Chapter 3 GENERAL MATERIALS AND METHODS
3.4 Soil analysis methods
All soil analysis was conducted at CSBP Limited laboratory according to standard analysis procedures outlined by (Loss 2012). The methods and references in this section are publicly available from this laboratory and permission to be published in this thesis has been authorised by the author Dr Stephen Loss (see Appendix 9.3). Unless specified otherwise, all soils were dried at 40C for 24 hours or until dry, then crushed and sieved to less than 2 mm. Most of the following methods are based on those described by (Rayment and Lyons 2011). These analytical methods are outlined in: P soil analysis Table 3.4, other soil nutrient analyses Table 3.5, soil physical and chemical analysis Table 3.6, and plant, water and fertiliser analysis methods (see Table 3.7).
3.4.1 International and Australia methods for soil, plant and water analysis
There is no single standard for the analysis of P in soil and the method varies depending on location (Rayment and Lyons 2011). The two currently used methods used in Australia are (Olsen et al. 1954; Colwell 1965; ISO 1994). For example, the United States of America prefers the Olsen Method as outlined by Hughes et al. (2000) for the analysis of P. Even within Australia the analysis of P varies, with the Eastern Australia states preferring the Olsen method while Colwell is the current standard method in Western Australia (Brennan and Bolland 2007). Analysis of water, plant and material for nutrient content is conducted using inductively coupled plasma (ICP) spectrometry (ISO 1987; Zarcinas et al. 1987).
3.4.2 Limitations and analytical methods for soil, plant and water analysis
In this thesis, the samples collected for analysis are only as accurate as the sampling techniques used. The samples collected for this research have been collected to provide a representative sample for a site and is meant to reflect the range of samples from a location since it was not possible to analyse every treatment in the experiment. All analytical methods have some inherent variability in the measurements that they produce because of small variations in sample preparation and instrumentation.
The accuracy of any analysis can vary depending on the type of analysis and as such methodologies have been developed to ensure that any error is minimised and ensure that results are consistence. The reporting limits for the following tests in Australia are: Colwell P (+/- 2), Total P (+/- 1), pH (CaCl2) (+/- 0.2), and EC (+/- 0.01). As analysis is conducted in batches, a number of control samples are included in the batch in random locations that have known results, and these are then checked to ensure that the methods and instrumentation are operating correctly and the results from each sample are accurate.
Table 3.4 Soil analysis methods for phosphorus*.
Analysis Method References
Colwell phosphorus and potassium (mg/kg)
“Using a soil to solution ratio of 1:100, soils are extracted with 0.5 M sodium bicarbonate solution adjusted to pH 8.5 for 16 hours. The acidified extract is treated with ammonium molybdate/antimony trichloride reagent and the phosphorus is measured colorimetrically at 880 nm using a discrete analyser. The potassium in the extract is determined using a flame atomic absorption spectrophotometer at 766.5 nm.”
(Colwell 1965; Rayment and Lyons 2011)
Olsen (mg/kg) “Soil are extracted at a ratio of 1:20 with NaHCOз (pH 8.5) for 30 minutes. The orthophosphate ion reacts with ammonium molybdate and antimonyl tartrate, under acidic conditions to form a phosphomolybdic acid complex. This complex is reduced with ascorbic acid to form a blue complex which adsorbs light at 880 nm and can be detected on a discrete analyser.”
(Olsen et al. 1954; Rayment and Lyons 2011) Total phosphorus “Soils are digested in sulphuric acid in the presence of a BDH Kjeldahl catalyst tablet in a microwave.
The total P concentration measured colourimetrically at 880 nm after incubation with the colouring reagent ammonium molybdate/potassium antimonyl tartrate in an acid medium.”
(Allen and Jeffery 1990b; Rayment and Lyons 2011) Acid extractable “Acid extractable P can be used to measure the labile P component in the pool. The test favour’s
extraction of Ca-bound P, many forms of which are not available to plants and, as such, the test can overestimate P availability. Soils are extracted for 16 hours with 0.005 M sulphuric acid and the P in the cleared extract is determined colourimetrically on a discrete analyser.”
(Rayment and Helyar 1980; Rayment and Lyons 2011) Phosphorus buffering
index (PBI)
“Phosphorus Buffering Index is measured by the amount of P sorbed by the soil when the solution concentration of P is increased by 100 (mg/mL). After extraction with a calcium chloride (+) sodium dihydrogen phosphate solution, this method determines the phosphorus buffering index of soils colorimetrically with ammonium molybdate/ammonium metavanadate reagent using a discrete analyser.”
(Allen and Jeffery 1990b; Rayment and Lyons 2011)
Phosphorus retention index (PRI)
“Phosphorus retention index is defined as the ratio of the adsorbed phosphorus to the equilibrium concentration. The amount of phosphorus adsorbed/desorbed by each gram of the soil pads is the difference between the initial concentration of phosphorus (Po) and the equilibrium concentration (Peq). Phosphorus in soils is extracted in a 0.02 M potassium chloride equilibrating solution in a ratio of 1:20 for 16 hours, and the concentration of P in the resulting solution (Peq) is determined colorimetrically on a discrete analyser.”
(Allen and Jeffery 1990b)
Table 3.5 Other soil nutrient analyses methods.
Analysis Method References
Nitrate and ammonium (mg/kg)
“Soil nitrate nitrogen and ammonium nitrogen are extracted with a 2 M potassium chloride solution for 1 hour at 25C. After dilution the resulting soil solution is measured on a Lachat Flow Injection Analyser. Ammonium is measured colorimetrically at 630 nm using the indo-phenol blue reaction. Nitrate is reduced to nitrite through a copperised-cadmium column and the nitrite is also measured colorimetrically at 520 nm.”
(Searle 1984; Rayment and Lyons 2011)
Sulphur (mg/kg) “Plant available sulphur in soils is determined by extraction with a 0.25M potassium chloride solution for 3 hours at 40C. The sulphur content of extracts are analysed by Inductively Coupled Plasma Spectrometry. This method is known as the KCI-40 or Blair/Lefroy Extractable Sulphur method.”
(Blair et al. 1991; Rayment and Lyons 2011)
Organic carbon (%) “In the (Walkley and Black 1934) method concentrated sulphuric acid is added to soil wetted with dichromate solution. The chromic ions produced are proportional to oxidised organic carbon and are measured colorimetrically at 600 nm on a plate reader.”
(Rayment and Lyons 2011) Total K (Kjeldahl) “Total K samples are digested with sulphuric acid and a Kjeldahl copper catalyst tablet in a microwave.
Diluted samples are read for K using a flame atomic absorption spectrophotometer at 766.5 nm.”
(Allen and Jeffery 1990a) Skene K (HCl) “This test is used to determine the available K fraction in the soil. Skene K values are typically lower
than Colwell K values when the soils being tested are alkaline. Soils are extracted for 1 hour in 0.05 M hydrochloric acid and the resulting extract is read for K using a flame atomic absorption spectrophotometer at 766.5 nm.”
(Haysom 1971; Rayment and Lyons 2011)
Total nitrogen “Total nitrogen is determined by Dumas high temperature combustion (LECO analyser), where soil samples are loaded into a combustion tube at 950 C and flushed with oxygen. All gases generated are collected and measured on an infrared detector for carbon and a thermal conductivity cell for nitrogen.”
Table 3.6 Soil physical and chemical analysis methods.
Analysis Method References
Soil pH and electrical conductivity (ds/cm)
“Using a soil to solution ratio of 1:5, soils are extracted in deionised water for 1 hour. The water pH and electrical conductivity of the extract are measured using a combination pH and conductivity electrode. After the water pH and EC have been measured, calcium chloride solution is added to the soil solution to the equivalent of 0.1 M and after thorough mixing for 10 minutes the calcium chloride pH is also measured. All measurements are recorded while the solution is stirred.”
(Rayment and Lyons 2011)
Texture “Texture is assessed by wetting the soil and feeling the wet soil between the forefinger and thumb. Six texture categories are used: sand (1.0), loamy sand (1.5), loam (2.0), clay loam (2.5), Clay (3.0) and heavy clay (3.5).”
Colour “Soils are classified into the following colours: white, grey, yellow, brown, orange, red, pink and black. More than one colour may be included (e.g. brown yellow) and light and dark may also be assigned.”
(Munsell 2000) Gravel content “The gravel content is estimated visually and by running the fingers through the soil. Approximate
figures are reported, e.g. 10‒15 (%).”
Physical observations “Unprepared soil samples are laid on a cardboard tray and soil texture, colour and gravel content are estimated by a quick physical observation. These procedures were devised to give an indication whether the soil type changed between samples, not as an accurate measurement of the soil characteristics. Our classification systems do not correspond to any other systems, e.g. Northcote classification”
(Northcote 1979)
Particle size “Prepared soil samples are treated with hydrogen peroxide to remove the organic matter, and then shaken with a 1:1 Calgon-Sodium Hydroxide mixture to disperse the soil particles. Using a table of particle sedimentation times, 25 mL aliquots of the solution are removed at the set times and the remaining sample is sieved. The aliquots are evaporated in an oven and weighed to determine the coarse and fine sand, silt and clay contents”
(Indorante et al. 1990)
Moisture, ash and organic matter (%)
“Fresh soil samples are weighed heated in a furnace to 100C overnight and weighed a second time to determine percentage moisture. Soils are then heated in a furnace at extreme temperatures to determine loss on ignition (400C for 4 hours) and percentage ash (600C for 2 hours) values. Loss on ignition and percentage ash measurements are used to estimate of organic matter content of the soil.”
(Rayment and Lyons 2011)
Exchange acidity (meq/100 g)
“Soils are extracted with 1 M KCl in a 1:5 ratio for 1 hour, and the exchangeable acidity Al³+ and H+ are measured by titration with NaOH and hydrochloric acid. Exchangeable Al³+ and H+ are held on exchange sites is largely dependent on the pH of the soil. The method for Al can be used to replace CaCl extractable method and the exchange acidity value is particularly useful for high pH soils.”