GB2128335A

GB2128335A - Measuring conductivity of a soil sample

Info

Publication number: GB2128335A
Application number: GB08223378A
Authority: GB
Inventors: Guy Wilson Grimes
Original assignee: OMNIUM ASSETS TRUST SYNDICATE
Current assignee: OMNIUM ASSETS TRUST SYNDICATE
Priority date: 1982-08-13
Filing date: 1982-08-13
Publication date: 1984-04-26

Abstract

A method for measuring the conductivity of a soil sample comprises the steps of: (a) introducing a predetermined quantity (up to mark 4) of the soil to be tested into a measuring cell (1) provided with a pair of electrodes (2) mounted in the side walls of the cell and spaced apart from the bottom of the cell; (b) introducing a predetermined quantity of an extractant liquid comprising deionised water into the cell (up to mark 5); (c) agitating the soil sample in contact with the extractant liquid to extract water-soluble salts from the sample; and (d) measuring the electrical conductivity of the liquid between the two electrodes of the cells. <IMAGE>

Description

SPECIFICATION Conductivity measurement This invention is concerned with improvements in and relating to methods for carrying out conductivity measurements, particuiarly on soil samples.

It is known to measure the electrical conductivity of a soil sample (or an aqueous extract from the sample) in order to estimate the content of ionisable inorganic salts in the sample and thereby form an estimate of its potential as a growth medium for plants (its "fertility").

It is an object of the present invention to provide a method for measuring the conductivity of a soil sample, which method is particularly designed to be used by amateur gardeners or small-scale growers.

According to the invention there is provided a method of measuring the conductivity of a soil sample which comprises the steps of: (a) introducing a predetermined quantity of the soil to be tested into a measuring cell provided with a pair of electrodes mounted in the side walls of the cell and spaced apart from the bottom of the cell; (b) introducing a predetermined quantity of an extractant liquid comprising deionised water into the cell; (c) agitating the soil sample in contact with the extractant liquid in the cell to extract watersoluble salts from the sample; and (d) measuring the electrical conductivity of the liquid between the two electrodes of the cell.

The term "soil" as used herein is intended to refer to any solid or liquid growth medium used in the cultivation of plants and thus refers not only to common or garden earths or soils but also to so-called "composts", especially peat-based and loamless "composts" containing added nutrients and to nutrient solutions.

Where the soil is a solid growth medium it is generally desirable that the agitated mixture produced in step (c) be allowed to settle so as to provide in the cell or lower layer of settled soil solids and an upper supernatant layer of extractant liquid containing extracted water-soluble salts.

The electrical conductivity of the supernatant liquid is then measured between the two electrodes. In order to facilitate settling of the said solids the extractant liquid preferably contains a coagulant material suspended or dissolved therein.

In the following description, reference will be made to the accompanying drawings in which: Figure 1 is a vertical cross section through a cell for use in accordance with the invention; and Figure 2 is a circuit of apparatus for use in measuring the conductivity of a liquid sample in a cell.

As shown in Figure 1 of the drawings, a conductivity cell for use in accordance with the invention comprises a generally tubular cell 1 having a closed bottom and made of an electrically non-conducting transparent material such as glass or a plastics material such as poly (methyl methacrylate), a polycarbonate or polystyrene.

A pair of spaced electrodes, 2,2', are mounted in the side walls of the cell each having a terminal 3, 3', outside the cell. The walls of the cell are provided with level marks 4 and 5 formed in or on the walls, with level mark 4 being below electrodes 2 and 2' and level mark 5 above the electrodes. Cell 1 is further provided with a removable cap 6.

In order to carry out a conductivity measurement in accordance with the invention, cap 6 is removed from the cell and a soil sample is introduced into the cell up to level mark 4, thereby to introduce a predetermined quantity of soil into the cell. Since the measure afforded by level mark 4 is a volumetric measure it will be appreciated that the weight of a sample may vary depending upon the density of the soil or growing medium and thus, in general, the weight of a sample of garden earth will be greater than the amount of a peat-based or loamless "compost".

Suitably the volume of sample introduced is from 0.8 to 1.2 ml, preferably about 1 ml, giving, on average, from 0.7 to 13 gm, preferably about 1 gm of garden earth and from 0.2 to 0.8 gm preferably about 0.5 gm of a conventional peatbased compost.

There is then introduced into the cell an extractant liquid comprising deionised water, preferably containing, in the case of a solid soil sample, a coagulant material such as barium sulphate, or a non-ionic (non-electrically conducting) or weakly ionic (weakly electrically conducting) organic coagulant material. The coagulant, when barium sulphate is employed is suitably present in the extractant liquid in a concentration of from 20-60 mg/ml, preferably about 40 mg/ml. The extractant liquid is introduced into the cell in an amount up to level mark 5, thereby to introduce a predetermined amount of extractant liquid, suitably from 2 to 3 ml, preferably about 2.5 ml.

Thus, as will be appreciated, the relative position of level marks 4 and 5 serves to define the ratios of sample and extractant and these can, of course, be varied depending upon the salt content of the sample to be measured. The electrodes 2 and 2' shail be spaced vertically intermediate level marks 4 and 5 especially when the cell is to be used to determine the conductivity of a solid soil sample.

Cap 6 is then placed on the cell and the whole is agitated, for example by hand, for a period of time sufficient to extract water-soluble salts into the extractant liquid, periods of 45 to 75 seconds being generally sufficient for hand shaking or agitation of the cell.

When the sample is a solid, the suspension in the cell is then allowed to settle whereby the soil solids settle and may be coagulated, by the action of the coagulant material, if any, in the extractant liquid, to provide a substantially clear supernatant liquid. Generally a time period of 45 to 75 seconds proves sufficient.

The conductivity of the supernatant liquid between the electrodes is then measured, for example using apparatus having a circuit as shown in Figure 2 of the drawings.

As shown in Figure 2, conductivity measuring apparatus comprises a source of DC electricity 7, suitably a dry cell, arranged in series with an ammeter 8 and a pair of contacts 9, 9' (to make contact with the electrical terminals 3, 3' of a conductivity measuring cell 1) and a switch 10. A resistance R1 is arranged in parallel with ammeter 8 and a second resistance R2 is also arranged in parallel with the ammeter via switch 11.

In operation, a cell 1 containing supernatant liquid is placed between contacts 9 and 9' and switch 10 is then operated whereby the meter 8 gives a reading proportional to the conductivity of the liquid sample in cell 1. Meter 8 may be calibrated simply to give the current flowing through the circuit and the conductivity of the sample, and its water-soluble salt content, may then be estimated from that current. Alternatively, meter 9 may be calibrated to show the conductivity of the sample or, more usefully, to give an estimate of the salt content, and hence the related fertility (growth potential), of the solution or of the sample from which it is derived.

The second resistance may be introduced into the circuit, by operating switch 11 to make it possible to measure the conductivities sample solutions of higher conductivity, for example those derived from materials of higher nutrient content such as loamless or peat-based "composts".

Alternatively a differently constructed cell, i.e.

one of different relative proportions and electrode spacing may be used with samples of, for example, loam less or peat-based composts.

While cell 1 may be reused it is conveniently formed as a disposable unit, the cell then being used for only one conductivity measurement and then discarded. If it is desired that the cell be reused it is, of course, necessary to thoroughly wash the cells with deionised water so that no water-soluble salts from a previous determination remain in the cell.

Claims

1. A method for measuring the conductivity of a soil sample which comprises the steps of: (a) introducing a predetermined quantity of the soil to be tested into a measuring cell provided wiht a pair of electrodes mounted in the side walls of the cell and spaced apart from the bottom of the cell; (b) introducing a predetermined quantity of an extractant liquid comprising deionised water into the cell; (c) agitating the soil sample in contact with the extractant liquid to extract water-soluble salts from the sample; and (d) measuring the electrical conductivity of the liquid between the two electrodes of the cells.

2. A method as claimed in claim 1 in which the measuring cell is formed of a transparent material and is provided with a pair of level marks respectively above and below the level of the electrodes in the cell; the lower level mark serving to indicate the predetermined amount of soil sample and the upper level mark serving to indicate the predetermined amount of extractant liquid.

3. A method as claimed in claim 1 or claim 2 in which the extractant liquid also contains a coagulant material.

4. A method as claimed in claim 1 substantially as hereinbefore described with reference to the drawings.