Clay Minerals (1976) 11, 81.

N O T E

E L E C T R O - O P T I C A L M E A S U R E M E N T O F

C L A Y S H R I N K A G E

Shrinkage is a very important physical property of soils owing to its influence on water balance and related consequences both on soil conservation and agronomic management. As a particular agricultural problem, shrinkage becomes important in soils with a high content of expanding types of clays, where large and deep cracks influence the root systems and therefore crop yields.

Several new methods have been devised in recent years to investigate soil shrinkage (Koepf, 1960; Franzmeier & Ross, 1968; Groenevelt, 1974). The present study was undertaken to accomplish a less time consuming measurement of shrinkage by means of electro optical equipment (Leitz Classimat).

The apparatus is similar to a Quantimet-B previously used by Jongerius (1972) for soil porosity investigation; it consists of an epidiascope, where the image of a shrunken sample is projected upon the photosensitive layer of a television camera, an electronic signal processing unit which analyses the video signals, and a monitor where the analysed image is also visible. The measured data can be digitally displayed and directly fed to an electronic computer (Olivetti P602). A simplified diagram is presented in Fig. 1.

Following the shrinkage development was considered the best way to estimate the performance of the instrument and clay with a high content of montmorillonite was selected as the most suitable sample in order to enhance this process.

The investigations were carried out using clay samples from the Uri Bentonite

Optical -,,,,-- Television macroscopic image ca mera

f

Object Monitor

Video signal processing unit

Digital display

Electronic computer

FIG. 1. Simplified diagram of Leitz Classimat.

82 Note

deposit of 'Pedra de Fogu', Sardinia (Italy), the average chemical composition of which has already been reported by Pietracaprina, Novelli & Rinaldi (1972).

Clay samples (10 g) were watered until they began to show signs of becoming plastic, a portion of the mass was then taken and moulded into a square box roughly 9 • 9 cm in size. The drying out process, at constant temperature (25~ was slowed down by the use of covers in order to obtain as uniform a loss of moisture as possible.

Measurements were effected on Leitz Classimat by 'grey scansion'; by this pro- cedure different grey ranges of the image are discriminated in order to bring the measurement only to a black and white determination.

In shrunken samples cracks appear black against a white framework of clay (or soils) such that the measurements involve determination of white percentage in respect of the total area. The field to be measured was previously limited by the use of an electronically formed rectangular mask; height and width in mm of the mask can be measured so that surfaces of white areas are quantitatively determined.

Plotting areas occupied by soil framework against weight of water present, gives the shrinkage curve (Fig. 2).

As is well known the shrinkage curve consists to a first approximation of two straight lines, intersecting at the minimum moisture content at which the pore space

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FIo. 2. Shrinkage curve of the Uri Bentonite.

Note 83

F~. 3 Example of discrilz~inated (a) and undiscriminated (b) sample through the television monitor.

84 Note

of the soil is filled only with water; at lower moisture contents the pore space is occupied simultaneously with air and water. By the present procedure areas instead of volumes are plotted on the y-axis; the curve obtained fits perfectly the expected pattern and the intersection with the coordinate axis of the dotted line of the figure identifies the area occupied only by the solid particles of Bentonite.

The application of Leitz Classimat for determination of soil shrinkage seems particularly suitable for laboratory trials though areas instead of volumes are involved in measurements. The method presents high accuracy and reproducibility; differences among individual measurements are practically negligible (less than 0.1 ~ ) and its rapidity facilitates serial determinations.

Shrinkage measurements by Leitz Classimat can be carried out from photographs of specimens too; the use of pictures opens therefore important perspectives for soil studies in the field. It is possible, for example, to follow seasonal variations of cracks, not only on a soil surface, but also along soil profiles, if photographic exposures are always taken from the same place at different times of the year.

Acknowledgment

The authors thank Mr M. La Marca for technical assistance.

Laboratorio per la Chimica del Terreno, C.N.R. Via Corridoni, 78, 56100 Pisa, Italy. 25 April 1975

G. PETRUZZELLI G. GUIDI P. SEQU!

REFERENCES

FRANZMEIER D.P. & ROSS S.J. (1968) Proc. Soil Sci. Soc. Am. 32, 573. GROENEVELT P.H. 0974) Geoderma 11, 287. KOEPF H. (1960)Z. Pflerniihr. Dung. Bodenk. 89, 159. JONGERIUS A., SCHOONDERBEEK D., JAGER A. • KOWALINSKI ST. (1972) Geoderma 7, 177. PIETRACAPRINA A., NOVELLI G. 8r RINALDI A. (1972) Clay Miner. 9, 351.