7.8 Subprogramme SW: Soil water chemistry

• 7.8.1 Introduction
• 7.8.2 Methods
• 7.8.2.1 Field methods and sampling
• 7.8.2.2 Laboratory analyses
• 7.8.3 Quality assurance/Quality control
• 7.8.4 Data handling
• 7.8.5 Data reporting
• 7.8.6 References

7.8.1 Introduction

Acidic water percolating through the soil dissolves and weathers minerals, releasing base cations for nutrient uptake by microbes and roots alike, for seepage to deeper layers and ground water, and ultimately for outflow to rivers and lakes. Soil water is intimately coupled with the chemical and biological processes in the upper soil layers and is sensitive to both acidification and nitrogen pollution. The SW subprogramme is therefore one of the most essential subprogrammes for understanding geohydrochemical interaction with biological/microbiological effects at both plot and catchment scales.

It is therefore unfortunate that soil water sampling should be so fraught with difficulties. The installation of soil water collectors, whether traditional zero-tension lysimeters or suction samplers, results in disturbance that affects the integrity of the soil water being sampled. As installation affects diminish with time, those associated with the plugging of the collector can be expected to increase. Separating the soil water phase from the soil matrix, with which it is continually attempting reach an equilibrium, can be expected to alter its chemistry. Similarly, the degassing of CO₂ from soil water brought to equilibrium with the atmosphere can be expected to alter its chemistry. The spatial variability in soil water chemistry and flow is known to be considerable resulting unrealistic numbers of samplers being required to meet true mean 95% confidence levels. It is not possible to measure the soil water flux with suction samplers since the soil volume from where the water is drawn is not known and varies with soil moisture content. Flux values based on zero-tension lysimeters are also probably suspect because of altered hydrological conditions resulting from the design and installation of the lysimeter. Tension samplers collects a different fraction of the soil water than zero-tension lysimeters, and both do not collect the soil water in which plant roots and microbes are in intimate contact. Nevertheless, soil water sampling provides important information about the status of the soil, indicating nutrient and toxicological conditions for plant roots and microbes, and—when coupled with water fluxes—nutrient, acidity, and pollutant leaching.

7.8.2 Methods

7.8.2.1 Field methods and sampling

Principles
Soil water can be sampled for monitoring purposes by using either zero-tension lysimeters, which collects percolate (gravitational) water, or with suction samplers (porous plates, cups), which collect percolate and water held by the soil up to the tension applied and that can reach the sampler during the time the suction is on. For a comparison of zero-tension lysimeters and suction samplers see e.g. Barbee & Brown (1986) and Haines et al.(1982). Furthermore, the suction (tension or negative pressure) applied to the suction sampler can be constant or a falling (see Nordic Council of Ministers 1989). Soil water can also be sampled by centrifugation (e.g. Reynolds 1984, Elkhatib et al. 1987), but this method involves removing a soil sample to the laboratory each time, and therefore unsuited for monitoring.

Choice of sampler
It is recommended to use suction cup samplers. Cups are made in a variety of materials, all of which may affect the sample to some extent (e.g. Nagpal 1982, Debyle et al. 1988, Raulund-Rasmussen 1989, Hughes & Reynolds 1990, Grossmann & Udluft 1991). It is therefore not possible to recommend one type of cup above another; only that they should be made of reasonably inert material and not weather easily. The IM Programme Centre will collect a list of samplers and suppliers that have been used to date within ICP IM.

Cup samplers come in a variety of forms (see Nordic Council of Ministers 1989 for illustrations). Usually it consists of a small cup made of porous material, the open end of which is attached to non-porous tubing through which the vacuum is applied and the sample retrieved. If the cup is attached to a non-porous tube of the same diameter as the cup, the sample can be collected within the body of the sampler itself (e.g. Soilmoisture Equipment Corporation soil water samplers, model 1900). Often, however, the buried cup is attached to a capillary tube which extends up through the soil surface and connects to a vessel in which both the sample is collected and the vacuum applied.

Sampling design and installation
The samplers should be located in upland (non-organic) SC plot(s) only; and at least in the SC plot considered the most important for the IM site. A randomized or systematic pattern may be used for the allocation of the samplers, although local factors (stones, low water yield) may make a more subjective allocation necessary. To avoid unnecessary walking on the plot, the samplers are best located around the edges of the plot so that they can be serviced from outside the plot, or in a separate area nearby.

Soil water chemistry and hydrology is extremely variable, both spatially and temporally. To be within the 95% confidence limit of the plot's true mean soil water solute concentrations would almost certainly require an unrealistic number of samplers. Therefore, rather than trying to determine true plot mean solute concentrations, emphasis should be on whether there is change in soil water chemistry collected at the same limited number of locations within the plot over time. The ratios of solutes, e.g. Ca/Al, NO₃N/NH₄N, probably show less spatial variability than the individual parameters.

Towards this aim, at least 3-6 samplers per depth should be installed. The depth to install the samplers should be within the 10-20 cm layer and the 30-50 cm layer, i.e. within and below the main rooting zone (0=mineral soil surface). If possible, record the horizon in which the cup is located. Suction samplers do not work well in humus layers because of difficulties in maintaining good soil contact. Therefore, if possible, it is recommended to install small zero-tension lysimeters immediately under the humus layer. A nest of samplers (one at each depth) can be installed at locations along the edge of the plot (see SC Chapter Fig. 7.7.2).

The installations should be made in such a way that disturbances are minimised, e.g. by using a soil auger. Ensure good contact with the cup and the soil by pouring a slurry made of local soil material, having first removed any stones and gravel, and water into the hole. It is also good practice to attach a piece of nylon string to the cup so that it can be relocated if the capillary tubing is severed below ground by animals. In addition, it is good to have the capillary tubing inside another tubing to protect it from burrowing animal damage.

Replace/reinstall a sampler if it persists in not collecting a sample. Rather than continuing on from abandoned samplers, new samplers start a new time series. This because of the high degree of spatial variability, even over short distances (m). The risk of increased weathering of ceramic cups and progressive plugging of samplers in general may also be a reasons for replacement after some years.

Sampling
Apply a suction of 0.3–0.6 bars to the sampler. Depending on the type of sampler, soil type and soil moisture conditions, the vacuum should be placed on the sampler for a period of 18 hours to two weeks. The samplers can be connected to large vacuum vessels (2 litres) which are able to maintain such a suction without the need for repeated pumping. Either a falling or a constant vacuum system may be used. Maintenance of the vacuum depends on whether the pores of the cup dry-out, letting air pass in. Therefore, pore size is important–the smaller the pores, the more difficult it is for the cup to dry out.

Attempt to take at least 1 sample per month. Record the volume of sample collected so that volume weighted monthly mean concentrations can be calculated (NB. the sample volume can not be used to calculate a water flux since the area from where the sample came is unknown). In sites with snow accumulation, samples are usually not collected during the snow period.

Samples of small volume (<25-50 ml) may be rejected because such samples often have extremely variable chemistries and are atypical of the bulk soil water chemistry (Starr 1985). Because of the large spatial variability likely to be encountered, it is recommended that the samples are not composited but analyzed individually. If this is not possible all the time, then bulk samples only from the same depth.

Use acid-washed collection vessels. These should be periodically replaced throughout the season. The samples should be transferred to acid-washed polyethylene bottles for transport to the laboratory (preferably in cold boxes) as soon as possible.

For details on handling water chemistry samples, see Chapter 8.2.

7.8.2.2 Laboratory analyses

The transport and storage period should be kept to a minimum. The set of parameters to determine are given in data reporting part 7.8.5. Total N has been added, and enables organic N to be calculated.

The analytical techniques described in the previous manual (EDC 1993) are still valid. Priority in the analysis schedule should be given to the non-metal determinations: pH, N compounds, DOC etc. Acid should be added to ensure desorption of metals from the walls of the storage bottle (e.g. 0.5 ml conc. HNO3 suprapur quality per 100 ml sample). During analysis, the samples should be kept in a dark and cold store (+4°C).

Because the porous cup itself acts as a filter, filtering of the sample for analysis is probably unnecessary. Tests in Sweden have found little effect on the concentrations of DOC and total Al collected with P80 cups (from Hoechst). Indeed filtering may actually contaminate the samples (REF). Further testing is advised before using expensive membrane filters. However, filtering for some analytical procedures may be necessary, e.g. with ion chromatography to preserve the exchange columns for longer. It is also important to note the limits of some techniques, e.g. the detection limits for metals determined by ICP, at least emission spectrometer models, are relatively high. It may be necessary to analyse a subset of samples by AAS/graphite furnace in order to get more exact concentrations.

7.8.3 Quality assurance/Quality control

Each NFP is expected to ensure that good laboratory practice is followed and are responsible for the quality of data reported to ICP IM Programme Centre. For methods on checking data, QC and precision and accuracy, see American Public Health Association. 1985. The results of quality controls, laboratory intercalibrations etc. undertaken (either with IM samples in particular or of the laboratory in general) should be reported to the IM Programme Centre. The Programme Centre also encourages participation in international intercalibration exercises.

A simple check can be made to see if the sum of cations is balanced by the sum of anions. If there is a difference that can not be explained by any missing ions, then this should be brought to the attention of the laboratory. Other simple checks include looking at scatter plots between the parameters, e.g. SO4S - Total S concentrations and strongly correlated, PO4P - Total P concentrations and strongly correlated, NO3N + NH4N - total N concentrations, and Total inorganic N (NO3N + NH4N) strongly correlated to DOC. Careful screening for outliers can substantially reduce the variability of the data. If any of the metal concentrations determined with an ICP in simultaneous mode are outliers, then there is reason to check the chemistry of the whole sample.

See data quality management in Chapter 8.

7.8.4 Data handling

In order to calculate solute leaching, soil water flow estimates are required. Although methods to measure soil water fluxes exist (e.g. tensiometry, TDR), they are not widespread or routine (Cassel & Nielsen 1986). There are a number of models which can compute runoff (drainage to ground water in this case) and since they rely on the laws of physics, they can be rather reliable (e.g. SOIL, Jansson 1991). However, such models require data that is difficult to obtain and considerable training in order to run them. Water balance/soil water deficit models probably offer the simplest approach (Nordic Council of Ministers 1989, Dingman 1994). The problem of obtaining soil water flux values for IM sites clearly needs to be further looked into. Any developments should be brought to the attention of the IM Programme Centre.

7.8.5 Data reporting

Mandatory and optional parameters

Example files

SW example Excel file
SW example ASCII file

• File identifier SUBPROG states the subprogramme.     
• MEDIUM refers to the type of soil coded according to the FAO soil classification (see SC subprogramme).     
• LEVEL refers to the depth of the base of the sampler from the mineral soil surface (cm).     
• Spatial pool SPOOL refers to the number of individual lysimeters used for discrete soil levels.     
• If the sample volume is recorded and the sampling is done more than once a month, concatenations values should be given as volume weighted means (see Annex 7), and flagged with W. Single monthly values are reported without status. Soil water flow, if available, is reported as a monthly mean. General information on flags is available in Chapter 4. For values below detection limit see Annex 7.
• Sampling year and month are given as YYYYMM, day field is left blank.

7.8.6 References

American Public Health Association. 1985. Standard methods for the examination of water and wastewater. 16th edition. ISBN 0-87553-131-8. xlix + 1268 pp.

Barbee, GC & Brown, KW , 1986. Comparison between suction and free-drainage soil solution samplers. Soil Sci. 141(2), 149-154.

Cassel, DK & Nielsen, DR, 1986. Field capacity and available water capacity. Chap. 36. In: Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods-Agronomy Monograph no. 9. 2nd ed. (Ed: Klute, Arnold) American Society of Agronomy-Soil Science Society of America, Madison, USA, 901-926.

Debyle, NV; Hennes, RW & Hart, GE, 1988. Evaluation of ceramic cups for determining soil solution chemistry. Soil Sci.146, No. 1, 30-36.

Dingman, SL, 1994. Physical Hydrology. Macmillan Publishing Company. ISBN 0-02-329745-x, XIV, 575 pp.

Elkhatib, EA; Hern, JL & Staley, TE, 1987. A rapid centrifugation method for obtaining soil solution. Soil Sci. Soc. Am. J. 51, 578-583.

Grossmann, J & Udluft, P, 1991. The extraction of soil water by the suction-cup method: A review. Journal of Soil Science 42, 83-93.

Haines, BL; Waide, JB & Todd, RL, 1982. Soil solution nutrient concentrations sampled with tension and zero-tension lysimeters: report of discrepancies. Soil Sci. Soc. Am. J. 46, 658-661.

Hughes,S & Reynolds, B, 1990. Evaluation of porous ceramic cups for monitoring soil-water aluminium in acid soils: comment on a paper by Raulund-Rasmussen (1989). Journal of Soil Science 41, 325-328.

Jansson, P-E. 1991. Simulation model for soil water and heat conditions. Description of the SOIL model. Swedish University of Agricultural Sciences, Dept. of soil Science, Uppsala. Report 165.72 pp.

Nagpal, NK, 1982. Comparison among and evaluation of ceramic porous cup soil water samplers for nutrient transport studies. Can. J. Soil Sci. 62, 685-694.

Nordic Council of Ministers, 1989. Methods for Integrated Monitoring in the Nordic countries. Miljørapport 1989:89. 280 pp.

Raulund-Rasmussen, K, 1989. Aluminium contamination and other changes of acid soil solution isolated by means of porcelain suction-cups. Journal of Soil Science 40, 95-101.

Reynolds, B, 1984. A simple method for the extraction of soil solution by high speed centrifugation. Plant and Soil 78, 437-440.

Starr, MR, 1985. Variation in the quality of tension lysimeter soil water samples from a Finnish forest soil. Soil Sci. 140(6), 453-461.