GB2172110A

GB2172110A - Electrical testing device for soils

Info

Publication number: GB2172110A
Application number: GB08505536A
Authority: GB
Inventors: Jacob Jackson
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-03-04
Filing date: 1985-03-04
Publication date: 1986-09-10
Also published as: GB2172110B; GB8505536D0

Abstract

A meter for determining in situ the dissolved electrolyte content of soils consists of a bimetallic probe connected through a switch to a current meter. The meter is based on a concept embodied in the US Bureau of Soils' cup, but the conductivity determination is made on soil saturated in situ rather than in a conductivity cell, and the conventional AC procedure is replaced by one using DC, the voltage being generated by the dissimilar metals of the electrodes. Both electrodes are tapered in the direction of entry into the soil to ensure good contact, creating a well-defined cell constant independent of immersion depth. since the stem does not pass current. The switch enables the user to place the electrode-pair in the desired test position before current is allowed to flow, and its magnitude is observed before any, possibly rapid, fall due to electrode polarisation.

Description

SPECIFICATION Electrical testing device for soils This invention has the object of providing a means for measuring in situ the dissolved electrolyte content, usually termed the salinity, of soils and composts used for growing plants.

Plants derive essential nutrients from the soil in the form of inorganic electrolytes, and normal growth is impossible without some minimal salinity. Salinity may also be excessive, resulting in scorching of the roots and impaired growth above ground. In areas where soils may have a high natural salinity it is therefore usual to carry out tests before bringing them into cultivation. The salinity of cultivated soils is usually fertiliser.

The common method of determining salinity is to prepare an extract of the soil with a fixed proportion of water and determine its conductivity, it being assumed that a rough relationship exists between conductivity and salt concentration expressed in terms of gmmols per litre.

A simpler official method has long been used in the U.S.A. employing the Bureau of Soils' cup. This is a cylindrical conductivity cell, the electrodes forming parts of the cylindrical surface, and their area and disposition being such as to give the cell a constant of 0.25. The cup is filled with soil, sufficient water is added just to saturate it, and the conductance is measured with an A.C. bridge.

The present invention is based on the same concept, but further simplifies it by performing the measurement in situ. The saturation occurs either naturally through rainfall or is effected artificially, and a reading is taken within a few minutes before appreciable drainage has taken place.

A crude measurement of soil conductivity in situ is already in use for a different but related purpose, determining moisture content, or, more correctly, the degree of moistness, which, at the lower end of its range, is known to be roughly proportional to conductivity. The simplest form of soil moisture meter consists of a bimetallic probe which is inserted in the soil, the leads to the two metals being connected through a microammeter. In the presence of water the dissimilar metals generate a roughly constant potential, and the magnitude of the current flowing through the indicator is a measure of the soil conductivity.

The probe thus constitutes a form of D.C.

conductivity electrode system. In the conventional construction the cathodic metal is a cylindrical tube of uniform external diameter, there being no differentiation between the cathode proper and the supporting stem. The co-axial anode, separated by an insulating ring, is generally tapered to facilitate insertion into the soil.

In BP 1368713 the present inventor disclosed a probe construction also using dissimilar metals which conforms much more closely to the theoretical requirements of conductivity electrode systems for semi-solids than those hitherto used in soil moisture meters, and in consequence gives much more consistent, i.e., repeatable, readings.

For the purpose of the present invention the claim for the modified probe design in respect of repeatability is important, and its validity has been confirmed in a comparative test. The comparison was made with a soil moisture meter having a conventional probe which was commended in a consumers' organisation report on soil moisture meters as giving "the most consistent readings". The test was carried out in a large mass of John Innes compost of uniform moisture content, and great care was taken to compact it uniform. To avoid subjective errors the meter currents were fed directly into a laboratory recorder, the readings according to the arbitrary scales of the two meters being obtained by separate calibrations. In the case of the conventional meter the total span of 23 readings was 1.9-4.3, or 77% of the average reading.The span of 24 readings of the meter conforming to BP 1368713 was 4.4-5.0, or 13% of the average reading. The standard deviation was 5.4%, which is adequately small for the present purpose.

In further tests a reasonable correlation was found between the conductivity as measured by the conventional AC procedure and the current flowing through the microammeter after allowing for the ohmic drop in the coil.

Wet soils with high fertiliser contents, however, have a far higher conductivity than those encountered in the usual use of the soil moisture meter with pot plants, and these give rise to far higher currents through the probe. A DC measurement is therefore more likely to be seriously affected by polarisation, since the principal cathodic depolariser normally present, oxygen, will be quickly exhausted locally.

However, even if the oxygen content of the soil air is well below 20%, sufficient will be available in the soil solution to provide an initial surge of current corresponding to the cell conductance. A feature of the present invention is to ensure that this surge is easily observed. Current is not allowed to flow until the probe is positioned where the measurement is desired. A switch is then operated to close the circuit, and the user notes the highest indication reached.

There is a second difference in probe behaviour in thoroughly wetted soils. These are more likely to make contact with areas of the stem immediately above the cathode proper than drier soils, adding to the cathodic current and perhaps significantly lowering the cell constant. The theoretical desirability of covering the entire stem with an insulating film, men tioned in BP 1368713 thus becomes of practi cal importance.

In one embodiment of the invention provision for the measurement of soil moisture is also made. Both measurements of course require the probe to be connected to the current meter, though through different circuits, and the switch has a central "OFF" position.

Claims

1. A meter for determining the dissolved electrolyte content of soils consisting of a bimetallic probe connected through a switch to a current meter. The probe functions as a dip conductivity electrode system, incorporating two clearly defined electrodes of dissimilar metals, both tapered in the direction of entry into the soil, supported by a stem which does not pass current.

2. A meter for determining the dissolved electrolyte content of soils as in Claim 1 which serves also as a soil moisture meter, the probe being connected to the meter through an additional circuit, either circuit being made through the switch, which has a third, "OFF" position.