A simple method for the extraction and preservation of an undisturbed root system from a soil
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Plant and Soil XXXI, no. 1 August ! 969
SHORT COMMUNICAT ION
A S imple Method for the Ext ract ion and Preservat ion of an Undisturbed Root System f rom a So i l
One of the great practical problems involved in the study of roots is the difficulty of removing intact root systems from the soil without destroying the spatial distribution of the roots which was originally present in the soil. Attempts have been made to overcome this difficulty in various ways, proba- bly the best known being the 'pin-board method', described by Weaver a in 1926, and more recently by Schuurman and Goedewaagen .o. The disad- vantage of this method is that the root system is presented in two dimensions and not three. If it may be assumed that a root system is horizontally sym- metric, it is possible to deduce a 3-dimensional presentation from the 2- dimensional one. However, it is probable that this assumption is not always valid.
In the study of soil plant relations, it is becoming of increasing importance to be able to quantify not only the geometry of individual roots (length, diameter, volume, and surface area), but also the spatial distribution of roots in the soil. Great steps have recently been taken in this field, notably by Hacket t 1, who has used photography of root systems to considerable ad- vantage.
The simple technique outlined here is designed to present in three di- mensions a root system 'in situ' so that it can be easily studied, photographed, and preserved.
The principle of the method is similar to that of the 'pin-board'. With a long needle and nylon thread (fishing line) layers of 1/4" square mesh are built up in the soil core by threading through the holes in the perspex tube (see Fig. 1), and this supports the root system 'in situ' after the soil has been removed. The support for the root system that is provided by the nylon net- work is replaced by gelatin, the whole being transferred to a perspex box to facilitate photography.
The soil core (7 inches by 21 inches diam.) is transferred to a perspex tube of similar internal dimensions, and in which holes have been drilled at " intervals. With a long needle, and nylon thread (fishing line), layers of }"
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202 SHORT COMMUNICATION
Fig. 1. Technique of building up the nylon mesh in the soil sample using a needle and perspex cylinder with the holes drilled in it.
square mesh at ~' vertical intervals are built up in the soil core by threading through the holes in the perspex tube. The sample, held by the nylon network in the tube, is suspended over a sink, and washed free from soil with a fine spray of water. In the case of some clay soils, it has been found helpful to freeze and thaw the moist sample before attempting to wash out the soil. The root system is left 'in situ' on the nylon mesh. The perspex tube, holding the root system and mesh, is enclosed in a tightly fitting polythene bag, and filled with 5% w/v gelatin (containing 0.1 ~o w/v thiomersal, a bacteriostat which inhibits microbial liquefaction of the gelatin). The polythene bag prevents the solution of gelatin from running out of the holes in the perspex tube. When the gelatin has set, the polythene bag, and ally excess gelatin on the outside of the tube, is removed. The nylon threads forming the mesh are cut on the outside of the perspex tube and are carefully pulled out of the gelatin cylinder, using a pair of tweezers. The tube is immersed in hot water, in order to melt the outside of the gelatin block, thus releasing it from the tube. The gelatin block, holding the root system in situ is transferred to either a rec- tangular perspex box (7 '~ 2" 21' internal dimensions (see Fig. 2) or a perspex tube (7" 2" I.D.) closed at one end. If the root system is to be photographed, a rectangular box, although more difficult to construct, has the advantage that the flat surfaces provided by the sides prevent the type of distortion associated with a curved surface.
SHORT COMMUNICATION 203
Fig. 2. The gelatin block holding the root system in situ, transferred to a perspex box.
After the transfer of the gelatin block to its container, gelatin solution, which has been allowed to cool almost to the point of setting is poured around the gelatin block, filling up the container. The container is carefully sealed, as much of the air as possible being excluded. By keeping the specimen in a refrigerator, liquefaction of the gelatin is reduced to a minimum, and any trapped, small bubbles of air will dissolve under the cool conditions.
The specimen may normally be kept for two or three months and longer without liquefaction of the gelatin. However, where there is all abundant rooting system, liquefaction takes place at an earlier stage, presumably due to the presence of a larger microbial population.
Apparatus I. Need le . (4" long). This may be conveniently made from 1/16" diam.
silver stem rod. A flat face is made on one end of the needle to facilitate the drilling of the 'eye'. The other end is easily sharpened on a grindstone.
2. Perspex tube. The internal dimensions of the tube must be similar to the dimensions of the soil core. A wall thickness of 1/8 inch has been found to
l i t be satisfactory. Holes (3/16" diam.) are drilled into the side of tile tube at
204 SHORT COMMUNICATION
intervals in such a way that when the holes are threaded, layers of ~" square mesh are bui l t up.
A convenient method for mark ing out and dri l l ing the holes is as follows. A rectangular open-ended box is made so that the tube exact ly fits inside it. The holes are marked out at }" intervals on the four sides of the box. Holes are dri l led through the sides of the box and through the tube held r igidly inside the box by two screws. The drill, once it has passed through the side of the box is then well supported for the dri l l ing of the tube. If th is suppor t is lacking when the obl ique holes are being dri l led into the tube, there is a tendency for the dril l to 'wander ' . This tendency can be fur ther reduced by gr inding the t ip of the drill square before carry ing out the obl ique drill ing.
A simple method is described for the presentat ion in three dimensions of an und is turbed root system which is ext racted from a soil core. A ny lon mesh is threaded into the soil core in order to support the root system whi lst the soil is removed by washing. The support for the root system provided by the ny lon mesh is replaced by gelatin, the whole being t ransferred to a perspex box. In this way, an und is turbed root system may be studied, photographed, and preserved.
P. T. G00DERHAM* Department of Soil Science, University of Reading, England
Received October 28, 1968
References l Hacket t , C., New Phytologist. 67, 287, (1968). 2 Sehuurman, J. J. and Goedewaagen, M. A. J., Methods for the Examination of
Root Systems and Roots. Vada, Wageningen, Holland (1964). 3 Weaver, J. E., Root Development of Field Crops. New York (1926).
* Present address: - Wye College, near Ashford, Kent, England.