especially root veggies. There are insecticides for managing these, but I find that most cases do not warrant treatment.

Also, be cautious when applying broadleaf weed killers. When applied to a lawn, many broadleaf weed killers can damage tree and shrub roots, especially if overapplied or when applied prior to a rainstorm that washes them down into the soil where tree roots are feeding.

You cannot have healthy plants without healthy roots, and healthy roots grow in healthy soil. So, the next time you get ready to plant a garden, spend a dollar on your soil before you spend a dollar on plants. Your plant’s roots will appreciate the extra effort and they will reward you with plants that are so big and beautifully healthy that people will think both of your thumbs are green!