Manual for Soil Analysis-Monitoring and Assessing Soil Bioremediation Phần 3 pps

60 B.-M. Wilke

• For assessment of the water retention characteristics

• To determine water content at specific matric pressures (e.g., for micro￾bial degradation studies)

• To ascertain the relationship between the negative matric pressures and

other soil physical properties (e.g., hydraulic conductivity, thermal con￾ductivity)

• To determine the drainable pore space (e.g., pollution risk assessment)

• To determine indices for plant-available water in the soil (e.g., for irri￾gation purposes)

Principle. Undisturbed soil samples (soil cores) are used for the measure￾ment at the high matric pressure range 0−100 kPa. The samples are satu￾rated with de-aerated water or calcium sulfate solution (0.005 mol/L) and

subsequently drained using sand, kaolin, or ceramic suction tables (for

pressures from 0 to 20 kPa) and pressure plate extractors (for determina￾tion of pressures from −5 to −1,500 kPa). At equilibrium status, soil samples

are weighed, oven dried and reweighed to determine the water content. The

results are given either as volume fraction or mass ratio. The differences in

volume fractions at different suction pressures give the pore volume (e.g.,

medium pores in vol%), the differences in mass fractions give the water

content retained in these pores. Two standardized (ISO 11274 1998) meth￾ods are described, namely use of sand, kaolin, or ceramic suction tables

for determination of water contents at pressures of 0 to −50 kPa, and use of

pressure plates for determination of pressures from −5 to −1,500 kPa.

Theory. Soil water content and matric pressure are related to each other. At

zero matric pressure the soil is saturated and all pores are filled with water.

As the soil dries matric pressure decreases and pores will empty according

to their equivalent diameter. Large coarse pores (> 50 µm) will drain at

a matric pressure of > −6 kPa, tight coarse pores (10−50 µm) at −6 to

30 kPa, medium pores at −30 to −1,500 kPa, and fine pores at < −1,500 kPa.

■ Sampling

1. It is essential that undisturbed soil samples be used for measurement

at the matric pressure range 0 to −100 kPa, since soil structure has

a strong influence on water-retention properties. Use either undisturbed

cores or, if appropriate, individual peds for low matric pressure methods

(< −100 kPa). Soil cores shall be taken in a metal or plastic cylinder of

a height and diameter such that they are representative of the natural

soil variability and structure. The dimensions of samples taken in the

field are dependent on the texture and structure of the soil and the test

2 Determination of Chemical and Physical Soil Properties 61

Table 2.1. Recommended sample sizes (height × diameter) for the different test methods

Test method Structure

Coarse Medium Fine

Suction table 50 × 100 mm 40 × 76 mm 24 × 50 mm

Pressure plate 10 × 76 mm 10 × 50 mm

method which is to be used. Table 2.1 gives guidance on suitable sample

sizes for the different methods and soil structure.

2. To ensure minimal compaction and disturbance to structure, take soil

cores carefully, either by hand pressure in suitable material or by using

a suitable soil corer. Take aminimum of three representative replicates for

each freshly exposed soil horizon or layer; more replicates are required

in stony soils. Dig out the cylinder carefully with a trowel, roughly trim

the two faces of the cylinder with a knife. If necessary adjust the sample

within the cylinder before fitting lids to each end, and label the top clearly

with the sample grid reference, the direction of the sampling (horizontal

or vertical), the horizon number, and the sample depth.

3. Wrap the samples (e.g., in plastic bags) to prevent drying. Wrap ag￾gregates (e.g., in aluminum foil or plastic film) to retain structure and

prevent drying. Alternatively, excavate undisturbed soil blocks measur￾ing approx. 30 cm3 in the field, wrap in metal foil, wax (to retain structure

and prevent drying), and take to the laboratory for subdivision. Store the

samples at 1−2 ◦C to reduce water loss and suppress biological activity

until they can be analyzed. Treat samples having obvious macrofaunal

activity with a suitable biocide, e.g., 0.05% copper sulfate solution.

■ Sample Preparation

1. To prepare samples for water-retention measurements at pressures great￾er than −50 kPa, trim undisturbed cores flush with the ends of the con￾tainer and replace one lid with a circle of polyamide (nylon) mesh (or

similar close-weave material or paper if the water-retention character￾istic is known) secured with an elastic band. The mesh will retain the

soil sample in the cylinder and enable direct contact with the soil and

the porous contact medium. Avoid smearing the surface of clayey soils.

Remove any small projecting stones to ensure maximum contact and

correct the soil volume if necessary. Replace the other lid to prevent

drying of the sample by evaporation. Prepare soil aggregates for high

matric pressure measurements by leveling one face and wrapping other

62 B.-M. Wilke

faces in aluminum foil to minimize water loss. Disturbed soils should be

packed into a cylinder with a mesh attached. Firm the soil by tapping

and gentle pressure to obtain a specified bulk density.

2. Weigh the prepared samples. Ensure that the samples are brought to

a pressure of less than the first equilibration point by wetting them, if

necessary, by capillary rise, mesh side or leveled face down on a sheet of

foam rubber saturated with de-aerated tap water or 0.005 mol/L calcium

sulfate solution. Weigh the wet sample when a thin film of water is seen

on the surface. The time required for wetting varies with initial soil

water content and texture. Soils are ideally field moist when the wetting

is commenced. General guidelines for wetting times are:

sand: 1–5 days

loam: 5–10 days

clay: 5–14 days or longer

peat: 5–20 days.

Very coarse pores are not water filled when the soil sample is saturated

by capillary rise.

2.4.1

Determination of Soil Water Characteristics

Using Sand, Kaolin, and Ceramic Suction Tables

Principle. Suction tables are suitable for measurement of water contents at

matric pressure from 0 to −50 kPa. A negative matric pressure is applied to

coarse silt or very fine sand held in a rigid watertight non-rusting container

(a ceramic sink is particularly suitable). Soil samples placed in contact

with the surface of the table lose pore water until their matric pressure is

equivalent to that of the suction table. Equilibrium status is determined by

weighing samples on a regular basis, and soil water content by weighing,

oven drying, and reweighing.

■ Equipment

• Large ceramic sink or other watertight, rigid, non-rusting container with

outlet in base (dimensions about (50×70×25 cm) and close-fitting cover

• Tubing and connecting pieces to construct a draining system for the

suction table

• Sand, silt, or kaolin, as packing material for the suction table (Com￾mercially available graded and washed industrial sands with a narrow