Searching for Roots

excerpt from the Soil Report Newsletter of Soilmoisture Equipment Corp.

Ever pull a clump of grass and try to shake off the soil? It won't all fall off, no matter how hard it is shaken. Not even water will remove it. Some soil always clings to the roots.

This observation, common to every farmer and gardener, has led to a startling and important discovery about how many plants survive and prosper. The ramifications for agriculture, forestry and horticulture, particularly in dry climates, may be huge. All it took was for someone to ask why all the dirt doesn't fall off a clump of grass.

One person who asked-at least she found the answer-was Michelle Watt, 28, a graduate student at Carleton University in Ottawa, Canada. Together with Dr. Clarke Topp, 57, of Agriculture Canada, the equivalent of the US Department of Agriculture, an experiment was designed to (forgive the pun) get to the root of the problem.

As Michelle explains, it had been known that some plants, especially grasses, together with soil bacteria, produce "soil sheaths" or "rhizosheaths" on the younger regions of their roots. The root tips--Michelle says it is limited to the outer 20 cms. of roots, about eight inches--produce "mucilage", a form of glue, which is left behind along the roots and binds to the surrounding soil. That's why the dirt won't shake off.

"The sheaths are particularly big and hard to remove when the soil is dry, as with desert grasses," Michelle explains. "However, the sheath requires water to be formed, because the mucilage needs water to expand initially and act as a glue. Where are the roots getting the water if the soil is dry? The only place it can come from is the plant itself. The plant must exude some moisture back into the soil through the roots."

Michelle explains that others have suggested this, but no one had been able to measure it. While working on her masters of science degree in biology at Carleton, Michelle discussed the problem with her faculty advisors, Dr. Margaret E. McCully and Dr. Martin Canny, who encouraged her to pursue the matter. This led her to another soil scientist at Carleton, Dr. Kenneth Torrance. He suggested she approach Dr. Clarke Topp of Agriculture Canada.

He was a fortunate choice. Clarke, as he prefers to be called, has long been interested in Time Domain Reflectometry (TDR) technology, a method for in situ measurement of soil moisture. His expertise in TDR brought him together with Whitney Skaling, president of the Soilmoisture Equipment Corp. (SEC). That collaboration resulted in development of SEC's Trase equipment (Trase stands for Time Reflectometry Analysis of Signal Energy). Skaling cites their joint endeavor as an example of university/government/industry cooperation to the mutual benefit of all.

"The study Michelle and I performed would not have been possible without TDR equipment such as Trase," says Clarke. "Among other things, it provides a constant reading and record of soil moisture, including changes that are extremely subtle. We hoped to discover that the roots of plants were indeed exuding water. Trase equipment made it possible to do that."

Easier said than done. Clarke and Michelle worked at Agriculture Canada's Central Experiment Farm, a thousand-acre facility in Ottawa--really in Ottawa. It originally was out in the greenswards, but the city grew and encompassed it. They designed an experiment using sweet corn, a grass. Seedlings were grown in plastic tubes, roughly three inches in diameter and l5 inches in height. The tube was filled with soil in three layers, each about four inches deep and divided by wax barriers penetrable by roots but impermeable to water. The layers on the top and bottom were moist, but the layer in the center, where the corn roots would be monitored, was dry.

The Trase waveguide was to be inserted into this dry section. Ordinarily, the waveguide on Trase is a heavy prong, perhaps a quarter inch in diameter. When inserted into the ground, it emits an electronic signal very much like radar. The time it takes for the signal to return permits TDR measurement of soil moisture. Obviously, a waveguide that size would not work in the tubes, so Clarke--he admits he likes to "tinker" with "gadgets"--developed a prong consisting of three wires about an eighth of an inch in diameter. This was inserted into the tube at a 45 degree angle to expose a longer length of the wires in the dry soil layer.

Michelle sprouted the corn for two days, then transplanted it in the top of the tubes, observing the seedlings for six days in the greenhouse. Results were nil, except for watching corn grow. The problem was to get the roots to grow along the waveguide so moisture could be measured. This was solved with fine nylon mesh, finer even that stockings, laying flat along the waveguide. That gave the roots something to cling to and grow along. Then, the tubes were tilted at an angle, because plant roots respond to gravity when growing. It worked. To the delight of Michelle and Clarke, the roots began to grow along the waveguide.

They were not out of the woods yet. The Trase equipment was not providing the moisture measurements they sought. A phone call was made to Whitney Skaling and Herb Fancher, Soilmoisture's sales manager. Could the problem be solved? It could. Meeting the needs of SEC equipment users receives the highest priority. As it turned out, the Trase machine was not sensitive to the specially-built waveguides Clarke had installed. In a few days a new electronic chip arrived in Ottawa, changing the software in Trase.

A new planting was made in February, l995. The roots grew along the waveguide, and Trase provided constant and accurate measurements of soil moisture in the dry section of soil. The soil moisture rose from five per cent to six and a half percent overnight, reaching a peak about 8 a.m. Then it declined during the day, rising again at night. The results are potentially earthshaking.

As was long known, a plant transpires water vapor through its leaves in daylight. But it also exudes moisture through its roots at night! The difference is extremely subtle, only one to one and a half per cent change, was able to monitor it.

"The experiment suggests that plants exude water at night to dissolve nutrients in the soil for its use," Clarke says. Michelle agrees, saying, "This has yet to be proven, but it would seem this is the reason."

The research done by Clark and Michelle in the lab was further enhanced by Dr. Margaret E. McCully, Michelle Watt's faculty advisor. She went into the field and collected samples of roots both in daylight and and at night. These samples were immediately frozen in liquid nitrogen, then examined under an electron microscope. The night samples showed minuscule globules of water and mucilage. No such globules were found on root samples taken in the afternoon.

The movement of the water in the plant is clearly diurnal, toward the leaves in daylight, toward the roots at night. What triggers the change? This has yet to be established.

This study was done with corn, but Michelle believes that something similar happens with all plants, from huge trees to simple mosses.

Asked the significance of this research, Dr. Topp replied that it tends to show that plants "play an active role in recovering nutrients from the soil, and water is the key to doing it." Plants are not simply passive creatures, responding only to the environment provided, but actively store and circulate water to obtain nutrients for survival and growth. Michelle agrees, adding that plants "seem driven by water potential" and that the soil itself is "influenced by the presence of the root."

That plants actively circulate water can be demonstrated by a large deciduous shade tree. Not much rain falls directly under its leaves. The soil is drier. Yet grasses, flowers and shrubs grow there. Clearly, they are obtaining water and nutrients from the tree roots.

Much work remains to be done, including what triggers the mechanism and how it works. Does the presence of water in the soil encourage or diminish the night time activity? If sheathing occurs only on the tips of new roots, what is the role of the older root structure? Could plants possibly circulate chemicals to attract specific nutrients they need from the soil? It would seem unlikely, but so was the whole idea of plants exuding water through its roots.

Some effects of the research seem apparent, however. To achieve maximum yields, moisture and fertilizer must reach the new root growth below the earth, rather than simply be spread on the surface or around the stalk. Cultivation and aeration appear to be the key to accomplishing that.

An example may be found in a commonly-used process for speeding the growth of a sapling into a shade tree. Holes are augured a few inches into the ground every foot or so around the perimeter of the leaves, then filled with fertilizer. If water is generously applied--and the auguring process continued as the limbs expand--the tree trunk will grow several inches in a year.

A native of Hamilton, Ontario, Clark Topp did his undergraduate work in agriculture at the University of Guelph, then earned his masters in physics at Guelph and his Ph.D. as a soil scientist at the University of Wisconsin in Madison. The father of three he now lives in Ottawa with his wife Ellie. Michelle Watt also lives in Ottawa, greatly enjoying her firstborn, Alexandra. She plans to seek her Ph.D. in Australia.