EQUIPMENT AVAILABLE

The Tropical Vegetation Dynamics Laboratory is a facility for the analysis of nutrients, particularly in plant and soil samples. We are also able to analyse photosynthetic pigments in algal and plant samples and crude protein content in aquaculture feeds and animal tissue. The laboratory contains a variety of equipment for the preparation, digestion and analysis of samples, including hammer mill, drying ovens, aluminium block digesters, UV-visible and atomic absorption spectrophotometers, centrifuges, end-over-end and reciprocating shakers, vacuum pump, pH and conductivity meters.

We have recently purchased a Molecular Devices SpectraMax Plus spectrophotometer with built in microplate reader, which can read 96 wells in 9 seconds. As well as colorimetric assays for nutrients, the instrument is equipped with protocols for enzyme kinetics, DNA quantitation, ELISAs/EIAs, and immunoassays.

CHARGES FOR USE OF NUTRIENT ANALYSIS LAB FACILITIES

Researchers are permitted to use our facilities for nutrient analysis on a cost recovery basis. We are equipped for the analysis of plant material for total nitrogen, phosphorus and cations. For soil samples, determinations can be made of total nitrogen, phosphorus, available phosphorus (bicarbonate or acid-extractable), pH, electrical conductivity, and organic carbon. We have adapted a simple hydrometer technique to estimate soil particle size (sand, silt and clay), and are also able to do analyses for soil cation-exchange capacity and exchangeable cations etc.

The costs for chemical reagents are:-

$25 per digestion run

$15 per total nitrogen run

$7.50 per total phosphorus run

$30 per total carbon run

$15 per bicarbonate-extractable P run

If you do the analyses yourself, the labour costs/bench fee for initial instruction is $40. We may be able to arrange for some-one to do the analyses for you, in which case the labour costs are $25 per hour, equivalent to $200 per TN/TP run and $100 per TOC run (not including sample preparation).

A maximum of 50 samples, including standards and blanks, can be processed in each run of samples. Broken digestion tubes cost $30 each to replace. In deference to other users, all care must be taken with glassware, dispensers, block digester and spectrophotometer. Breakages will be charged for. All reagent spills must be cleaned up immediately.

We may also be able to assist with other analyses of soil, water or plant material.

SYNOPSIS OF METHODS

We use the single digestion method of Anderson & Ingram (1989) for the determination of N, P and cations. Air-dry soil or plant samples are ground to 0.2 mm, digested with sulphuric acid and hydrogen peroxide. Nitrogen is determined colorimetrically by the salicylate-hypochlorite method of Baethgen & Alley (1989), and phosphorus by an adaptation of Murphy and Riley's (1967) single solution method (Anderson & Ingram 1989). Aliquots of the digest can be analysed for cations (Na, K, Ca, Mg) using atomic absorption spectrophotometry. Some of the mineral forms of the cations are probably not present in the digest, so these do not strictly represent "total" concentrations, but are a useful index of what is potentially available for plants for comparative purposes. Subsamples of the air-dry soils are dried in an oven to determine moisture content, and nutrients are reported as % on an oven-dry basis.

Soil pH and electrical conductivity are determined using the standard method based on a 1:5 water extract (Rayment & Higginson 1992). Soil organic carbon is determined by the Heanes wet oxidation method - based on heating the sample with H₂SO₄ in the presence of dichromate (Rayment & Higginson 1992). External heating ensures complete oxidation, unlike the traditional Walkley & Black method. The carbon is determined from the concentration of chromic ions determined with a spectrophotometer.

Available P is determined by the modified Olsen method using an extracting solution of 0.5M NaHCO₃, adjusted to pH 8.5, a soil/solution ratio of 1:100 and an extraction time of 16 hours (Rayment & Higginson 1992). Soil nitrate can be determined simply and accurately using second derivative spectroscopy (Sempere et al.. 1993). Soil texture as % sand, silt and clay is estimated from the changes in specific gravity of a soil suspension using a Bouyoucos soil hydrometer (Anderson & Ingram 1989), after removal of organic matter with hydrogen peroxide, and dispersion of the particles by shaking overnight with sodium hexametaphosphate.

REFERENCES

Anderson S.E. & Ingram J.S.I. (1989) Tropical Soil Biology and Fertility: A Handbook of Methods. p. 171. C.A.B. International, Aberystwyth.

Baethgen W.E. & Alley M.M. (1989) A manual colorimetric procedure for measuring ammonium nitrogen in soil and plant Kjeldahl digests. Communications in Soil Science and Plant Analysis 20, 961-969.

Murphy J. & Riley J.P. (1967) A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27, 31-36.

Rayment G.E. & Higginson F.R. (1992) Australian Laboratory Handbook of Soil and Water Chemical Methods. Inkata Press, Melbourne.

Sempere A., Oliver J. & Ramos C. (1993) Simple determination of nitrate in soils by second-derivative spectroscopy. Journal of Soil Science 44, 633-639.

Content created by Bob Congdon

Updated on 15-February-2008