Hydroponic enthusiast and scientist Dr. J. Benton Jones Jr. conducted research to determine which factors contribute to optimal essential element absorption, and which factors (and plant qualities) may actually be detrimental to the process. What did he conclude? Read on to discover.

By DR. J . Benton Jones JR.

The Root Whisperer

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68 Maximum Yield USA | August 2011

Roots perform two essential functions—they anchor the plant in the rooting medium and are the means by which water and essential nutrient ele-

ments enter the plant.Some may not fully appreciate the anchoring aspect until

their field or garden plants dislodge or trees are blown over in high winds, illustrating the importance of sufficient root penetration into the rooting medium.Since the means by which water and essential nutrients

enter the plant is a biological function, those factors that correlate with biological activity apply, such as pH, temper-ature, energy and oxygen supply. Roots will function under a fairly wide pH range—5.5 to 7.0—while root function is at its optimum when the root temperature is within 68 to 86°F, or within the same range as that experienced by the aerial portion of the plant. Plants will wilt if the root temperature differs too much from that existing around the aerial portions.Functional roots require two things—oxygen and a continu-

ous supply of carbohydrates—which are needed to generate the energy required for water and ion absorption to take place. With a large or enlarging root system, significant quantities of carbohydrates are needed and the likelihood of an anaerobic con-dition developing increases, because carbohydrates that form in plant leaves by photosynthesis must be divided between those needed for vegetative growth, including the formation of flowers and fruit, and those required by the roots.

the root whisperer

“Functional roots require two things—oxygen and a continuous supply of carbohydrates—which are needed to generate the energy required for water and ion absorption to take place.”

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The balance between root and vegetative plant growth has been the subject of many investiga-

tions, although no definitive understanding of the relation-ship has yet

been reached. The unanswered

questions are, first: “who is in the driver’s seat for growth—the roots or the vegetative portion

of the plant?” And second: “where do the photosyn-thetically-generated carbohydrates go first?”Root death is not an uncommon occurrence in some grow-

ing systems and it was the factor that doomed the nutrient film technique (NFT) for the growing of long-term crops such as tomatoes, cucumbers and peppers. Once the NFT trough was filled with roots, the flow of nutrient solution down the trough was impeded and there was insufficient oxygen within the root mass to sustain root function. As portions of the root system began to die due to oxygen star-vation, plant growth and fruit formation slowed or stopped

and with increasing oxygen starvation the plants died.The size of the root mass is not a

significant factor in a plant’s ability to absorb water and essential nutrients—what is important is that the roots are functional. The only exception would be when the rooting environment is minimal in terms of water and ele-mental availability, making a large root soil contact surface necessary in order for the plant to meet all its water and elemental needs.In mineral soils as well as for some types

of soilless mixes, contact between ions in solution and plant roots is governed by three physical processes—mass flow, diffusion and root interception. Those elements dissolved in the soil solution will move as water moves within the rooting medium, bringing them into contact with plant roots. Ion absorption from the solution surrounding the root creates a concentration gradient that results

the root whisperer

in the movement of ions some distance from the root, moving by diffusion into the area where the absorption occurred. The plant also plays a role as its roots grow into the rooting medium, increasing the contact surface between plant roots and the rooting medium.Maintaining constant conditions within the rooting me-

dium will allow plant roots to seek out that portion hav-ing a desirable balance between water and oxygen supply. If there are constant changes in the rooting environment then root development and function will be adversely af-fected, which will be reflected in periodic wilting under conditions of high atmospheric demand, coupled with nutrient element insufficiencies that will slow vegetative growth and lower product yield and quality.Root absorption of essential elemental ions is a complex

process that is not yet entirely understood. For an element to be absorbed by the roots it must exist in ionic form, as plant roots are not generally able to absorb molecules. For ion absorption to occur energy is required, which is ob-tained by root respiration, which in turn requires a supply of carbohydrates and the presence of oxygen. Ion absorp-tion will not occur under anaerobic conditions around the root. Since water and ion absorption occur at the same time, some scientists believe that a portion of the ions in solution are carried into the root with the incom-ing water, bypassing the mechanisms previously thought to be required for ion absorption to occur. Water is pulled into the plant root by pressure generated by the transpira-tion of water from leaf surfaces, but ion absorption can occur even when water is not being absorbed.It is the area immediately behind the growing root tip

where most of the water and ion absorption occurs. As the root matures, absorption activity declines. For hy-

droponic growing, where the entire root mass is exposed to

a nutrient solution, root maturity will not be as much of a factor in water and ion absorption.Root hair development occurs in a moist (not wet)

rooting environment, particularly when the essential plant nutrient element supply is low. Root hair development does not occur in adequately fertil-

ized soils, soilless media or hydroponic growing systems.

the root whisperer

“Root hair development occurs in a moist (not wet) rooting environment, particularly when the essential plant nutrient element supply is low.“

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74 Maximum Yield USA | August 2011

When both nitrogen and phosphorus are in abundant supply in the rooting medium, root hairs will not develop.A unique zone can exist

around the root known as the rhizosphere, a thin

cylindrical zone immediately around the root serving as the interface between the root

surface and the surrounding rooting medium. Rhizosphere

development requires a stable rooting environment that is not provided by

most hydroponic growing systems, where a nutrient solution is delivered periodically to

the rooting medium.For some woody plants, like trees, there is no ra-

dial redistribution of absorbed essential elements—or of water either, at least to some degree—so that all the roots must be functioning equally for normal growth to occur.At the entrance to a bank there was a large raised

bed with a young pin oak tree planted in its center, providing beautiful fall color. The area around the tree was covered with

mulch. Wanting to add flowering plant color appropriate for each season of the year, the bank manager contracted a local garden club to select and set appropriate plants in the bed around the pin oak tree.

the root whisperer

“For trees and most woody plants, what are known as feeder roots grow up toward the soil surface, not down, the feeder roots forming from lateral roots extending from the base of

the plant .“

For most plants it has been demonstrated that just one actively functioning root is capable of supplying most, if not all, of its essential nutrient elements. I have also conducted research to determine what effect rooting vessel size and shape can have on tomato plant growth and fruit yield. I found that a tomato plant will grow well and produce fruit when rooted in a one quart beverage bottle, with perlite being the rooting medium and the nutrient solution being introduced into the bottom of the bottle.Root size is not a significant factor in essential element

absorption if there is an adequate supply of carbohydrates and oxygen available for the functioning roots. However, root mass can be a factor—depending on plant water demands—when the plant leaf area is large and the atmo-spheric demand high. In general, there is no consistent positive correlation between root mass and plant growth and product yield. In fact, a large root mass may actually be detrimental to plant growth and product yield due to the amount of carbohydrates and oxygen necessary to sustain the function of such large roots. MY

“For most plants it has been demonstrated that just one actively functioning root is capable of supplying most, if not all, of its essential nutrient elements.“

the root whisperer

Over the next several years, everyone admired the beau-tiful array of flowering plants with the changing seasons, but no one paid attention to the pin oak tree—which stopped growing and then, several years later, died during the winter. Why?For trees and most woody plants, what are known as

feeder roots grow up toward the soil surface, not down, the feeder roots forming from lateral roots extending from the base of the plant. The constant disturbance of the soil surface around the pin oak kept the feeder roots from functioning—roots that were essential for the tree to sus-tain growth. Another factor that probably had an effect was the rooting medium used for the flowering plants, which brought in fertilizer elements (NPK) not needed by the tree and possibly contributed to its death.A similar root-affecting situation existed at a field crop

research station that I frequently visited during the grow-ing season. There were groves of pecan trees scattered about the station grounds. In one area, four large pecan trees provided shade for the workers, who parked their trucks and field equipment under them during lunch and rest breaks. The general appearance of the pecan trees was not affected by this practice, but none of these pecan trees ever produced nuts.

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