Rhizosphere – the Unseen World

Sometimes you really have to debate with yourself on whether to buy something or not. I had this ongoing debate with myself when I was trying to decide if I should buy this book in the picture. It was recently released, it is by an author I know, Dr. Gentry, and it is on a subject I enjoy reading about. The problems though are that it is a textbook, it is expensive, and I had to ask, would it be practical or theoretical? I took the plunge, and I am now on my second reading through the over 700 pages!

This book is absolutely written for anybody that works in organic agriculture and it certainly has applications for everybody in agriculture. The chapters are written by many different authors and explain real world stuff. I am by no means a microbiologist, but I still enjoy understanding plants and soils and how they interact with the microbiome that generally inhabit both.

There are many wonderful chapters but the one I want to talk about is written by Dr. Linsey Slaughter, Department of Plant and Soil Science, Texas Tech University and is simply entitled, “Rhizosphere.” On page 269 she writes, “The rhizosphere is the zone of soil immediately influenced by the root with altered microbial diversity, increased activity and number of organisms, and complex interactions between soil microorganisms and the root. The significance of the rhizosphere arises from the release of organic material from the root and the subsequent effect of increased microbial activity on nutrient cycling and plant growth. The unique assemblage of microorganisms in the rhizosphere, known as the rhizosphere microbiome, where microbial community composition, abundance, and functional attributes are distinct from the bulk soil microbiome of the surrounding environment, can influence plant growth in beneficial, neutral, variable, or harmful ways.”

This “influence” starts with the seed. As she explains, “the seeds exude organic compounds such as carbohydrates and amino acids during imbibition, or adsorption of water to rehydrate the seed, at the onset of germination and as the seed coat ruptures to form the primary root. Soil microbes immediately begin to colonize and compete for these resources. Other antimicrobial and signaling compounds are also exuded (by the seed) to protect the seed against pathogenic (disease causing) microbes.”

Dr. Slaughter next discusses roots and root formation. “The root tips at the apex of each root type are responsible for directional growth of the root and are the primary location for water and nutrient absorption and rhizodeposition. The success of the plant depends in large part on the ability of the root tips to sense environmental signals and direct root tip growth in response. The massive number of root tips in the soil accounts for a considerable portion of root surface area and is the most significant location for root interactions with rhizosphere microbial communities. Because of the importance of root tips for root development and function, they are also the site where microbes may have the most significant effect on plant root systems and ultimately the development and physiology of the entire plant.

Okay this is the part of the chapter I want to emphasize and is the first time I have seen a picture that actually shows what a plant can do. I hope I am not breaking copyright law or any other law by putting this picture here, but it really shows the influence plants have on the rhizosphere surrounding the plant. Here is Dr. Slaughter’s comments, “The rhizosphere is a spatially and temporally heterogeneous zone created by active roots. Physical effects of root growth increase microscale compaction and impact aggregation as soil is pushed aside by the root or enmeshed by root hairs. Nutrient absorption, respiration, and exudation by plant roots create zones of nutrient depletion or enrichment as well and changes in pH and aeration.” 

Impact of plant species on rhizosphere pH. Note lower rhizosphere pH for chickpea (left & right) versus maize (corn) (center). Chickpea is well adapted to growth in nutrient-poor soils because of its ability to acidify the rhizosphere through root exudation. Bar = 10 mm.

From Marschner and Römheld (1983).

Page 275, “Principles and Applications of Soil Microbiology.” Elsevier Publishing

As you can see, the roots are changing the pH of the soil surrounding the two different plants. The chickpea is lowering the soil pH which allows it and its microbiome to uptake nutrients it needs while the corn is raising the pH to a 7.5. These changes to the rhizosphere allow the plant to absorb the nutrients it needs.

There is so much more to talk about from this book. Chapters on soil fungi, bacteria, and protozoa. Soil properties and their effect on microbes, soil fauna or insects, composts, etc., etc. I do recommend the book if you want to go deeper into what you do in organic agriculture – it is fascinating to understand and it is our future!

Author: Bob Whitney, Regents Fellow & Extension Organic Specialist

Agriculturalist, extension educator and researcher, organic agriculture enthusiast and promoter, international program developer, Christian, husband, father and friend.

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