WO2002044694A2

WO2002044694A2 - Permeability measurement

Info

Publication number: WO2002044694A2
Application number: PCT/GB2001/005263
Authority: WO
Inventors: John Featherstone-Witty
Original assignee: Iger
Priority date: 2000-11-28
Filing date: 2001-11-28
Publication date: 2002-06-06
Also published as: GB0028986D0; AU2002223094A1; WO2002044694A3

Abstract

A method and apparatus for determining the permeability of a sample. The method includes providing the sample (6) in a receptacle (2, 4) so as to provide a headspace (7) between the sample (6) and part of the receptacle (2), introducing a predetermined volume of a first gas into the headspace and subsequently monitoring the change in concentration of the first gas and/or a second gas in the headspace. The method and apparatus are particularly suitable for determining the permeability of a soil sample.

Description

Peirmecabilitv Measurement

The present invention is concerned with a method and apparatus for determining the permeability of a sample of porous material.

It is frequently desirable to monitor the permeability of porous materials such as soil, filters or textile materials, so that, if appropriate, adjustments can be made to the environment or manufacturing process so as to make corresponding adjustment to the permeability.

For example, the permeability of soil to oxygen is a major factor directing those soil processes which determine productivity, sustainability and durability of pastures, agricultural crops and sports turfs. As the permeability decreases, soil oxygen concentrations fall.

Two processes critical to continuing soil fertility are mineralisation and denitrification. Mineralisation, the process by which soil organic matter is slowly broken down to release plant nutrients, decreases as soil oxygen falls, while under the same conditions denitrification leads to an increasing loss of soil nitrate to the atmosphere as nitrogen oxides. In addition, apart from the environmental implications of denitrification, fertiliser lost from grasslands by denitrification in the UK alone is estimated to cost £100 million annually. As this problem worsens, plant growth may be severely hampered by the mechanical properties of the soil as well as decreased gas flux and waterlogging. Under extreme conditions, plants are killed by products such as hydrogen sulphide arising from anaerobic soil. In all but light sandy soils, good permeability and friability result from the biological formation of soil crumbs which pack together leaving spaces between the crumbs. Once this structure breaks down, it cannot be re- established by mechanical means; for example, treatments such as ploughing or motor powered soil tilling only serve to worsen the situation. Re-establishment of structure is essentially a biological process which may take several years. The best strategy to avoiding difficulties would rely on monitoring soil permeability to obtain early warning of degradation so that land use management can be modified.

A number of methods are known for measuring the permeability of soil; such methods include measurement of air spaces, measurement of gas flow, and estimation of gas diffusion.

Measurement of air spaces is based upon laboratory measurements of porosity (open pore space) which are based on estimates of volume, moist and dry weight and particle density. Determination of particle density by weighing and water displacement is particularly slow. Furthermore, although estimates obtained by such methods can enable estimation of the volume of soil pores, these estimates may not correlate with permeability because, for example, the pores may not be interconnected.

Measurement of gas flow is a method based upon gas flow through soils; the method uses either cylinder gases (as disclosed by Green and Fordham in MAFF Technical Bulletin 29, 272-288 1975) , or a miniature gasometer (as disclosed by Evans in Methods in Soil Analysis, Part I. 319-330 1965) to apply air at a constant pressure to in situ or exhumed soil cores. The air permeability of the soil is inferred from the gas flow rate.

Estimates of gas diffusivity have been based on the diffusion into soil of chlorotrifluoromethane from the headspace of in situ cores and subsequent analysis of samples by gas chromatography (as disclosed by Rolston et al in Soil Science Society of America 55, 1536-1542 1986) . Other methods include the injection of N₂0 into soil and subsequent chromatographic analysis of the concentration of this gas as it moves away from the point of injection (as disclosed by Jellick and Schnabel in Soil Science Society of America 50, 18-23; 1986) . Radioactive krypton (⁸⁵Kr) has been used in a similar way (as disclosed by Ball et al in European Journal of Soil Science 45, 3-13 1994) .

Two further methods known in the art are based on measurements of change in oxygen concentration. These involve the use of a Carlo Erba portable gas analyser in the field to measure oxygen and nitrogen flux through cores (as disclosed by Sung-Ho et al Soil Science Society of America 40, 3-6 1976) and the use of oxygen specific micro- electrodes to follow the oxygen concentration change within a core as the head space gas is alternated between oxygen and nitrogen (as disclosed by Rappoldt in European Journal of Soil Science 45, 169-177 1995) .

Compaction is commonly seen as a major problem for golfing greens, football pitches and tennis courts as well as agricultural soils. Remedial treatments such as spiking can be effective if applied in time, but severe deterioration in soil permeability may require the turf and subsoil to be replaced. In these examples a simple method of measuring the permeability would provide a valuable tool for predicting and avoiding problems and monitoring treatment.

A further area in which monitoring of permeability is desirable is in silage making. A major factor in successful silage making is the exclusion of oxygen during fermentation. Failures in this area result in a poor

(often unusable) product and a huge (>50%) loss in feed dry matter. Several manufacturers have recently introduced new laminated plastic wrapping films and it is desirable to evaluate the oxygen permeability of these membranes under a range of conditions (temperature, humidity, effects of UV light etc. ) .

Furthermore, the permeability of the silage itself is a very important factor. Traditionally, silage has been made by compressing large amounts of grass in silage bays. The results obtained in this way are different from those obtained by baling and one likely . reason for this is related to the state of compression and hence the oxygen permeability.

It is therefore an aim of the present invention to alleviate at least some of the disadvantages identified above .

It is a further aim of the present invention to provide a simple, rapid and portable method for measuring the permeability of soil, which preferably overcomes former in accuracies resulting from soil respiration.

It is yet a further aim of the present invention to provide apparatus for measuring the permeability of soil. It is still yet a further aim of the present invention to provide an apparatus for monitoring soil respiration.

It is still yet a further aim of the present invention to provide apparatus for providing an output which is indicative of the porosity of a sample.

Therefore according to a first aspect of the present invention there is provided a method of determining the permeability of a sample, which method includes:

(a) providing the sample in a receptacle so as to produce a head space between the sample and part of the receptacle;

(b) introducing a predetermined volume of a first gas into the head space; and

(c) monitoring the change in concentration of a gas in the headspace, the monitored gas being the first gas or a second gas.

The headspace produced in step (a) is preferably of a known volume (or alternatively it may be calculated) .

It is particularly preferred that the second gas is oxygen. The first gas introduced into the headspace may be oxygen or alternatively an inert gas such as nitrogen, Helium or Argon, which advantageously serves to displace oxygen may be used. It is also envisaged that the first gas and the second gas are the same gas, for example both may be oxygen.

It is envisaged that when the first gas is oxygen the change in concentration of the second gas (such as oxygen) is a result of the oxygen diffusing through the sample from the headspace. Alternatively, when the second gas is a gas other than oxygen (for example nitrogen) an amount of gas (such as oxygen) is removed from the headspace. Preferably, the headspace is purged of oxygen therefore the change in concentration of the second gas (such as oxygen) is a result of the second gas diffusing from the atmosphere through the sample and to the headspace.

In a particularly preferred embodiment the first gas is nitrogen; therefore nitrogen is used to purge the headspace. Advantageously, the nitrogen may be obtained from the atmosphere by, for example, the use of an oxygen absorbing agent arranged in a cartridge or the like, preferably attached to the inlet port.

Advantageously, the volume of the headspace may be predetermined, measured directly, or inferred from the increase in gas concentration which results from the introduction into the headspace of a known amount of gas.

The headspace volume may therefore be calculated utilising the concentration of gas which results when a known volume of gas is introduced into the headspace.

Advantageously, the second gas may be oxygen. However, by replacing the sensor, it is envisaged that the gas may be, for example, ammonia, carbon dioxide, carbon monoxide, chlorine, ethylene, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen sulphide, nitric oxide, nitrogen dioxide, ozone, helium, argon or sulphur dioxide. However, when the sample is of soil, it is preferred that the gas is oxygen.

The sample to be monitored is typically a plug of soil (including associated plant matter) . However it is envisaged that other porous samples (such as textiles, or the like) may be monitored according to the present invention.

According to a further aspect of the present invention there is provided an apparatus for determining the permeability of a sample which apparatus includes a receptacle arranged to receive a sample such that the receptacle and the sample define a head space between the sample and the receptacle (substantially as described hereinbefore) ; means for introducing a predetermined volume of a first gas into the headspace; and means for monitoring the change in concentration of the first gas and/or a second gas in the headspace.

The first gas and the second gas are substantially as described hereinbefore with reference to the first aspect of the invention.

The apparatus typically includes means for determining (by calculation) the change in oxygen concentration in the headspace due to respiratory 0₂ (soil 0₂ use) consumption using the equation C_t = (Co - C e^~κt + C_∞

Where:

K = 1/(RV)

V = (Volume of 0₂ added x32.959) / (C₀-8.708) Soil 0₂ use = 2 ( (Atmospheric 0₂ - CJ /R) t = time; C_t = headspace 0₂ concentration at time t;

C₀ = concentration at time zero; C„ = concentration at time infinity; K is a constant determined, for a particular core; R is the diffusion resistance of soil and V = headspace volume. Therefore, the present invention extends to apparatus for determining the permeability of soil, which apparatus includes : a receptacle arranged to receive a sample so that the sample and the receptacle define a headspace; means for introducing a predetermined volume of a first gas into the headspace; monitoring means for monitoring the change in concentration of gas in the headspace (the monitored gas may be the first gas or a second gas) ; and means for determining the change in oxygen concentration in the headspace due to respiratory 0₂ (soil 0₂ use) consumption using the equation

C_t = (C₀ - CJ e^"κt + C_∞

The apparatus further comprises an inlet port and an outlet port. The inlet port is typically arranged to permit the entry of the predetermined volume of the first gas into the headspace. The inlet port and/or the outlet port are typically closable.

The receptacle preferably includes a tubular member; such a tubular member may have a cross-section which is square, rectangular or octagonal for example. A circular cross- section is preferred; it is further preferred that the cross-section is substantially uniform along the length of the tubular member.

Typically, the length of receptacle is varied depending on the permeability of the sample. For example, when samples have relatively high permeability, it is preferred to use a longer receptacle, whereas if the permeability of the sample is relatively low, it is preferred to use a shorter receptacle. It is preferred that the receptacle has an open end distal to the headspace, and a closable end proximal to the headspace.

The receptacle advantageously further includes a cap member which is arranged to engage with the tubular member and thereby form part of the receptacle at the closable end thereof so as to create a substantially gas tight seal. The cap member is preferably removable from the tubular member as described above and replaceable thereon, thereby forming the closable end. For example, the cap member may be provided with an internal screw thread for complementary engagement with an external screw thread on the tubular member (or vice versa) . Alternatively an O-ring may be provided for engagement between an internal face of the cap member and an external face of the tubular member.

The sample receptacle is typically of non-porous material such as stainless steel or plastics, for example. The tubular member and the cap member may be of the same non- porous material, or they may be of different non-porous materials .

According to a first embodiment of the present invention, the sample is a plug of soil. Advantageously, the sample fits snugly in the receptacle. Typically, the receptacle has a cutting surface, generally at the closable end. Advantageously, the soil sample may be taken as a plug of soil, whereby the cutting surface is pushed into a lawn, field or the like, such that the plug of soil forms a core which fits snugly in the receptacle. The core is advantageously cut off flush with the closable end of the receptacle so that the volume in the receptacle remains substantially constant for successive tests according to the present invention.

According to a particularly preferred embodiment of the present invention, there is provided a method of determining the permeability of a soil sample which method includes :

(a) providing the sample in a receptacle so as to produce a headspace between the sample and part of the receptacle;

(b) introducing a predetermined volume of a first gas into the headspace; and

(c) monitoring the change in concentration of gas in the headspace, the monitored gas being the first gas and/or a second gas.

According to a second embodiment of the present invention, the sample is of a porous material, such as a textile. Advantageously, such a sample is positioned at the open end of the receptacle and arranged such that a single layer of the sample spans the open end.

The sample may be a plug of soil from land undergoing bioremediation. Advantageously, the permeability is measured for the purpose of achieving optimal air/water mixtures in the soil.

It is also envisaged that the apparatus may include more than one gas sensor, so that the permeability of the sample to more than one gas may be measured simultaneously (for example, oxygen, carbon dioxide and/or H₂S) . For example, when the apparatus is arranged to measure soil, the apparatus includes a C0₂ sensor and an oxygen sensor. Advantageously, the apparatus may include a C0₂ Sensor. The C0₂ Sensor is preferably arranged to measure soil respiration. The measurement of soil respiration may be separate to the measurement of soil permeability or it may be performed concurrently. This measurement would provide a quick check as to when a particular soil being tested was about to enter an anaerobic state. This would be especially useful prior to the apparatus being fully calibrated for a particular soil type.

The headspace preferably has a volume which is predetermined or measured directly.

The receptacle and the oxygen monitoring means are substantially as described hereinbefore. The carbon dioxide monitoring means is preferably a carbon dioxide sensor. A suitable sensor is the model Figaro TGS 4160, which is manufactured by Envin Scientific products Limited, Gloucestershire, United Kingdom.

The amount of the gas which diffuses from the head space into the sample is typically monitored using a gas sensor such as the oxygen sensor type CAG-250 FIS, which is manufactured by Ceramatec Inc, 2425 South 900 West, Salt Lake City, UT 84119, USA.

It is preferred that the first gas is nitrogen. The nitrogen is preferably obtained from the atmosphere using an oxygen absorbing cartridge or the like. The cartridge preferably includes a gas inlet arranged to permit air to enter the cartridge, oxygen absorbing means and a nitrogen outlet arranged to permit nitrogen removed from the air to enter the apparatus. Advantageously, the outlet port is arranged to permit an excess amount of gas to escape; the port is then generally closed prior to the start of measurement. Therefore, the outlet port and/or the inlet port have respective closure means .

It is preferred that the apparatus (or at least the receptacle) is portable (especially hand portable) .

Typically, the means for monitoring the amount of gas which diffuses from the headspace into the sample includes a gas sensor (for example, a gas sensor substantially as described hereinbefore) . Such a gas sensor is typically battery operated.

The apparatus may preferably include a data processing means arranged to produce an output indicative of the permeability of the sample.

When the sample is soil and/or plant matter, the change in oxygen concentration may be due to one or more of two processes. Firstly, the change may be a result of diffusion. (The diffusion may either be from the headspace to the atmosphere through the sample) if oxygen has been introduced, or from the atmosphere to the headspace if a gas other than oxygen has been introduced. Secondly, the oxygen may be used in respiration (this process will, of course, only apply when the sample is soil) .

The rate of diffusive loss or gain from the headspace is determined by the concentration gradient driving the diffusion process (Fick's Law of Gaseous Diffusion) so that concentration change with time is an exponential function determined by soil permeability. Respiratory oxygen consumption effectively remains constant with time.

Therefore, the two processes can be separated mathematically and determined independently by curve fitting oxygen concentration data to Equation 1 and Equation 2 given below.

C_t = (Co - CJe^"Kt + C_∞ Equation (1)

Where t = time (S) ;

C_t = concentration at time t (moles irf³) ; Co = concentration at time zero (moles irf³) ;

C_. = concentration at time infinity (moles m^~3) ;

K is a constant defined by: K = 1/(RV)

R = Diffusion resistance of soil to oxygen (rns^-1 core^-1) ; V = Headspace volume (m³)

(Permeability is the reciprocal of diffusion resistance. Permeability = 1/R and has SI units of m^"1s core) .

The values of all the above variables (C₀, C_t and CJ except for V are provided by the curve fitting procedure. The value for headspace volume (V) can be measured directly so that R can be determined. Alternatively this value may be calculated from the initial oxygen concentration (Co, Equation 1) which results from the addition of or removal from the headspace of a known volume of oxygen;

If concentrations are expressed as mol.πf³ then, at 20°c: Headspace volume (V) = (Volume of 0₂ added x 32.959) / (C₀-8.708)

Where 0₂ is displaced from the headspace by another gas the term ^ΛVolume of 0₂ added' in the above equation becomes negative. In normal soil oxygen is used by the micro-flora and fauna in normal soils the final headspace oxygen concentration (C_∞, Equation 1) will be below atmospheric. The rate of respiratory oxygen use can be calculated from this value and diffusion resistance of the soil (R, Equation 1) .

0₂ use = 2 ( (Atmospheric 0 - CJ /R) Equation 2

Advantageously, the method permits increased accuracy, by enabling changes in oxygen concentration due to respiration by soil organisms to be distinguished from changes in oxygen concentration due to sample permeability. This enables the actual parameter of interest, permeability to oxygen to be measured, which is important in avoiding the development of anaerobic/reducing conditions within the soil and thus poor aesthetics and productivity. Previous methods have measured the diffusion of gasses other than oxygen and assumed similar rates of diffusion. Such a method used in the prior art may be disadvantageous as some gases would result in toxicity problems and necessitate use of a lab.

Therefore, according to a further aspect of the present invention, there is provided a method of determining the permeability of a soil sample, which method includes:

(a) introducing the soil sample into a receptacle so as to produce a headspace between the sample and a portion of the receptacle;

(b) introducing a predetermined volume of a first gas into the headspace;

(c) monitoring the change in oxygen concentration in the headspace; and

(d) determining the change in oxygen concentration in the headspace due to respiratory 0₂ consumption using the equation

C_t = (C₀ - CJ e^"κt + C_∞

Where:

K = 1/(RV)

Headspace volume V = (Volume of 0 added x 32.959) / (C₀-8.708)

Soil 0₂ use = 2 ( (Atmospheric 0₂ - CJ /R)

Where t = time; C_t = headspace 0₂ concentration at time t; C₀ = headspace 0₂ concentration at time zero; C_∞ = 0₂ ^' concentration at time infinity; K is a parameter determined by curve fitting which is characteristic of a particular soil core; R is the diffusion resistance of soil and V = headspace volume.

The permeability of the soil is therefore obtained by calculating the amount of oxygen permeating through the sample, and measuring the total change in oxygen concentration. The amount of oxygen used by the respiration of the soil is therefore calculated. The method is substantially as described hereinbefore.

According to a further aspect of the present invention, there is provided apparatus for determining the porosity of soil, which apparatus includes monitoring means arranged to monitor the change in oxygen concentration in a headspace; means for determining the change in oxygen concentration in the headspace due to respiratory 0₂ (soil 0₂ use) consumption using the equation C_t = (C₀ - CJ e^_κt + C_∞

Where:

K = 1/(RV)

V = (Volume of 0₂ added x32. 959 ) / (C_Q-8.708) Soil 0₂ use = 2 ( (Atmospheric 0₂ - CJ /R) t = time; C_t - headspace 0₂ concentration at time t;

C₀ = concentration at time zero; C„ = concentration at time infinity; K is a constant determined, for a particular core; R is the diffusion resistance of soil and V = headspace volume.

The apparatus^' is preferably substantially as described hereinbefore.

According to yet a further aspect of the present invention, there is provided an apparatus suitable for measuring when a soil is about to enter an anaerobic state, which apparatus includes, a receptacle arranged to receive a sample such that the receptacle and the sample define a head space between the sample and the receptacle, oxygen monitoring means and a carbon dioxide monitoring means.

The receptacle and the oxygen monitoring means are substantially as described before.

Accordingly, this aspect of the present invention further extends to a method of determining when a soil is near an anaerobic state, which method includes:

(a) obtaining a measurement of soil respiration so as to provide a value for oxygen used by the soil in respiration (0₂ Use)

(b) obtaining a value for soil diffusion resistance (R)

(c) calculating an anaerobic state value (ASV) according to the equation:

ASV = R x Soil 0₂ se Wherein the soil 0₂ is at or near an anaerobic state when ASV > 2x atmospheric 0 pressure.

Advantageously, the soil 0₂ use may be calculated from the equation

Soil 0₂ use = 2 ( (atmospheric 0 - CJ /R) Where C_∞ is oxygen concentration at time infinity.

The present invention therefore extends to a method of monitoring soil respiration, which method includes:

(a) providing a sample of the soil in a receptacle so as to produce a headspace between the sample and part of the receptacle; (b) monitoring the change in concentration of oxygen and/or carbon dioxide

According to a particularly preferred embodiment, there is provided a method of monitoring soil permeability or respiration, which method includes:

(a) providing a sample of the soil in a receptacle so as to produce a headspace between the sample and part of the receptacle;

(b) monitoring the change in concentration of oxygen and/or carbon dioxide;

(c) calculating a K value for oxygen (K0₂) and carbon dioxide (KC0₂) using the equation

Ct = (Co - CJ e^"κt + C_∞

(d) using the ratio of K0₂ to KC0₂ to obtain a value of permeability for the soil.

Preferably, the change in concentration of oxygen and carbon dioxide is measured.

According to yet a further aspect of the present invention, there is provided a computer program arranged to calculate 0₂ concentration at time t, using the equation

C_t = (Co ~ CJ e^"κt + C_∞

The computer program is typically for use in a method and/or apparatus substantially as described hereinbefore.

Accordingly, there is further provided according to the present invention a data processing means arranged to produce an output indicative of the permeability of a sample (such as soil) in accordance with the equation

C_t = (Co - CJ e^"κt + C_M

There is further provided a data processing means arranged to produce an output indicative of the permeability of a sample (such as soil) in accordance with the equation

C_t = (Co - CJ e^"κt + C_∞

The data processing means is particularly suitable for use with the method and/or apparatus substantially described hereinbefore.

The output may be audible and/or visual, and may, if desired, be arranged on a display monitor or the like, or sent to printing means for subsequent printing.

The data processing means may include (but is not limited to) a data acquisition package known under the trade mark "Labtech Notebook" manufactured by Strawberry Tree Incorporated of Sunnyvale, California.

Furthermore, whilst the invention is defined above in terms of apparatus and method, the invention also extends to programs or software, particularly software on or in a carrier, adapted for operating the apparatus for putting the invention into effect.

Accordingly, there is provided software arranged to operate an apparatus to determine the porosity of a sample by introducing a predetermined volume of a first gas into a headspace, and monitoring the change in concentration of the first gas and/or a second gas in the headspace. The software is preferably arranged to calculate the porosity using the equation C_t = (C_Q - CJ e^"κt + C_∞.

The program may be in the form of a source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for putting the invention into effect.

The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium such as a ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disc or a hard disk. Further, the carrier may be on a transmissible carrier such as electronic, electromagnetic or optical signal which may be conveyed via electrical or optical cable or by radio or other means.

When the program is embodied in a signal which may be conveyed by a cable or other device or means, the carrier may be constituted by such cable or other device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for putting the invention into effect.

The invention may be more clearly understood from the following illustrative drawings, which are given by way of example only.

Figure 1 is a cross-sectional view of a soil permeability apparatus according to the present invention;

Figure 2 shows exemplary curves for 0₂ diffusion out of the headspace of a soil core having the same diffusion resistance with and without soil 0 consumption;

Figure 3 shows results obtained indicating changes in headspace oxygen with time associated with mechanical compaction of a soil; and

Figure 4 shows results obtained indicating beneficial effects of clover growth.

Referring to Figure 1, a monitoring apparatus is generally indicated by the numeral 1. A plastic cap 2 of circular- section containing a coaxial oxygen sensor 3 (also of circular cross-section) is engageable with a stainless steel tube 4; the space between tube 4 and cap 2 is sealed by an 0-ring 10.

The oxygen sensor 3 is also sealed to cap 2 by a smaller 0- ring 8. Oxygen enters the apparatus (and especially the interior of tube 4) via injection ports 5a and 5b which pass through cap 2. In use, a plug 6 of soil to be measured is removed from, for example, a grass lawn or field; grass 9 growing in the plug 6 of soil (including its associated roots) is allowed to remain intact. Stainless steel tube 4 has a cutting edge 11 which is pushed into a grass lawn, field or the like, such that the cutting edge is distal to the growing grass 9. The plug 6 thereby forms a core which fits snugly in tube 4, advantageously without disturbing the grass 9. The plug 6 is cut off flush with the cutting edge 11, and the cap 2 with the oxygen sensor, is sealed to the tube by the O-ring 10. The tube is prevented from pushing completely into the cap by a spacer 12 so as to produce a headspace 7 is produced between the end of the plug 6 and the oxygen sensor 3.

The mV output from the apparatus is linearly related to 0₂ concentration over a range 0 to 100%. At the start of measurement, a small amount of 0₂ is introduced into the headspace 7 to increase concentration from atmospheric to 25 or 30% and the headspace 7 is sealed. The injection ports 5a and 5b are closed and the progressive decrease in concentration due to outward diffusion and respiratory consumption by the soil sample is measured.

Plugs of soil and growing plant matter are not harmed during measurement and grass or other plant matter growing on the top of the plug can remain intact; thus, a plug can be replaced back into the hole from which it has been removed, without damage, if required.

Referring to Figure 2, it is shown that the curves both without and with respiration fit the following relationship: 0₂ = (25 - 20.9)*e^~κt + 20.9 and 0 = (25 ~18)*e^_κt + 18 respectively.

The value for the decay constant K is determined by soil permeability and is 0.02 for both curves. The difference between curves in this example is determined only by soil respiration. Assuming that head space volume is unity (R = 1/K) . Thus R = 40 and respiration in the second case can be calculated according to Fick's Law as: 0₂ consumption = (2(25- 20.9)/40)

The following Examples illustrate results obtained using apparatus according to the invention; the results are given in schematic form in Figures 3 and 4 of the accompanying drawings. Concentrations in the figures where present are shown as percentages of the oxygen concentration in air i.e 8.71 mol m^~3 or 20.9%.

Examples

1. Effects of mechanical compaction.

The data presented in Figure 3 were generated by samples from pasture land in Cheshire.

Changes in head space p0₂ with time in soil samples fit to the equations: headland 0₂ = (30.74 -20.63) e -°-^003iαt + 20.63 field 0₂ = (28.16 -20.84) e -°-°^0010t + 20.84

The correlation coefficient for the fit of experimental data to these equations is in the excess of r² = 0.9995. Relative permeabilities based on the exponent value are about 30:1.

The very poor permeability in the field samples were the result of mechanical compaction by farm equipment and by over cultivation. Good permeability in samples from uncultivated headlands highlights the extent of the problem and shows that the poor field structure was not intrinsic to the soil type itself. Pasture yields were reduced by 60% but no remedial action had been taken because the specific case was not recognised.

2. Beneficial effects of clover on soil structure.

The effect of clover on structuring problems in soil was analysed. Traces from soil cores from beneath either clover or grass growing in adjacent areas in the same field are shown. Data from three replicates of each treatment are shown in Figure 4; the results give an indication of variability in curve shape. Relative permeability values are presented in the Figure. Correlation coefficients for all exponential curve fits r² > 0.9995.

3. Validation of method in the absence and presence of respiration.

In order to validate the new method a series of experiments were carried out in which estimates resistance to oxygen R0₂ and 0₂ uptake determined as described previously were compared with independent estimates of the same parameters measured in a different way.

The most direct method to check that the exponent in the decay Equation (1) provides a correct measure of soil permeability to 0₂ is to compare estimates of R0₂ based on this decline constant with values calculated directly from measured rates of 0₂ flux across the core. The latter measurements are however complicated in normal soil because 0₂ flux through the core is reduced by the respiratory consumption of this gas. However such measurements can be made in soil with little biological activity and negligible 0₂ uptake. A second possibility for assessing the permeability of normal soil is to measure the flux rate of an inert gas such as He which is not metabolised but possible difficulties in this case can arise in calculating permeability to 0₂ from permeability of another gas of differing molecular weight and water solubility.

The validity of 0₂ uptake estimates obtained with the proposed method can be checked directly by measuring rate of 0₂ depletion in sealed cores

To address the above problems two separate experiments were carried out in biologically inactive soil with and without addition of sucrose and soil innoculant. The rationale for these experiments is as follows: In soil without respiration

(1) Values for resistance to 0 can be calculated directly from measurements of 0 flux through the core using mass spectrometry under steady state conditions (R_3S in Tabs. 1 and 2) and compared with estimates obtained using the decline method (Rdec in Tabs. 1 and 2)

(2) The resistance of the core to He can also be measured using mass spectrometry under steady state conditions (R_Hθ in Tabs. 1 and 2) so that the ratio for the diffusion of these two gases can be obtained.

In amended soil with normal biological activity and respiration

(1) The resistance of the core to He can be measured and used (with the conversion ratio obtained in (1) above) to calculate 0₂ flux and hence R0₂ and these values can be compared with those obtained using the decline method.

(2) 0 uptake values measured directly from the rate of depletion of this gas in sealed cores (Oup _dt in Tabs, land 2) can be compared with those obtained using the decline method as Equation 2 Table 1. Resistance of biologically inactive soil cores to 02 diffusion estimated from either exponential decline in headspace pθ2 or from steady state O2 flux through core. .

3 analytical replicates. Values for C., and K were obtained by curve fitting the decline data to

Equation 1 and, in all cases the correlation coefficient was > 0.999.

Values presented are the mean of six replicates ± standard error. The mathematical steps used to obtain the derived values are shown in the footnote

From decline in headspace O2 concentration

Measured Values Units

Final 0₂ concentration (C_∞) mol rn³ 8.698 ± 0.0023

Decline constant (K) mol s^'N^"1 x 10^"3 3.56 ± 0.123

Headspace volume (V) m³ x 10^"6 11.00

Derived values ^aDiffusivity per core (D) m^"1 s core x 10^"8 3.92 ± 0.136

^■•Diffusion resistance of core to O_z (R^.) ms ¹ core ^x x 10⁶ 25.7 ±0.897

From steady state measurements of 0₂ flux

Measured values 0₂ flux mol s-' x 10^"6 0.359 ± 0.065

He Flux mol s-' x 10^"6 1.07 ± 0.058 Derived values

^■Diffusion resistance of core to 0₂( -_S) ms'¹ x 10⁶ 24.5 + 1.08

"Diffusion resistance of core to He (R_He) ms-'x lO⁶ 8.23 ± 0.432 Ratio Resistance 02/Resistance He 2.96 + 0.0480 ^a D =K xV ^bR_dK = l/ D ^c R.. = (8.71/ 0₂ flux) where 8.71 is the 0₂ gradient (mol m"³) across the core ^d R_He = (8.73/ He flux) where 8J3 is the He gradient (mol rn³) across the core

Measurements of C0 by inactive cores indicated that respiration was negligible (<2 mol s^"1 core-¹ x 10^-10) . This is confirmed by the observed final 0₂ concentration following the diffusive decline in headspace concentration which is not significantly different from atmospheric concentration (8.698 mol m^"3, Tab 1, cf 8.707 mol m^"3 for atmosphere) . Values for the diffusion resistance of the cores to 0₂ derived from the decline in headspace concentration of 25.7 ms^-1 x 10⁶ (Rdec in Table 1) is similar to the estimate obtained from steady state measurements of 0₂ flux of 24.5 ms^-1 x 10⁶ (R_ss in Table 1 ). Diffusion resistance of the core to He derived from steady state measurements (R_HS Table 1) is, as would be predicted on theoretical grounds, considerably lower than resistance to 0₂ . The ratio R_ss/ R_He of 2.96 is slightly higher than that predicted by Graham's Law of 2.83 (the relative rates of diffusion of gases under the same conditions are inversely proportional to the square roots of the molecular weight of those gases) .

Table 2. Diffusion resistance of biologically active soil cores to 02 and He and of respiratory O2 use and CO2 production obtained in several ways.

Values presented are the mean of six replicates + standard error.

From decline in headspace O2 concentration

Measured Values Units

Final 0₂ concentration (CJ mol m^"3 8.56 + 0.028

Decline constant (K) mol s-'V¹ x lO-³ 3.34 ± 0.282

Headspace volume (V) m³ x l0^"6 11.00

Rate of change in p0₂ of capped core (dO/dt) mol m^' x 10^"4 9.88 ± 0.652

Derived values ^aDiffusivity per core (D) m^"1 s core 10^"83.68 ± 0.310 ^bDiffusion resistance of core to O- (R_d 28.1 ± 2.07

O₂ gradient (Δ 0₂ ) mol m^"" 0.148 ± 0.028 ^d0₂ uptake based on 0₂ diffusion gradient and R (Oup_dec) mol core s x 10 0.97 ± 0.137 O₂ uptake based on dO/dt (Oup_dt) mol core^"1 s^"1 x 10 1.09 ± 0.072

From steady state measurements

Measured values

0₂ flux through core mol s^"1 core ^"1 x 10^"6 1.15 ± 0.184 He flux through core mol s^'1 core ^_1 x 10^"δ 0.87 ± 0.113

C0₂ production by core mol s^"1 core ^_1 x 10^'8 1.29 ± 0.160

Derived values ^fApparent diffusion resistance to 0₂ based upon flux (RO_ss) ms^"1 core ^"' x 10⁶ 8.64 ± 1.53 diffusion resistance to He (RHe) ms"¹ core ^_1 x 10⁶ 10.4 ± 1.45 ^hDiffusion resistance to 0₂ calculated from RHe/RO, ratio ms"¹ core ^_1 x 10^s 29.3 ± 4.11 ^aD = K x V ^bR_{d c} = l D ⁰ Δ 0₂₌, 8.708 - C,

O₂ up_d._c = Δ O₂ /(0.5 R) ^e Ou_Pdt = (dO/dt x V) ^fRO_ss = (8.71/ 0₂ flux) as per Tab 1. This estimate is invalid because of respiratory 0₂ consumption (see text) ^sRHe = (8.73/He Flux ) where 8.73 is the He gradient (mol m^"3) across the core. ^hDiffusion resistance of core to 0₂ calculated from conversion ratio in Tab 1 as R0_2(ratio) = RHe x 2.96

Claims

Clai s

1. A method of determining the permeability of a sample which method includes : (a) providing the sample in a receptacle so as to produce a headspace between the sample and part of the receptacle; (b) introducing a predetermined volume of a first gas into the headspace; and (c) monitoring the change in concentration of a gas in the headspace, the monitored gas being the first gas and/or a second gas.

2. A method according to claim 1, wherein the second gas is ammonia, carbon dioxide, carbon monoxide, chlorine, ethylene, hydrogen, hydrogen chloride, hydrogen cyanide, hydrogen sulphide, nitric oxide, nitrogen dioxide, ozone, sulphur dioxide or oxygen (which is preferred) .

3. A method according to claim 1 or 2, wherein the first gas is oxygen, nitrogen, ammonia, carbon dioxide, carbon monoxide, chlorine, ethylene, hydrogen, hydrogen chloride, hydrogen, cyanide, hydrogen sulphide, nitric oxide, nitrogen dioxide, ozone, sulphur dioxide, helium or argon.

4. A method according to any preceding claim, wherein the first gas and the second gas are the same gas.

A method according to any preceding claim, wherein the volume of the headspace is predetermined, measured directly, or inferred from the increase in gas concentration which results from the introduction into the headspace of a known amount of the first gas.

5. A method according to any preceding claim, wherein the sample is a plug of soil (which may include associated plant matter) .

7. A method according to claim 6, wherein the sample and the receptacle form a gas tight seal between the sample and an internal surface of the receptacle.

8. A method according to claim 6 or 7, wherein the receptacle has a cutting surface which is pushed onto the soil such that the sample forms a core which fits substantially in the receptacle (preferably forming the gas tight seal) .

9. A method according to any of claims 6 to 8, wherein the core is cut off substantially flush with a closable end of the receptacle.

10. A method according to any of claims 1 to 5, wherein the sample is a textile.

11. A method according to claim 10, wherein the sample is arranged at an open end of the receptacle, preferably having a single layer of the sample spanning the open end.

12. Apparatus for determining the permeability of a sample, which apparatus includes: a receptacle arranged to receive a sample so that the sample and the receptacle define a headspace; means for introducing a predetermined volume of a first gas into the headspace; and onitoring means for monitoring the change in concentration of the first gas and/or a second gas in the headspace.

13. Apparatus according to claim 12 which includes means for determining the change in oxygen concentration in the headspace due to respiratory 0₂ (soil 0₂ use) consumption using the equation:

C_t = (C₀ - CJ e^"κt + C_∞ Where:

K = 1/(RV)

V = (Volume of 0₂ added x32.959) / (C„-8.708)

Soil 0₂ use = 2 ( (Atmospheric 0₂ - CJ /R) t = time; Ct = headspace 0₂ concentration at time t; C₀ = concentration at time zero; C„ = concentration at time infinity; K is a constant determined, for a particular core; R is the diffusion resistance of soil and V = headspace volume.

14. Apparatus according to claim 12 or 13, which further includes an inlet port and an outlet port, preferably the inlet port and/or the outlet port have respective closing means.

15. Apparatus according to any of claims 12 to 14, wherein the inlet port is arranged to permit entry of the predetermined volume of the first gas into the headspace.

16. Apparatus according to any of claims 13 to 15, wherein the receptacle includes a tubular member.

17. Apparatus according to claim 16, wherein the tubular member has a cross section which is circular, square, rectangular or octagonal.

18. Apparatus according to claim 16 or 17, wherein the cross-section is substantially uniform along the length of the tubular member.

19. Apparatus according to any of claims 13 to 18, wherein the receptacle has an open end distal to the headspace, and a closable 'end proximal to the headspace.

20. Apparatus according to any of claims 13 to 19, wherein the receptacle further includes a cap member arranged to engage with the tubular member and thereby form part of the receptacle at a closable end thereof.

21. Apparatus according to claim 20, wherein the cap member is removable from the tubular member and replaceable thereon, thereby forming a closable end.

22. Apparatus according to claim 20 or 21, wherein the cap member is provided with an internal screw thread for complimentary engagement with an external thread on the tubular member, or an 0-ring is provided for engagement between an internal face of the cap member and an external face of the tubular member.

23. Apparatus according to any of claims 20 to 22, wherein the cap member is of a non-porous material such as steel (preferably stainless steel) or plastics material.

24. Apparatus according to any of claims 12 to 22, wherein the receptacle is of a non-porous material such as steel (preferably stainless steel) or plastics material.

25. Apparatus according to any of claims 12 to 24, wherein the monitoring means includes a gas sensor arranged to monitor the first gas and/or the second gas.

26. Apparatus according to any of claims 12 to 25, wherein the apparatus includes a second monitoring means arranged to monitor a gas.

27. Apparatus according to claim 26, wherein the first monitoring means is an oxygen sensor and the second monitoring means is a carbon dioxide sensor

28. Apparatus according to claim 27, wherein the carbon dioxide sensor and/or the oxygen sensor are arranged to monitor respiration.

29. Apparatus according to any of claims 12 to 28, wherein the receptacle is portable.

30. Apparatus according to any of claims 12 to 29 which is portable.

31. Apparatus according to any of claims 12 to 30, which is removeably connectable to an oxygen absorbing cartridge, preferably the cartridge includes a gas inlet arranged to permit air to enter the cartridge, an oxygen absorbing means and a nitrogen outlet arranged to permit nitrogen removed from the air to enter the apparatus .

32. Apparatus according to any of claims 12 to 31, which includes a data processing means arranged to produce an output indicative of the permeability of the sample.

33. Apparatus for monitoring soil respiration, which apparatus includes: a receptacle arranged to receive a sample so that the sample and the receptacle define a headspace, oxygen monitoring means and carbon dioxide monitoring means.

34. A method of determining the permeability of a soil sample which method includes:

(a) providing the sample in a receptacle so as to produce a headspace of a known volume between the sample and part of the receptacle;

(b) introducing a predetermined volume of a first gas into the headspace; and

(c) monitoring the change in concentration of the first gas and/or a second gas in the headspace.

35. A method of determining the permeability of soil, which method includes

(a) providing the soil sample in a receptacle so as to produce a headspace between the sample and a portion of the receptacle;

(b) introducing a predetermined volume of gas into the headspace;

(c) monitoring the change in oxygen concentration in the headspace; and (d) determining the change in oxygen concentration in the headspace due to respiratory 0₂ (soil 0₂ use) consumption using the equation C_t = (Co - CJ e^"κt + C_∞ Where:

K = 1/(RV)

V = (Volume of 0₂ added x32.959) / (C₀-8.708) Soil 0₂ use = 2 ( (Atmospheric 0₂ - CJ /R) t = time; C = headspace 0₂ concentration at time t; C₀ = concentration at time zero; C_∞ = concentration at time infinity; K is a constant determined, for a particular core; R is the diffusion resistance of soil and V = headspace volume.

36. A method of monitoring soil permeability or respiration, which method includes:

(b) monitoring the change in concentration of oxygen and/or carbon dioxide;

(c) calculating a K value for oxygen (K0₂) and carbon dioxide (KC0₂) using the equation C_t = (C₀ - CJ e~^κt + C„

37. A method of determining when a soil is near an anaerobic state, which method includes:

(a) obtaining a measurement of soil respiration by measuring C0₂ concentration, so as to provide a value for oxygen use by the soil in respiration (0₂ use) ;

(b) obtaining a value of soil diffusion resistance

(R) ( c) calculating an anaerobic state value (ASV) according to the equation

ASV = R x 0₂ use wherein the soil is at or near an anaerobic state when ASV ≥_. 2 x atmospheric 0₂ pressure .

38. Apparatus for determining the porosity of a sample of soil, which apparatus includes monitoring means arranged to monitor the change in oxygen concentration in a headspace; means for determining the change in oxygen concentration in the headspace due to respiratory 0 (soil 0₂ use) consumption using the equation

C_t = (C₀ - CJ e^'κt + C_∞ Where:

K = 1/(RV)

V = (Volume of 0₂ added x32.959) / (C₀-8.708) Soil 0₂ use = 2 ( (Atmospheric 0₂ - C_K) /R) t = time; Ct = headspace 0₂ concentration at time t; C₀ = concentration at time zero; C_∞ = concentration at time infinity; K is a constant determined, for a particular core; R is the diffusion resistance of soil and V = headspace volume.

39. Data processing means arranged to produce an output indicative of the permeability of a sample in accordance with the equation

Ct - (C₀ - CJ e^"κt + C_∞

40 . Data processing means according to claim 39, wherein the output is audible and/or visual .

41. Software arranged to operate an apparatus to determine the porosity of a sample by introducing a predetermined volume of a first gas into a headspace, and monitoring the change in concentration of the first gas and/or a second gas in the headspace .