Executive Summary
What the report is about
This report describes some preliminary pot and field experiments, which scope the use of electricity as a rapid, non-destructive method for the measurement of the spatial distribution of tree and crop roots.

Intended audience
This report is relevant to forestry, agroforestry and plant breeding researchers, in projects where root mass and root location are measured.

Background
Why measure roots?

Roots are the plant organs responsible for the uptake of water and nutrients from soil and to a large degree the storage of below ground carbon. The spatial extent of roots, and their distribution within that zone, is critical to the growth of crops, forests, plantations and agroforestry systems, and the water and carbon balances of the land they occupy. Measurements of root extent and distribution, therefore, are essential for the understanding and modelling of plant growth and water and carbon balances.

Why electrical techniques?
The measurement of plant roots by conventional methods is a notoriously slow and laborious procedure, particularly under field conditions. Electrical methods offer an opportunity to develop a rapid, non-destructive method that could significantly reduce the cost of root investigations and facilitate a significant advance in our knowledge of plant-soil-water interactions.

Methods and results
Low frequency
We undertook field and laboratory experiments to evaluate the use of capacitance measurements on trees and to evaluate the potential for the use of this technique to measure the horizontal and vertical extent of roots.

We demonstrated that conventional capacitance techniques can be used on trees and have applied a new technique to increase the accuracy and repeatability of the measurements. This has also eliminated most of the error associated with electrical resistance between the tree stem and the electric probes. We did, however, show no relationship between capacitance and the physical extent of the roots using low frequency measurements.

High frequency
By altering the conceptual model of roots as electrical components within a soil medium, and undertaking measurements at high frequency, we successfully detected via remote methods, the presence of shallow individual tree root, under laboratory and field conditions. This technique was adapted for use with a ground probe and used to measure the electric field associated with the roots of faba beans (Vicia faba) and a small ribbon gum (Eucalyptus viminalis) in pots.

The electric field measurements were shown to vary according to root mass density and root length density, in both vertical and horizontal directions, and were able to detect fine roots (< 1 mm diameter) and with relatively low root length densities (< 0.5 cm cm-3).

Implications and recommendations
This study has shown that the low frequency capacitance technique for the measurement of the size of plant root systems is probably equally applicable to trees as it is to smaller plants.

More significantly, we showed that the high frequency technique can rapidly and non-destructively map the spatial distribution of roots in soil. Subsequent to this project, further investigations were carried out under Australian Provisional Patent No. 2003905289, jointly held by CSIRO and RIRDC. This patent has now been allowed to lapse and the results of the current and subsequent project (CSL-22A) have been released for public use.

If an electrical technique can be developed into a reliable method for field conditions, it would have immediate and widespread applications for agronomy, forestry, plant breeding and hydrology.

Of specific interest is the measurement of maximum rooting depth, as this has a specific application with regard to characterising root zone water storage capacities, a critical determinant of plant water use.

There are numerous other potential agronomic and ecological applications for the technique in the rapid detection of roots growing closer to the soil surface. These include the exploitation by crops of applied fertilizers, belowground competition between different plant species and water use dynamics in natural vegetation communities.

The preliminary work undertaken in this study justifies a structured and detailed appraisal of these techniques to determine their potential for rapid root investigations under field conditions.