24
Appendix 4: Soil sampling
To determine the moisture retention characteristic or the pF-curve of a specific soil, undisturbed core samples
must be collected. This is because of the major influences of both pore size distribution and soil structure on
moisture retention, especially at the high matrix potentials of the operating range of suction tables.
There is no explicit prescription in literature for recommended sample sizes. Optimal sizes for core rings are
determined by the size of structural elements in the soil. To obtain representative data, sample sizes should be
large with respect to the size of soil aggregates, cracks, root channels or animal holes. From a practical point
of view, sample diameters should not be too large as not to reduce the amount of simultaneously analysable
samples, and sample height should be constrained to several centimeters; so that equilibrium conditions are
reached in a reasonable period of time.
According to the Dutch NEN 5787 standard, samples with a volume between100 and 300 cm³ are usually used for
the suction tables, while samples with a height of more than 5 cm are discouraged, because the time needed to
establish equilibrium will be long, and the accuracy of determination of pF-values near saturation will be low.
In the procedures for soil analyses of the International Soil Reference and Information Centre (ISRIC), sample
rings with a diameter of 5 cm and a volume of 100 cm³ are recommended, while in other publications heights
of 2 or 3 cm are preferred.
Eijkelkamp Soil & Water recommends the use of a 100 cm³ volume core ring, with an inner diameter of 50 mm
(outer diameter 53 mm) and a height of 51 mm.
When pressing the core rings into the soil, care should be taken not to disturb the original setting of the soil
and to completely fill the ring. Sampling conditions are best when the soil is approximately at field capacity.
Eijkelkamp Soil & Water supplies a number of standard soil sample ring kits (with art. no: 0753SA, 0753SC and
0753SE (for rings Ø 53 mm), 0760SC for rings Ø 60 mm and 0784SC for rings with Ø 84 mm).
Ring holders may be used to facilitate insertion, especially in the subsoil. After insertion to the desired depth,
the rings are carefully dug out (e.g. using the spatula provided with the Eijkelkamp sample ring sets), at some
centimeters below the ring itself. The surplus of soil is reduced to a few millimeters, trimming it carefully with
a fine iron saw, and the caps are placed on the ring for protection and to minimise evaporation losses. The
remaining surplus of soil will protect the sample during transport and will be removed in the laboratory, prior
to analysis. Transport the core rings in a protective case (art. no. 070201 for Ø 53 mm or 070202 for Ø 60/84 mm).
Since soil structure and pore size distribution have significant influence on soil water retention, several replicate
samples are needed to obtain a representative pF-value. Depending on natural variability of the study area,
three to six replicate samples per unit are advised.
In case the samples cannot be analysed on short notice, store the samples in a refrigerator to reduce microbial
activity which might cause non-representative changes in soil structure.
Do not freeze the samples because soil structure will be influenced.