Soil testing

Note: tests marked * are not part of the scope of accreditation.

Methods

102 Sample Preparation

Test Method no. 102(i) Dry Soil Sample Preparation

Soil for chemical analyses is dried by being spread out in a tray and placed in a forced air drying cabinet kept at around 35 °C. Dry soil is ground in a roller mill until it passes a 2-mm sieve. Where a test uses a small sample weight, a more homogeneous subsample is prepared by further grinding using a ring and puck mill to pass a 0.25-mm sieve. The procedure is based on that described by Metson (1971).

• Metson, A.J. 1971. Methods of Chemical Analysis for Soil Survey Soils. New Zealand Soil Bureau Bulletin 12.

Test Method no. 102(ii) Wet Soil Sample Preparation

Soil for biochemical analyses is sieved in a field-moist condition by gently pushing through a sieve of mesh size in the range 4 – 6mm. If the soil is too wet, such that it smears on the sieve, it is first partially air-dried by spreading out in a thin layer and blowing room-temperature air across the surface with a domestic fan and turning regularly by hand. During sieving, material such as grass and stones are picked out and removed together with anything that may have been live at the time of sampling such as roots and earthworms. The procedure is based on that described by Ross (1971).

• Ross, D.J. 1971. Modifications to the fumigation procedure to measure microbial biomass C in wet soils under pasture: influence on estimates of seasonal fluctuations in the soil biomass. Soil Biology and Biochemistry 20(3):377 – 383.

104 Moisture Factor

Test Method no. 104(i) Moisture Factor
(Formerly 104)

Since the moisture content of an air-dry soil varies with time and conditions of storage such as air temperature and humidity, it is more satisfactory to express analytical results on an oven-dry rather than an air-dry basis.

However, as oven-drying at 105 °C may cause changes in some chemical properties air-dry samples are analysed and a moisture factor is applied in the calculation to convert results to an oven-dry basis. A subsample of air-dried soil is weighed into a can and oven-dried overnight at 105 °C. The loss in weight due to moisture driven off is used to calculate a moisture factor for that sample. The procedure is based on that described by Blakemore et al (1987).

• Blakemore, L.C.;Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. NewZealand Soil Bureau Scientific Report 80.

Test Method no. 104(ii) Water Content

A subsample of as-received soil is weighed into a can and oven-dried overnight at 105 °C. The loss in weight due to moisture driven off is used to calculate a water content for that sample. The procedure is based on that described by Rayment & Lyons (2011).The result is an “as-received” water content. It is not the true field-moist water content as significant wetting or drying may have occurred in the interval between sampling and receipt at the laboratory.

• Rayment, G.E. andLyons, D.J. 2011. Soil Chemical Methods – Australasia.

Test Method no. 104(iii) Water Holding Capacity

Water holding capacity (WHC) is the amount of water retained by the soil after it has been saturated and allowed to drain overnight. Soil biochemical tests are routinely carried out at a target soil water content equivalent to a certain percentage (usually 60%) of the sample’s WHC, as results can be affected by very wet or dry conditions.

Water holding capacity is determined by keeping fresh soil overnight with water at a 1:2 ratio in a filter funnel plugged with glass wool and stoppered. The stopper is then removed and the soil allowed to drain for 3 hours. The water content of the saturated soil is then determined using method 104(ii). The procedure is as described by Harding and Ross (1964).

• Harding, D.E. and Ross, D.J. 1964. Some factors in low-temperature storage influencing the mineralisable nitrogen of soils. Journal of the Science of Food and Agriculture15: 829 – 834.

105 Bulk Density

Samples for bulk density measurement are collected by coring, where a thin-walled metal cylinder of known volume is pushed into the soil at right angles to the ground surface. The cylinder is then dug out and the soil at either end pared to a plane surface. The soil core is gently pushed out of the cylinder using a plunger, bagged and transported to the laboratory where it is oven-dried at 105 °C and weighed. The procedure is based on that described by Gradwell & Birrell (1979).

• Gradwell, M.W. and Birrell, K.S. 1979. C. Methods for Physical Analysis of Soils. New Zealand Soil Bureau Scientific Report 10C.

106 pH

Test Method no. 106(i) pH in Water 1:2.5 ratio
(formerly 106)

In this method, a 1:2.5 suspension is stirred vigorously then left to stand overnight before measurement with a pH electrode. The method is based on that described by Blakemore et al (1987).

This ratio is that recommended in 1930 by the Soil Reaction Committee of the International Society of Soil Science. Variation of the ratio of soil to water within the range 1:1 to 1:10 does not alter the pH much, values being around 0.1 to 0.3 pH unit higher for the more dilute suspensions (Piper, 1942) probably owing to dilution of CO₂ absorbed in the soil sample (Whitney & Gardner, 1943). Standing overnight allows more reproducible results (Metson, 1971.)

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Metson, A.J. 1971. Methods of Chemical Analysis for Soil Survey Soils. New Zealand Soil Bureau Bulletin 12.
• Piper, C.S. 1942. Soil and plant analysis. University of Adelaide, Adelaide. 368p.
• Whitney, R.S. and Gardner, R. 1943. The effects of carbon dioxide on soil reaction. Soil Science 55:127 – 141.

Test Method no. 106(ii) pH in Water 1:5 ratio
(formerly 106)

In this method, a 1:5 suspension is shaken for 30 minutes then measured with a pH electrode. The extract can also be used for the measurement of conductivity. The method is based on that described by Blakemore et al (1987).

Variation of the ratio of soil to water within the range 1:1 to 1:10 does not alter the pH much, values being around 0.1 to 0.3 pH unit higher for the more dilute suspensions (Piper, 1942) probably owing to dilution of CO₂ absorbed in the soil sample (Whitney & Gardner, 1943).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Piper, C.S. 1942. Soil and plant analysis. University of Adelaide, Adelaide. 368p.
• Whitney, R.S. and Gardner, R. 1943. The effects of carbon dioxide on soil reaction. Soil Science 55:127 – 141.

Test Method no. 106(iii) pH in 0.01M CaCl₂
(formerly 106)

Water is the usual suspension medium for soil pH measurements; however electrolyte solutions are often preferred because fluctuations in soluble salts due to seasonal conditions, soil moisture content, and fertiliser inputs cause less variation in pH values.

Peech (1965) advocated the use of 0.01M CaCl₂ in place of water as the suspension medium for soil pH measurements because it is similar in electrolyte composition to soil solutions found at optimum moisture conditions for plant growth in soils with low salt concentration. The overall effect of using salts is to lower the measured pH values. For New Zealand soils results with 0.01M CaCl₂ are about 0.5 to 1 pH unit lower than with water.

In this method, a 1:2.5 suspension of soil and 0.01M CaCl₂ is stirred vigorously then left to stand overnight before measurement with a pH electrode. The method is based on that described by Blakemore et al (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Peech, M. 1965. Hydrogen-ion activity. Agronomy 9: 914 – 926.

Test Method no. 106(iv) pH in 1 M KCl
(formerly 106)

Water is the usual suspension medium for soil pH measurements; however electrolyte solutions are often preferred because fluctuations in soluble salts due to seasonal conditions, soil moisture content, and fertiliser inputs cause less variation in pH values.

When 1M KCl is used as the suspension medium, extensive ion exchange takes place including the release of aluminium. Hydronium and other proton donors are brought back into solution thus lowering the measured pH. It has been suggested (Black, 1968) that pH values measured with 1M KCl may approach those in the ion atmosphere of the original soil. The overall effect of using salts is to lower the measured pH values. With KCl the effect varies considerably, and the pH is often more than 1 pH unit lower than results obtained with water.

In this method, a 1:2.5 suspension of soil and 1 M KCl is stirred vigorously then left to stand overnight before measurement with a pH electrode. The method is based on that described by Blakemore et al (1987).

• Black, C.A. 1968. Soil- Plant Relationships. 2nd edn. Wiley, New York. 792p.
• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.

108 pH in H₂0₂

This test is used to indicate the presence of oxidisable sulphides in soils and sediments. It is used in Australia to identify acid sulphate soils where soil and drainage water acidity problems will emerge when the soil or sediment is exposed to air. In New Zealand it is used to identify similar problems related to mining overburdens. If present, sulphide ions are oxidised with heated hydrogen peroxide to form sulphuric acid, with a consequential lowering of pH. If the pH declines to 3.5 or less then problems with acidity in the sample can be expected.

In this method, field moist soil is heated with 30% H₂O₂ then left to cool before measurement with a pH electrode. The method is based on that described by Ford & Calvert (1970).

• Ford, H. W.; Calvert, D. V. 1970. A method for estimating the acid sulphate potential of Florida soils. Soil and Crop Science Society of Florida Proceedings 30: 304-307.

110 Electrical Conductivity

In this method, a 1:5 soil:water mixture is shaken for 30 minutes, allowed to settle then the conductivity measured with a temperature-compensated probe. An approximate soluble salts value may be derived from the conductivity using the empirical relationship:

Soluble salts (%) = Conductivity (dS/m) × 0.35

The method is based on that described by Blakemore et al (1987). Reporting is now in line with SI convention and uses units of dS/m rather than the historic mS/cm. Numerically though, dS/m and mS/cm are equivalent.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.

Test Method no. 112 Loss on Ignition (LOI)

Combustion time and temperature are critical factors that vary between published LOI methods and may affect the comparability of results. This method uses 500 °C for 4 hours (Blakemore et al, 1987), but these parameters may be specified otherwise.

Various factors may be used to convert soil organic matter, as determined by loss on ignition, to soil organic carbon; however these are strongly dependent on soil properties, such as the amount and type of clay and the amount of carbonates and sesquioxides (Goldin, 1987). Results therefore are not reported as soil organic matter or carbon content, but only as loss on ignition at a specified temperature and time.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Goldin, A. 1987. Reassessing the use of loss-on-ignition for estimating organic matter content in non-calcareous soils. Communications in Soil Science and Plant Analysis 18: 1111 – 1116.

114 Organic Carbon and Total Nitrogen

Soils are analysed using a Leco TruMac  which utilises the Dumas dry combustion principle. Samples are combusted in a stream of pure oxygen at 1050 °C. Moisture is removed from the combustion gases via a thermo-electric cooler and the gases are routed into a ballast to equilibrate. An aliquot is subsampled and passed through a heated copper catalyst which converts the various forms of nitrogen to N₂ which is then measured by a thermal conductivity detector. At the same time the CO₂ produced from carbon present is measured in an infrared detector cell (Leco, 2003).

Inorganic carbon (e.g. carbonate and bicarbonate) is not combusted at the temperatures used in this method. For most New Zealand soils with a pH below 7, the free carbonate content is negligible (Miller, 1968) and therefore the total carbon content obtained using Dumas combustion without acid pretreatment may be taken as the total organic carbon content of the soil (Metson et al, 1979.) Charcoal, however, is measured by this technique and if present will contribute to the result.

A conversion factor of 1.72 can be used to convert organic carbon to organic matter, based on the assumption that organic matter contains 58% organic C (Nelson and Sommers, 1996).

• Leco, 2003. Total/organic carbon and nitrogen in soils. LECO Corporation, St. Joseph, MO, Organic Application Note 203-821-165.
• Metson, A.J., Blakemore, L.C. and Rhoades, D.A. 1979. Methods for the determination of soil organic carbon: a review, and application to New Zealand soils. NZ Journal of Science 22:205 -228.
• Miller, R.B. Soil pH, calcium carbonate and soluble salts. Soils of New Zealand, Part 2. NZ Soil Bureau Bulletin 26(2):50 – 54.
• Nelson, D.W. and L.E. Sommers. 1996. Total carbon, organic carbon, and organic matter. In: Methods of Soil Analysis, Part 2, 2nd ed., A.L. Page et al., Ed. Agronomy. 9:961-1010. Am. Soc. of Agron., Inc. Madison, WI.

116 Total Kjeldahl Nitrogen, Phosphorus & Potassium (formerly Total Kjeldahl Nitrogen)

Samples are digested using the Kjeldahl wet oxidation process as described by Blakemore et al (1987). Nitrogen in the sample is converted to NH₄-N by sulphuric acid digestion in the presence of a copper catalyst and sodium sulphate to raise the boiling point of the mixture. The completed digest is diluted to avoid precipitationer, nitrogen in the digest is determined colorimetrically as NH₄-N using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998a).

Taylor (2000) reported that a reasonable estimation of total phosphorus can be determined using this digestion. Using a QuikChem 8500 flow injection analyser, orthophosphate (PO₄-P) in the digest reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998b). Overall results from this digestion method average 4% lower than the fusion and dry ashing methods and are expressed as Total Kjeldahl Phosphorus.

Total Kjeldahl Potassium may also be measured in the digest using optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES. It is not always a good approximation of total soil potassium as recoveries may be as low as 5 – 10% of the true total (Rayment & Lyons 2011.)

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998a Quik Chem Method 13-107-06-2-D.
• Lachat Instruments, Milwaukee, WI, USA. 1998b Quik Chem Method 13-115-01-1-B.
• Rayment, G.E. and Lyons, D.J. 2011. Soil Chemical Methods – Australasia. 495p.
• Taylor, M.D. 2000. Determination of total phosphorus in soil using simple Kjeldahl digestion. Communications in Soil Science and Plant Analysis 31(15/16): 2665-2670.

118 Mineral Nitrogen

Mineral nitrogen (i.e. NH₄-N and NO₃-N) levels represent immediately plant-available nitrogen and are often used as indicators of biological activity and soil fertility. However, as these forms are extremely labile levels can fluctuate rapidly and results must be interpreted with caution.

In this method, ammonium and nitrate are extracted with 2M KCl using a 1:10 soil:extractant ratio and a 1 hour end-over-end shake followed by filtration. The procedure is that described by Blakemore et al (1987). NH₄-N is determined colorimetrically using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998c) and NO₃-N by Cd reduction and NEDD colorimetry (Lachat 1998d), both using a QuikChem 8500 flow injection analyser.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998c Quik Chem Method 12-107-06-3-A.
• Lachat Instruments, Milwaukee, WI, USA. 1998d Quik Chem Method 12-107-04-1-B.

120 Anaerobic Mineralisable Nitrogen

The complexity of the nitrogen cycle and the instability of mineral forms mean that the mineral nitrogen test is a snapshot of levels present in a single sampling. Measuring potentially mineralisable nitrogen can provide a more realistic indication of soil nitrogen bioavailability.

Anaerobic mineralisable nitrogen is determined as the difference between the NH₄-N measured in two separate soil extracts using 2M KCl. The first is extracted immediately and the second after a 7-day anaerobic incubation at 40 °C with the soil covered by water. The incubation is carried out in sealed containers with minimal headspace. The warm temperature accelerates the rate of microbial activity and thus the rate of conversion of organic matter to NH₄-N, while the anaerobic conditions prevent conversion of the NH₄-N to NO₃-N. The incubation method is adapted from that of Keeney and Bremner (1966) with extractions as described by Blakemore et al (1987).

After incubation, 2.5M KCl is added to bring the solution up to 2M KCl before extraction by shaking end-over-end for 1 hour then filtering. Both the initial and the day 7 extracts are analysed for NH₄-N colorimetrically on a QuikChem 8500 flow injection analyser using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998c).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Keeney, D.R. and Bremner, J.M. 1966. Comparison and evaluation of laboratory methods of obtaining an index of soil nitrogen availability. Agron. J. 58: 498–503.
• Lachat Instruments, Milwaukee, WI, USA. 1998c Quik Chem Method 12-107-06-3-A.

121 Aerobic Mineralisable Nitrogen

The complexity of the nitrogen cycle and the instability of mineral forms mean that the mineral nitrogen test is a snapshot of levels present in a single sampling. Measuring potentially mineralisable nitrogen provides a more realistic indication of soil nitrogen bioavailability.

Aerobic Mineralisable Nitrogen is determined as the difference between the ammonium and nitrate measured in two separate soil extracts using 2M KCl. The first is extracted immediately and the second after aerobic incubation for 56 days at 25 °C. The incubation is carried out at a target soil water content equivalent to about 60% of the sample’s water holding capacity (WHC), as mineralisation can be affected by very wet or dry conditions. The method is described by Parfitt et al (2005).

After incubation, ammonium and nitrate are extracted with 2M KCl using a 1:10 soil:extractant ratio and a 1 hour end-over-end shake followed by filtration. NH₄-N is determined colorimetrically using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998c) and NO₃-N by Cd reduction and NEDD colorimetry (Lachat 1998d), both using a QuikChem 8500 flow injection analyser.

• Lachat Instruments, Milwaukee, WI, USA. 1998c Quik Chem Method 12-107-06-3-A.
• Lachat Instruments, Milwaukee, WI, USA. 1998d Quik Chem Method 12-107-04-1-B.
• Parfitt, R.L., Yeates, G.W., Ross, D.J., Mackay, A.D. and Budding, P.J. 2005. Relationships between soil biota, nitrogen and phosphorus availability and pasture growth under organic and conventional management. Applied Soil Ecology 28:1 – 13.

124 Olsen-Available Phosphorus

Available phosphorus tests attempt to extract a similar fraction of phosphate to that accessible to plants. This method is based on that of Olsen et al (1954), as described by Blakemore et al (1987). In this method, soils are shaken end-over-end at a 1:20 ratio, with 0.5M sodium hydrogen carbonate adjusted to pH 8.5, for 30 minutes, then filtered. This process is carried out at 25 °C. Using a QuikChem 8500 flow injection analyser, orthophosphate, PO₄-P, in the extract reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998e).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998e Quik Chem Method 12-115-01-1-G.
• Olsen, S.R.; Cole, C.V.; Watanabe, F.S.; Dean, L.A. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Department Circular 939.

126 Bray 2-Soluble Phosphorus

Available phosphorus tests attempt to extract a similar fraction of phosphate to that accessible to plants. This method is based on the phosphorus extraction method of Bray and Kurtz (1945), as described by Blakemore et al. (1987). Soils are shaken by hand for 40 seconds with a reagent which is 0.3 M NH₄F and 0.1 M HCl, at a ratio of 1:10 soil:extractant. Using a QuikChem 8500 flow injection analyser, orthophosphate, PO₄-P, in the extract reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998f).

Davis (1994) has used the Bray-2 extract to also measure extractable potassium and magnesium, where flame atomic absorption spectrophotometry is used to measure the extracted cations.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Bray, R.H.; Kurtz, L.T. 1945. Determination of total, organic and available forms of phosphorus in soils. Soil Science 59: 39 – 45.
• Davis, M.R. 1994. Topsoil properties under tussock grassland and adjoining pine forest in Otago, New Zealand. New Zealand Journal of Agricultural Research, 37: 465 – 469.
• Lachat Instruments, Milwaukee, WI, USA. 1998f Quik Chem Method 12-115-01-1-A.

128 0.5M H₂SO₄-Soluble Phosphorus

Phosphorus removed by this extractant represents the amount of inorganic phosphorus that is present in the soil but not strongly occluded. The extraction is that described by Blakemore et al. (1987). Soils are shaken end-over-end with 0.5M H₂SO₄ at a ratio of 1:200 soil:extractant for 16 hours, then filtered. Using a QuikChem 8500 flow injection analyser, orthophosphate, PO₄-P, in the extract reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998f).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998f Quik Chem Method 12-115-01-1-A.

130 Organic Phosphorus

The organic phosphorus fraction is determined from the increase in 0.5M H₂SO₄-soluble phosphorus caused by ignition of the soil at 550 °C for 60 minutes, which converts organic phosphorus to inorganic phosphate. The ignited soil is then extracted by shaking end-over-end with 0.5M H₂SO₄ at a ratio of 1:200 soil:extractant for 16 hours. The extraction is that described by Blakemore et al (1987). Using a QuikChem  8500 flow injection analyser, orthophosphate, PO₄-P, in the extract reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998f).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998f Quik Chem Method 12-115-01-1-A.

132 Phosphate Retention

Phosphate retention is an empirical measure of the ability of a soil to remove phosphorus rapidly from solution. The process is considered to be a precursor to the much slower process of phosphorus fixation, which renders phosphorus unavailable to plants. The test is sometimes referred to as anion storage capacity (ASC), particularly in the soil fertility context where it is used as a measure of the possibility of rapid leaching of mobile anions.

Samples are shaken for 16 hours with a 1,000 mg/L P solution at pH 4.6. The method was devised by Saunders (1965) so that the concentration of phosphorus used gives a high degree of differentiation between soils of low and high phosphorus retention ability, and the pH used is close to the point of maximum phosphate retention in many soils. After centrifuging the phosphorus left in solution is determined using a QuikChem 8500 flow injection analyser. Orthophosphate (PO₄-P) in the extract reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998g).

• Lachat Instruments, Milwaukee, WI, USA. 1998g Quik Chem Method 12-115-01-1-J.
• Saunders, W.M.H. 1965. Phosphate retention in New Zealand soils and its relationship to free sesquioxides, organic matter and other soil properties. New Zealand Journal of Agricultural Research 8:30 – 57.

134 Phosphate Sorption Curves

Phosphate sorption isotherms as originally described by Fox and Kamprath (1970) were used in estimating whether a soil is able to maintain a level of phosphate (arbitrarily around 0.08 mg/L) adequate for crop growth in the soil solution. There is now increasing use of the isotherms to estimate sludge or effluent loadings to soil at an allowable output phosphate concentration of leachate to the environment (Barry et al, 1995).

This method is adapted from that described by Daly and Wainiqolo (1993). It involves equilibrating samples of a soil for 16 hours with different amounts of phosphate in 0.01 M CaCl₂. The amount of phosphorus left in solution is measured using a QuikChem 8500 flow injection analyser, where orthophosphate (PO₄-P) in the solution reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998g). Phosphorus left in solution is plotted against phosphorus added.

• Barry, G.A., Chudek, P.J. Best, E.K. and Moody, P.W. 1995. Estimating sludge application rates to land based on heavy metal and phosphorus sorption characteristics of soil. Water Research 29 (9): 2031-2034.
• Daly B.K.; Wainiqolo, J.L. 1993. Methods of Analysis for Agricultural Samples: Soil, Plant, Animal Feed and Water. Fiji Agricultural Chemistry Laboratory Tech Report 03/93. 115p.
• Fox R.L., Kamprath, E.J. 1970. Phosphate sorption isotherms for evaluating the phosphate requirements of soils. Soil Science Society of America Proceedings 34(5):902-907.Lachat Instruments, Milwaukee, WI, USA. 1998g Quik Chem Method 12-115-01-1-J.

140 Bases - Shake

Test Method no. 140(i) Bases - 1:50 Shake

When a sample of soil is shaken with molar ammonium acetate buffered to pH 7 the ammonium ions occupy all available anionic sites by displacing the charge-balancing cations into solution. This method uses a 1:50 soil:extractant ratio and a 60-minute shake. Due to the shorter contact time between the extractant and the soil and the mode of extraction, levels of exchangeable bases measured may be slightly lower than found using the traditional and more strictly correct leaching method. The differences are not considered significant in practical terms.

The ammonium acetate extracts are filtered then analysed for the bases K⁺, Na⁺, Ca²⁺ and Mg²⁺ by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES. The method is based on that described by Blakemore et al (1987).

Traditionally results for exchangeable cations were expressed in milliequivalents per 100 g soil (me/100g or me%), but as equivalent weight (molecular weight divided by charge) is not used in the SI system, the unit now used is centimoles of positive charge per kg (cmol(+)/kg). This has the same numeric value as the old me/100g.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.

Test Method no. 140(ii) Bases - 1:20 Shake

When molar ammonium acetate buffered to pH 7 is shaken with a sample of soil the ammonium ions occupy all available anionic sites by displacing the cations into solution. This method uses a 1:20 soil:extractant ratio and a 30-minute shake. It is possible that for soils with high levels of calcium and magnesium the 1:20 ratio will not quantitatively extract all of these elements. The method is only used for measuring bases in conjunction with CEC estimation procedure by pH drop (test method no. 145).

The ammonium acetate extracts are filtered then analysed for the bases K⁺, Na⁺, Ca²⁺ and Mg²⁺ by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES. The method is derived from that described by Blakemore et al. (1987).

Traditionally results for exchangeable cations were expressed in milliequivalents per 100 g soil (me/100g or me%), but as equivalent weight (molecular weight divided by charge) is not used in the SI system, the unit now used is centimoles of positive charge per kg (cmol(+)/kg). This has the same numeric value as the old me/100g.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.

142 Bases - Leaching

When a sample of soil is leached with molar ammonium acetate buffered to pH 7 the ammonium ions occupy all available anionic sites by displacing the charge-balancing cations into solution. This method uses an automatic extractor to leach molar ammonium acetate through the soil at a steady rate over a 2-hour period. The extractor has been described by Holmgren et al (1977). The leachates are then analysed for the bases K⁺, Na⁺, Ca²⁺ and Mg²⁺ by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES. The method is derived from that described by Blakemore et al. (1987).

Traditionally results for exchangeable cations were expressed in milliequivalents per 100 g soil (me/100g or me%), but as equivalent weight (molecular weight divided by charge) is not used in the SI system, the unit now used is centimoles of positive charge per kg (cmol(+)/kg). This has the same numeric value as the old me/100g.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Holmgren, G.G.S., Juve, R.L. and Geschwender, R.C. 1977. A mechanically controlled variable rate leaching device. Soil Science Society of America Journal 41:1207 – 1208.

144 Cation Exchange Capacity

Test Method no. 144(i) Cation Exchange Capacity (CEC)

Soil is covered with ammonium acetate and left to soak for about 15 minutes then leached over a 2-hour period using a mechanical extractor to remove exchangeable bases. Excess ammonium acetate is washed from the sample with alcohol. The adsorbed ammonium ions are then displaced from the exchange sites by a 2-hour leach with molar sodium chloride. NH₄-N in the sodium chloride leachate are determined colorimetrically on a QuikChem 8500 flow injection analyser using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998) to give the CEC. The method is based on that described by Blakemore et al (1987).

Traditionally results for cation exchange capacity were expressed in milliequivalents per 100 g soil (me/100g or me%), but as equivalent weight (molecular weight divided by charge) is not used in the SI system, the unit now used is centimoles of positive charge per kg (cmol(+)/kg). This has the same numeric value as the old me/100g.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Lachat Instruments, Milwaukee, WI, USA. 1998b Quik Chem Method 12-107-06-1-B.

Test Method no. 144(ii) Cation Exchange Capacity (CEC) by pH Drop
(formerly 145)

When molar ammonium acetate buffered to pH 7 is shaken with a sample of soil the ammonium ions occupy all available anionic sites by displacing the cations into solution. This method uses a 1:20 soil:extractant ratio and a 30-minute shake. The lower ratio is used to give adequate sensitivity to the CEC estimation procedure but it means that the measurement of exchangeable bases in the same extract may not be quantitative.

The pH of the ammonium acetate extract is measured and the amount of exchangeable hydrogen in the soil is derived from a relationship published by Brown (1943). By adding the exchangeable hydrogen to the exchangeable bases (both expressed as cmol(+)/kg) the CEC can be estimated.

Traditionally results for cation exchange capacity were expressed in milliequivalents per 100 g soil (me/100g or me%), but as equivalent weight (molecular weight divided by charge) is not used in the SI system, the unit now used is centimoles of positive charge per kg (cmol(+)/kg). This has the same numeric value as the old me/100g.

• Brown, I C. 1943: Rapid method of determining exchangeable hydrogen and total exchangeable bases of soils. Soil Science 56: 353-357.

146 Exchangeable Aluminium

The four exchangeable bases plus aluminium represent virtually all the exchangeable cations normally present in soil. Measurement of exchangeable aluminium is of importance as high levels can be potentially harmful to plant roots. This method uses 1M KCl in a ratio of 1:5 soil:extractant. After 16 hours standing the extract is filtered and analysed for aluminium by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES. It is adapted from that described by Blakemore (1987).

Exchangeable aluminium values decline to negligible amounts in soils with a pH above about 5.6 (Rayment & Lyons, 2011). If this test is performed on such soils erroneous values may be obtained, as clay is dispersed in the extract leading to errors in the aluminium measurement.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.Rayment, G.E. and Lyons, D.J. 2011.Soil Chemical Methods – Australasia. 495p.

148 Reserve Potassium

This value, which was proposed by Metson et al. (1956), represents the long­ term potassium-supplying power of the soil. The method disregards the more readily extractable fraction of the non-exchangeable potassium and represents the nearly constant rate of release after several extractions.

In order to determine this value, the more readily soluble potassium is removed by a single extraction with 1M HNO₃, at a wide acid:soil ratio. This fraction is discarded. Two or three successive extractions with 1 M HNO₃, at a narrower acid:soil ratio are then made and the K concentration in these extracts is deter­mined using optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES). The results of these two or three extractions, which should be fairly simi­lar, are averaged to give the reserve potassium (K_c) value. The method is adapted from that described by Blakemore et al (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Metson, A. J.; Arbuckle, R. H.; Saunders, M. l. 1956. The potassium-supplying power of New Zealand soils as determined by a modified normal-nitric-acid method. Transactions of the 6th International Congress of Soil Science B: 619-627.

150 Acid Soluble & Reserve Magnesium

This value represents the acid-soluble 'reserve' magnesium (Mgr) of the soil (Metson and Brooks 1975). It is calculated by determining the magnesium soluble in boiling 1 M HCl (Mgas) and subtracting the exchangeable magnesium (Mgex) obtained by leaching with 1 M ammonium acetate (see Method 142). Magnesium is measured using optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES). The method is adapted from that described by Blakemore et al (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Metson, A. J.; Brooks, Jean, M. 1975: Magnesium in New Zealand soils. 11. Distribution of exchangeable and reserve magnesium in the main soil groups. NZ Journal of Agriculture Research 18: 317-335.

152 Calcium Carbonate

This weight loss method uses 1 + 1 HCl to evolve CO₂ from carbonate in the sample, the weight loss being recorded using a top-pan balance. A correction for evaporation weight loss is applied to allow for the loss of water vapour and HCl. Recoveries of approximately 97% are obtained from added CaCO₃. The method is adapted from that described by Blakemore (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.

154 Calcium Nitrate-Extractable Metals (formerly 159)

Calcium nitrate is used as a neutral salt extractant as a means of estimating immediately bioavailable metal. Metals extracted by the reagent (1:6 soil:extractant, 0.01M CaNO₃, 2 hours shaking) are measured either by ICP-OES or subcontracted for ICP-MS. The method is described by McLaren et al (2005).

• McLaren, R.G., Clucas, L.M. and Taylor, M.D. 2005. Leaching of macronutrients and metals from undisturbed soils treated with metal-spiked sewage sludge. 3. Distribution of residual metals. Australian Journal of Soil Research 43: 159-170

156 DTPA-Extractable Metals (formerly 160)

The DTPA (diethylenetriamine penta-acetic acid) method, although designed for slightly acid or alkaline soils, has been widely used in the US to estimate potential bioavailability of metals. Metals extracted by the reagent (1:2 soil:extractant, 0.005M DTPA, pH 7.3, 2 hours shaking) are measured by ICP-OES. The method is described by Lindsay and Norvell (1978).

• Lindsay, W.L.; Norvell, W.A. 1978. Development of a DTPA soil test for zinc, iron, manganese and copper. Soil Science Society of America Journal 42: 421 428.

158 EDTA-Extractable Metals (formerly 161)

Haynes & Swift (1983) showed EDTA to be more useful than DTPA in indicating potential bioavailability of metals in New Zealand soils and in later research Haynes (1995) related EDTA-extractable metal levels to plant responses. Metals extracted by the reagent (1:2.5 soil:extractant, 0.04M EDTA, pH 6.0, 2 hours shaking) are measured by ICP-OES. The method is described by McLaren et al (2005).

• Haynes, R.J. and Swift, R.S. 1983. An evaluation of the use of DTPA and EDTA as extractants for micronutrients in moderately acid soils. Plant and Soil 74: 111-122.
• Haynes, R.J.1995. Micronutrient Status of A Group Of Canterbury Cropping Soils And Its Relationship With Plant Response To Applied Cu And Zn. In Fertilizer Requirements of Grazed Pasture and Field Crops: Macro- and Micro-Nutrients. (Editors: L D Currie and P Loganathan).Occasional report No. 8. Fertilizer and Lime Research Centre, Massey University, Palmerston North, pp.292-299.
• McLaren, R.G., Clucas, L.M. and Taylor, M.D. 2005. Leaching of macronutrients and metals from undisturbed soils treated with metal-spiked sewage sludge. 3. Distribution of residual metals. Australian Journal of Soil Research 43: 159-170.

160 Total Metals (formerly 163)

This method is that of Kovacs et al (2000), and is very similar to EPA SW 846 Method 3050B. It uses nitric acid and hydrogen peroxide to dissolve “almost all elements that could become environmentally available” and gives a matrix suitable for analysis by ICP-OES or ICP-MS depending on analyte levels.

• Kovacs, B., Prokisch, J., Gyori, Z., Kovacs, A.B., and Palencsar, A. 2000. Communications in Soil and Plant Analysis, Vol 31(3): 1949 – 1963.
• USEPA SW-846 Test Methods for Evaluating Solid Waste, Physical/Chemical Methods; Method 3050 Acid digestion of sediments, sludges and soils.

164 Acid-oxalate-extractable Fe, Al & Si

The acid oxalate extraction method depends mainly on the complexing affinity of acid oxalate to extract the Fe, Al, and Si from short-range order materials. The data can also be used to estimate the allophane and ferrihydrite contents of soils (Parfitt and Childs 1988). The Fe, Al & Si extracted by the reagent (1:100 soil:extractant, 0.2M oxalate, pH 3, 4 hours shaking) are measured by ICP-OES. The method is adapted from that described by Blakemore et al (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Parfitt, R.L., Childs, C.W. 1988. Estimation of forms of Fe and Al: A review and analysis of contrasting soils using dissolution and Moessbauer methods. Aust. J. Soil Res. 26: 121 144.

166 Pyrophosphate-extractable Fe & Al

It was considered that pyrophosphate extracted Fe and Al from organic complexes but only relatively small amounts from amorphous (short-range order) minerals (McKeague 1967). Parfitt and Childs (1988) however indicate that while the reagent extracts Al from organic complexes, the Fe is extracted from ferrihydrite. Values from this method are used in a number of soil classification systems to identify podzolised soils.

The Fe & Al extracted by the reagent (1:100 soil:extractant, 0.1M pyrophosphate, 16 hours shaking) are measured by ICP-OES. The procedure combines centrifugation and the addition of superfloc to provide suitably clear extracts. The presence of superfloc helps avoid the resuspension of the solid material after centrifuging. The method is adapted from that described by Blakemore et al (1987).

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• McKeague, J. A.; Day, J. H. 1966. Dithionite- and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Canadian Journal of Soil Science 46: 13-22.
• Parfitt, R.L., Childs, C.W. 1988. Estimation of forms of Fe and Al: A review and analysis of contrasting soils using dissolution and Mossbauer methods. Australian Journal of Soil Research 26: 121- 144.

168 Dithionite-citrate-extractable Fe & Al

Dithionite-citrate reagent extracts 'free' (non-silicate) iron, which includes some inorganic crystalline forms, plus the fractions extracted by pyrophosphate and acid oxalate (organic complexes and short-range order inorganic forms). The amounts of aluminium extracted by this reagent are not easy to interpret however, and are usually similar to the amount removed by acid oxalate.

The method is adapted from that described by Blakemore (1987), which in turn is based on that of Holmgren (1967) and involves an overnight shaking at room temperature. This is more convenient than the traditional methods in which the sample is extracted a number of times, with heating. The Fe & Al extracted by the reagent (1:50 soil:extractant, 22% sodium citrate solution plus 1g sodium dithionite, 16 hours shaking) are measured by ICP-OES.

• Blakemore, L.C.; Searle, P.L.; Daly, B.K. 1987. Methods for Chemical Analysis of Soils. New Zealand Soil Bureau Scientific Report 80. 103 p.
• Holmgren, G. G. C. 1967. A rapid citrate-dithionite extractable iron procedure. Soil Science Society of America Proceedings 31: 210-211.

172 Soil Basal Respiration

Soil basal respiration is the level of carbon dioxide given off by a soil sample and is a measure of the total biological activity of micro-organisms, macro-organisms, and roots.

Samples are adjusted to 60% WHC if necessary and then pre-incubated in plastic bags at 25 °C for 7days.

Samples are then placed in a sealed glass jar at 25 °C in the dark for 7 days. A gas sample is then removed by syringe and CO₂ concentration is determined on a gas chromatograph using a thermal conductivity detector (TCD).

• West, A.W., and Sparling, G.P., (1986). Modifications to the substrate-induced respiration method to permit measurement of microbial biomass in soils of differing water contents.  J. Microbiol.  Methods 5: 177-89.

174 Microbial Biomass Carbon

This method is adapted from Vance et al (1987). Soils are fumigated and the resulting increase in carbon extracted in 0.5M K₂SO₄, compared to non-fumigated soils, is used to estimate the amounts of carbon held in the soil microbial biomass. Extracted carbon is measured using an Elementar Vario TOC Cube. The microbial biomass carbon is then estimated by multiplying the carbon "flush" by a factor, k_EC, which represents the efficiency of extraction. The factor will vary for different soil types and methodologies used. In this method a k_EC factor of 0.41 (Sparling et al, 1990) is used.

• Sparling, G.P., Feltham, C.W. Reynolds, J., West, A.W. and Singleton, P.L. 1990. Estimates of soil microbial C by a fumigation-extraction method: use on soils of high organic matter content, and a reassessment of the k_EC factor. Soil Biology and Biochemistry 22: 301 – 307.
• Vance E.D., P.C. Brookes and D.S.Jenkinson. 1987. An Extraction Method for Measuring Soil Microbial Biomass C. Soil Biol. Biochem 19: 703-707.

176 Microbial Biomass Nitrogen

Soils are fumigated and the resulting increase in nitrogen extracted in 0.5M K₂SO₄, compared to non-fumigated soils, is used to estimate the amounts of nitrogen held in the soil microbial biomass. This procedure is adapted from Jenkinson (1988). Extracted nitrogen is measured using an Elementar Vario TOC Cube.

Microbial biomass nitrogen is estimated by multiplying the nitrogen "flush" by a factor, k_EN, which represents the efficiency of extraction. This factor will vary for different soil types and methodologies used. This method uses a factor of 0.45 (Jenkinson, 1988).

• Cabrera, M.J. & Beare, M.H. Alkaline persulfate oxidation for determining total nitrogen in microbial biomass extracts. Soil Science Society of America Journal 57:1007-1012.
• Jenkinson D.S. 1988 Determination of Microbial Biomass Carbon and Nitrogen in Soil. In Advances in Nitrogen Cycling in Agricultural Ecosystems (J.R.Wilson, Ed.): 368-386 CAB International, Wallingford, UK.
• Lachat Instruments, Milwaukee, WI, USA. 1998d Quik Chem Method 12-107-04-1-B.

185 Hot Water-Extractable Carbon

Soil (field-moist or air-dry) is extracted in hot water overnight then filtered. Total carbon in the filtrate is determined using a Total Organic Carbon (TOC) analyser. The procedure is as described by Ghani et al (2003), except that the cold-water extractable carbon fraction is not removed prior to the hot water extraction. This means that readily-soluble carbon, such as from recent liming, animal excreta and soluble plant residues will be included.

Ghani, A., Dexter, M., & Perrott, K.W. Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilisation, grazing and cultivation. Soil Biology & Biochemistry 35 1231- 1243 (2003).

186 Cold Water-Extractable Carbon

Field-moist soil is extracted in cold water for 30 minutes then filtered. Total carbon in the filtrate is determined using a Total Organic Carbon (TOC) analyser. The procedure is as described by Ghani et al (2003).

Ghani, A., Dexter, M., & Perrott, K.W. Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilisation, grazing and cultivation. Soil Biology & Biochemistry 35 1231- 1243 (2003).

187 Hot Water-Extractable Nitrogen

Soil (field-moist or air-dry) is extracted in hot water overnight, then filtered. The filtrate is oxidised by alkaline potassium persulfate with autoclave heating. Nitrate-N is measured in the extract at 520 nm by Cd reduction and diazotising with sulphanilamide followed by coupling with N-(1-naphthyl)ethylenediamine dihydrochloride using a flow injection analyser. The procedure is as described by Curtin et al (2006).

Curtin, D., Wright, C.E., Beare, M.H., McCallum, F.M. 2006. Hot Water-Extractable Nitrogen as an Indicator of Soil Nitrogen Availability. Soil Science Society of America Journal 70: 1512 - 1521.

190 Particle Size

Distributions and proportions of the various sizes of primary mineral soil particles are usually determined by their capacity to pass through sieves of various mesh sizes or by their rates of settling in suspension. If present, the coarse fraction (>2.0 mm) is analysed by the wet sieving of a large sample through a 2.0 mm sieve, to remove sand, silt and clay. The material retained is dried and then sieved through a stack of larger aperture sieves.

The pipette method (Day, 1965) is used for determining particle size distribution of the fine earth fraction (<2.0 mm) of soils. A soil suspension is placed in a column and, after an initial shaking, a pipette is used to withdraw samples at various times from a set depth as described by Claydon (1989).

The two methods are carried out separately and if necessary the figures obtained for the fine earth fraction by the pipette method can be adjusted to a whole soil basis by using the percent % <2 mm figure produced for the coarse fraction by the wet sieving method.

• Claydon, J.J. 1989. Determination of particle-size distribution in fine-grained soils - pipette method. DSIR Division of Land and Soil Sciences Technical Record LH5. 10p.
• Day, P.R. 1965. Particle fractionation and particle size analysis. Pp. 545-567 In C.A.Black (ed) Methods of Soil Analysis, Part 1. (Ed.) Agronomy, No. 9, American Society of Agronomy, Madison, Wisconsin.