Plant testing

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

Methods

202 Plant Sample Preparation

Plant samples are dried at 80 °C in a forced air oven then ground in a Udy cyclone mill to pass a 1-mm screen. As the dry material can re-absorb moisture, samples are briefly re-heated at 80 °C immediately prior to weighing out for analyses. The procedure is based on that described by Campbell and Plank (1997).

• Campbell, C.R. and Plank, C.O. 1997. Preparation of Plant Tissue for Laboratory Analysis. Pp 37 - in Y.P. Kalra (ed) Handbook of Reference Methods for Plant Analysis. CRC Press.

204 Carbon & Nitrogen

Plants are analysed using a Leco TruMac Analyzer 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. Carbon and nitrogen in plant tissue. LECO Corporation, St. Joseph, MO, Organic Application Note 203-821-171.

206 Nitrogen, Phosphorus, Potassium, Calcium and Magnesium

Samples are block digested using the Kjeldahl wet oxidation process as described by Blakemore et al (1987). The sample is first treated with salicylic acid in sulphuric acid to form nitro compounds from any nitrates present, and then these are reduced to the corresponding amino compounds with sodium thiosulphate (Bremner and Mulvaney, 1982). 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 precipitation of salts and left overnight to allow solids to settle.

Using a QuikChem 8500 flow injection analyser, nitrogen in the digest is determined colorimetrically as NH₄-N using the indophenol reaction with sodium salicylate and hypochlorite (Lachat 1998a). Orthophosphate (PO₄-P) reacts with ammonium molybdate and antimony potassium tartrate under acidic conditions then a molybdenum blue complex is formed after ascorbic acid reduction (Lachat 1998b).

Potassium, calcium and magnesium are determined by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis 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.
• Bremner, J.M. and Mulvaney, C.S. 1982: Nitrogen - Total in A.L.Page et al (ed). Methods of Soil Analysis Part 2. Chemical and Microbiological Properties: 711-733. American Society of Agronomy, Madison, Wisconsin.
• 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.

208 Calcium, Magnesium, Sodium, Iron, Manganese, Copper and Zinc

Test Method no. 208(i) Calcium, Magnesium, Sodium, Iron, Manganese, Copper and Zinc by dry ashing
Formerly 208

This method is based on that of Prasad and Spiers (1978), and involves dry ashing the sample at 475 °C in a muffle furnace and dissolving the ash in 2 M HCl. The metals are then determined by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES.

• Prasad, M.; Spiers, M. 1978. Comparative study of ashing techniques for the digestion of horticultural plant samples. Agricultural and Food Chemistry 26: 824 827.

Test Method no. 208(ii) Calcium, Magnesium, Sodium, Iron, Manganese, Copper and Zinc by acid/peroxide digestions
Formerly 210

This method is that of Kovacs (1996) which uses nitric acid and hydrogen peroxide to digest plant samples. The metals are then determined by optical emission spectroscopy with inductively coupled plasma excitation (ICP-OES) using a Spectro Genesis ICP-OES.

• Kovacs, B., Gyori, Z., Prokisch, J., Loch, J. and Daniel, P. Communications in Soil and Plant Analysis, Vol 27 (5-8), pp 1177 – 1198, 1996

218 Acid detergent fibre, cellulose & lignin

The acid detergent fibre-sulphuric acid procedure (Rowland and Roberts 1994) utilises sequential treatment of the sample with reagent to destroy various fractions, followed by gravimetric determination of the residues. In the first step protein is hydrolysed using boiling acid detergent. The fraction remaining is defined as acid-detergent fibre, and is comprised of cellulose, lignin and ash. This residue is then treated with 72% sulphuric acid to destroy the cellulose, leaving a fraction comprised of lignin and ash. The difference between these first two fractions is defined as cellulose. Finally the residue is ignited at 550 °C to destroy all remaining organic matter, leaving the inorganic or ash fraction. The difference between this fraction and the second fraction is defined as acid-detergent lignin.

• Rowland, A.P. and Roberts, J.D. (1994) Lignin and cellulose fractions in decomposition studies using acid detergent fibre methods. Communications in Soil Science and Plant Analysis 25: 269-277.