How do different root structures affect soil?

Plant roots modify soil in different ways – depending on the root’s architecture.

Most of us think of the soil as the natural habitat for plants, and therefore soils must provide a nurturing and supportive environment for them, right? Well…most of us could not be more wrong about that.

The soil is not a very welcoming environment for plant growth. It does not provide everything a plant needs freely and without reservation. In fact, left as is, the soil probably would not produce very many plants at all. Proof of this is found in the tremendous amount of soil modification plants engage in just to improve their chances of survival. 

Plants modify soil. That is a fact. They spend a lot of energy doing it, and they do it to their own advantage. Organisms (which, of course, include plants) are even one of the five soil formation factors, along with climate, relief/topography, parent material, and time. Plants modify the soil chemically, biologically, and physically in very substantive ways. This blog entry focuses mostly on the physical side of things by considering how root structures affect the soil.

mustard root system and ruler with first and second order laterals and seminal root labeled
Image 1: A mustard root with single taproot system and root orders shown. Credit: Jake Mowrer

Root structure is called root architecture. The term can include physical arrangement of roots, number, thickness, length, depth, angles of branching, and distribution of root orders. Different plants have different root architectures. General architecture can tell you a lot about a plant species’ survival strategy, and changes in that architecture can tell you a lot about the specific stresses that plant is exposed to.

There are many reasons why scientists ‘go down the rabbit hole’ with roots. They are the unseen half of the plants, and so represent a great mystery even in today’s technologically advanced world. We don’t know all the functions they perform and what benefits we may gain from understanding them better. We do know that roots are the primary means for resource acquisition by plants, that they release many complex chemical compounds into the soil that affect carbon storage and influence other soil organisms, and that they leave the soil changed after the interaction. 

And they look cool.

An easy place to get started with understanding root architecture is the concept of root order. In image 1, we have a young mustard plant excavated from field, its roots washed free of soil. The largest central root is the primary or seminal root. A root that branches off the seminal root is classified as a first order lateral. A root the branches from a first order lateral is classified as a second order lateral. This may continue, but usually not too far, because small roots tend to begin producing the very fine root hairs that maximize surface area to volume for the uptake of water and nutrients.

corn root system and ruler with brace roots and seminal roots labeled
Image 2: Corn root system with fibrous root system featuring brace roots and multiple seminal roots. Credit: Jake Mowrer

Another basic idea in root architecture is that there are two basic types of root systems, the tap root system, and the fibrous root system.  The mustard plant in Image 1, above, features a tap root system with a dominant and easily recognizable downward growing single root from which branch all the other roots.  In a fibrous root system (Image 2), such as that of the corn plant, there are many more seminal roots and often no clear single primary root.  You can see that the roots develop from the stem, rather from a downward growing root.  Taproots represent a survival strategy that relies on access to water nutrients held deeper in the soil.  Fibrous roots represent a strategy that emphasizes exploration of soil closer to the surface and increased overall contact with soil particles to acquire nutrients with low mobility.

The portion of the soil most explored, the depth, and the lateral reach of a plant’s root system all affect how different plants physically modify soil in different ways.  The distribution of the architecture results in different distributions of physical disturbances in the soil, such as macro channels, voids, and aggregate formation from decomposing tissue and root exudates.  All these affect water infiltration and retention properties of soil.  Furthermore, the tissue of roots contains much more lignin than above ground plant tissues.  Lignin takes longer to decompose than other plant residue components.  So, roots can leave longer lasting carbon in different spatial patterns that result in different patterns of channels and voids in the soil.

fava bean plant roots and ruler with nodules labeled
Image 3: Nodules formed on the roots of a Fava Bean plant. These structures are necessary to protect the anaerobic activity of nitrogen fixing bacteria, which form a mutualistic symbiosis with the plant. Credit: Jake Mowrer

Of course, there are modifications and specializations within these two major systems that contribute to greater variety in architecture.  Potatoes and carrots, for instance, are structures of taproot systems designed to store food for the plant.  Their formation, and even removal, leaves larger single voids in the soil. Onions have a fibrous root system with a group of subsurface leaves that form the onion itself on top.  And legumes create structures called nodules that house nitrogen fixing bacteria (Image 3). 

A soil with good tilth* allows for the full realization of a plant’s root architecture.  In turn, a plant’s root architecture contributes to and maintains proper tilth.  Hopefully, you can see how different architectures will result in different physical modifications of the soil.  Please further explore the myriad shapes, functions, and beautiful structures that are plant root systems.

Answered by Jake Mowrer, Texas A&M AgriLife Extension

Read here about how root depth affects soil.

*Tilth: a generalization of soil properties that promote plant growth, especially the below ground portion of the plant (i.e. roots).  A soil with proper tilth will be friable, will feature well-developed aggregates and macroporosity, and above all will allow for the proliferation of roots.

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