Plant and Soil XXXI, no. 1 August ! 969

S H O R T C O M M U N I C A T I O N

A S i m p l e M e t h o d for the E x t r a c t i o n a n d P r e s e r v a t i o n of a n

Undisturbed Root System f r o m a S o i l

Introduction

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 W e a v e r a in 1926, and more recently by S c h u u r m a n and G o e d e w a a g e n .o. The disad- vantage of this method is tha t 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), bu t also the spatial distribution of roots in the soil. Great steps have recently been taken in this field, notably by H a c k e t 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.

Outline o/method

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 ¼" intervals. With a long needle, and nylon thread (fishing line), layers of }"

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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 ¼~' 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 at tempting 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 '~ × 2½" × 2½1' internal dimensions (see Fig. 2) or a perspex tube (7" × 2½" 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.

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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. N e e d l e . (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. P e r s p e x t u b e . 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

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in t e rva l s in such a w ay t h a t w h e n t he holes are th readed , layers of ~" square m e s h are bu i l t up.

A c o n v e n i e n t m e t h o d for m a r k i n g ou t a n d dr i l l ing t he holes is as follows. A r e c t a n g u l a r open-ended box is m a d e so t h a t t h e t u b e exac t ly fi ts ins ide it. T h e holes are m a r k e d o u t a t }" i n t e rva l s on t h e four sides of t h e box. Holes are dr i l led t h r o u g h t h e sides of t h e box a n d t h r o u g h t he t u b e he ld r ig id ly ins ide t he box b y two screws. T he drill, once i t h a s passed t h r o u g h t h e side of t h e box is t h e n well s u p p o r t e d for t h e dr i l l ing of t h e tube . I f t h i s s u p p o r t is l ack ing w h e n t h e obl ique holes are be ing dr i l led in to t h e tube , t h e r e is a t e n d e n c y for t he dri l l to 'wande r ' . This t e n d e n c y can be f u r t h e r r educed b y gr ind ing t he t ip of t h e dril l square before ca r ry ing ou t t h e obl ique dril l ing.

Summary

A simple m e t h o d is descr ibed for t h e p r e s e n t a t i o n in t h r e e d imens ions of a n u n d i s t u r b e d roo t s y s t e m w h i c h is e x t r a c t e d f rom a soil core. A n y l o n m e s h is t h r e a d e d in to t h e soil core in o rder to s u p p o r t t h e roo t s y s t e m whi l s t t he soil is r e m o v e d b y washing . T h e s u p p o r t for t h e roo t s y s t e m p r o v i d e d b y t h e n y l o n m e s h is rep laced b y gelat in, t he whole be ing t r a n s f e r r e d to a pe r spex box. I n th i s way, an u n d i s t u r b e d roo t s y s t e m m a y be s tud ied , p h o t o g r a p h e d , a n d preserved.

P. T. G00DERHAM* Department of Soil Science, University of Reading, England

Received October 28, 1968

References l H a c k e t t , C., New Phytologist. 67, 287, (1968). 2 S e h u u r m a n , J. J. and G o e d e w a a g e n , 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.