Soil quality — Guidance on soil temperature measurement

This document outlines methodologies for soil temperature measurement and provides guidance on the selection of a measurement method depending on measurement purposes. It also gives guidance on characteristics, performance and use of infrared (IR) thermometers which is now widely applied to obtain rapid measurements and thermistors which have been commonly used to obtain more accurate measurements.

Qualité du sol — Recommandations relatives au mesurage de la température du sol

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ISO 4974:2023 - Soil quality — Guidance on soil temperature measurement Released:11. 07. 2023
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Soil quality — Guidance on soil
temperature measurement
Qualité du sol — Recommandations relatives au mesurage de la
température du sol
Reference number
ISO 4974:2023(E)
© ISO 2023

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ISO 4974:2023(E)
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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  © ISO 2023 – All rights reserved

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ISO 4974:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Fundamentals .1
4.1 Principle . 1
4.2 Soil temperature measurement methods . 2
4.2.1 General . 2
4.2.2 Conventional measurement . 3
4.2.3 Quick measurement . 4
5 Selection of measurement methods . 5
5.1 General . 5
5.2 Method selection in the soil quality field . 5
5.3 Typical application in the soil quality field . 6
5.3.1 Conventional measurement . 6
5.3.2 Quick measurement . 6
6 Quality assurance and quality control . 6
7 Test report . 6
Annex A (informative) Soil temperature measured by thermistor and IR thermometer .7
Bibliography . 9
© ISO 2023 – All rights reserved

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ISO 4974:2023(E)
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bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see
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This document was prepared by Technical Committee ISO/190, Soil quality.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at
  © ISO 2023 – All rights reserved

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ISO 4974:2023(E)
Soil temperature varies continuously in response to climate and meteorological changes and the
interaction of soil and atmosphere. Some of the factors that affect soil temperature include diurnal and
annual cycles, and irregular episodic changes in weather (i.e. radiation, cloudiness, drought, humidity,
atmospheric temperature, rainfall, cold events). Landscape formation, regional differences, vegetation
and soil management practices by humans also affect soil temperature. The other major influence on
soil temperature variation is depth below the ground surface, with the soil temperature shifting in
peaks moving deeper in the soil profile .
Soil temperature variations across day and year can be quite important. In boreal conditions within
the year, soil temperature can pass from −15 °C to more than 15 °C at 5 cm depth. If in drylands in
Mediterranean areas, the temperature can reach 40 °C at depth and more than 50 °C at the ground
surface. With global warming, soil temperature tends also to increase in the same order as atmospheric
temperature as demonstrated in various studies .
Soil temperature needs to be measured being crucial for biogeochemical processes such as length of
growing season, decomposition of soil organic matter, rates of mineralization and nutrient assimilation
by plants as well as plant productivity. Soil temperature changes with depth influence many soil
characteristics, such as microbial activity, chemical reactions, nutrient cycling, and many other
processes. It is known that soil organic matter is more easily decomposed by microbes activated
when the temperature is raised if appropriate water content is available and temperature stays in a
range favourable for microbes. Soil temperature can then be used as a proxy for the risk of carbon
dioxide emission from target land.
Soil temperature is, as mentioned earlier, also a basic parameter in agricultural sciences and industries
since it directly affects plant growth. Soil temperature has a strong influence on the microbial
mineralization such as ammonification of nitrogen contained in organic forms to ammonium and
nitrification thereof to nitrite and nitrate in agricultural soil. This contributes to growth, especially
during sprouting. In agricultural businesses, using greenhouses, controlling soil temperature is a key
to successfully growing vegetables for a good harvest by optimizing the growth speed and depressing
the activities of worms and other soil organisms that eat vegetables. In golf courses, soil temperature
is continuously monitored for proper maintenance of greens to condition the grass growth, leaf colour
and green density.
In the civil engineering field for ground disaster prevention, soil temperature is used as a useful
indicator suggesting groundwater flow routes. By carefully measuring soil temperature at many points
in sloping ground to map the values 3-dimensionally, water streams underground can be tracked. As
groundwater can be a trigger for the start of massive soil blocks sliding on a slope and an accelerator of
the movement, draining groundwater therefrom reduces the risk of landslides. Thus, soil temperature
is helpful in efficiently managing landslide prevention work.
The procedure for soil temperature measurement depends on the accuracy required for the given
purpose for which the data are required. To know a soil temperature range in target land for climate
change studies, the measurement can be done in a simple and quick way which indicates soil temperature
as an approximate value. In contrast, to precisely map soil temperature distributions in the ground for
slope disaster prevention, the temperature should be carefully measured to obtain not approximate but
exact values. Selection of appropriate measurement methods for soil temperature should be discussed
prior to site investigation.
© ISO 2023 – All rights reserved

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Soil quality — Guidance on soil temperature measurement
1 Scope
This document outlines methodologies for soil temperature measurement and provides guidance on
the selection of a measurement method depending on measurement purposes.
It also gives guidance on characteristics, performance and use of infrared (IR) thermometers which is
now widely applied to obtain rapid measurements and thermistors which have been commonly used to
obtain more accurate measurements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 11074, Soil quality — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 11074 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
soil temperature
temperature measured at a targeted spot in the soil or at the soil surface
4 Fundamentals
4.1 Principle
Soil temperatures are measured for specific purposes. As is well known, carbon dioxide is emitted
from the ground as a result of organic carbon decomposition in soil in a temperature range where
microbes are active and able to decompose the organic matter. The soil temperature range, therefore,
works as a good indicator of carbon emission risks. In this context, soil temperature can be measured
as an approximate value, rather than as an exact value of high accuracy. For the purpose of climate
change studies, quick measurement of soil temperature is thus allowable, the precision of which is not
necessarily the same compared to other methods conventionally applied.
When the primary interest is in soil temperature distributions in target land, obtainable information
can include indication of groundwater flow locations that directly affect soil temperature. On slopes,
where groundwater streams channel the ground, they destabilize it and can cause the surface ground
to move. Differences in soil temperature between spots, even if very small, can indicate spatial locations
of groundwater flows. Soil temperature should thus be determined at high accuracy in order to make
the differences meaningful. The higher the accuracies of obtained temperature values, the more clearly
water flows can be indicated in the ground.
© ISO 2023 – All rights reserved

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ISO 4974:2023(E)
As described in 4.2.2, there are a variety of direct temperature measuring devices such as thermometers
that can be used to measure soil temperatures. The most convenient to use are often thermistors. They
typically give the value to an accuracy of 0,01 °C and have been widely used for a long time as a practical
instrument when exactly determining the temperature of target objects including soil.
IR detection has been applied to check fever in humans by applying an IR thermometer at the exit of
the ear canal or simply pointing the measuring device directly at the skin surface on the forehead or
wrist. The measurement can be completed in less than 10 s at an accuracy of 0,1 °C at least. By applying
an IR thermometer to a hole drilled in the ground, soil temperature can be similarly, easily and quickly
measured. It is not necessary for an IR thermometer to be attached directly to the wall or bottom
of a hole in the ground. Instead, it can be placed at the top of a hole. This operability enables remote
measurement of soil temperature.
The relationship between soil temperature values measured by thermistor and IR thermometer is
illustrated in Annex A, Figure A.1. No significant difference is found between the two values but the gap
in the accuracy exists as mentioned above.
As described in 4.2.2 and 4.2.3, soil temperature measurement methods can be classified into two
options. One is a conventional method, and the other a quick method. The former is applied for given
purposes to obtain exact temperature values, even if taking time, while the latter to obtain approximate
values quickly. Quick methods can be characterized as screening methods, the term of which is defined
in ISO 12404:2021, 3.2.
4.2 Soil temperature measurement methods
4.2.1 General
Temperature cannot be measured directly. It can only be estimated by its influence on some properties
of matter that responds to variation in the intensity of heat in an object’s body where the examples
of matter include mercury, bimetal strips, platinum wires, thermocouples and gas. Changes with
temperature in properties such as volume, length, electric resistance, thermal electromotive force (emf)
and pressure have been found most useful for practical temperature measurement. In order for all of
these properties to give the same indication of the temperature of a given body, calibration techniques
have been established, and certain standard references have been agreed upon. Thermometers are
instruments built to take advantage of any of these properties of matter.
NOTE Mercury is no longer used as such matter due to its toxicity. However, for good understanding of
temperature measuring principles, it is mentioned as matter which was typically used historically.
When estimating the temperature of a

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