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Analytical methods used in the Soil Fertility Laboratory of the KwaZulu-Natal Department of Agriculture and Rural Development

The Soil Fertility Laboratory routinely performs the following analyses as part of the Department’s Fertilizer Advisory Service, using the rapid procedures described by Hunter (1975) and Farina (1981): Ambic-2-extractable P, K and Zn, KCl-extractable Ca, Mg and acidity, and pH (KCl). These methods are briefly described below (Manson and Roberts, 2011).

Sample preparation

Soil samples are air dried at room temperature; they are spread out in drying trays and air is forced over them. When dry, the samples are crushed between rubber belts on a soil crusher and passed through a 1-mm sieve. Material coarser than 1 mm that cannot be crushed (such as stones, gravel and concretions), is discarded.

Batch handling

Samples are scooped into trays which each contain 11 PVC cups (capacity 70 mL); a tray is used for nine unknown samples, one standard soil sample (for quality control) and one blank. For operations such as dispensing and stirring, and for quality control, batches of three trays (27 samples, three unknowns, and three blanks) are used. Multiple dispensers and diluter/dispensers are used to dispense aliquots of extractant or reagent to three samples at a time.

Sample density

Soil samples are analysed on a volume rather than a mass basis. To enable the conversion of the results to a mass basis, the mass of a 10-mL scoop of a dried and milled sample is measured and the calculated sample density is reported.

pH (KCl)

10 mL of soil is scooped into sample cups. 25 mL of 1 M KCl solution is added and the suspension is stirred at 400 r.p.m. for 5 min using a multiple stirrer. The suspension is allowed to stand for about 30 minutes, and the pH is measured using a gel-filled combination glass electrode while stirring. De-ionised water is substituted for the 1 M KCl solution if pH (water) is required.

Extractable (1 M KCl) calcium, magnesium and acidity

2.5 mL of soil is scooped into sample cups. 25 mL of 1 M KCl solution is added and the suspension is stirred at 400 r.p.m. for 10 min using a multiple stirrer. The extracts are filtered using Whatman No.1 paper. 5 mL of the filtrate is diluted with 20 mL of 0.0356 M SrCl2, and Ca and Mg determined by atomic absorption. To determine extractable acidity, 10 mL of the filtrate is diluted with 10 mL of de-ionised water containing 2-4 drops of phenolphthalein, and titrated with 0.005 M NaOH.

Extractable (Ambic-2) phosphorus, potassium, zinc and manganese

The Ambic-2 extracting solution consists of 0.25 M NH4CO3 + 0.01 M Na2EDTA + 0.01 M NH4F + 0.05 g L-1 Superfloc (N100), adjusted to pH 8 with a concentrated ammonia solution. 25 mL of this solution is added to 2.5 mL soil, and the suspension is stirred at 400 r.p.m. for 10 min using a multiple stirrer. The extracts are filtered using Whatman No.1 paper. Phosphorus is determined on a 2 mL aliquot of filtrate using a modification of the Murphy and Riley (1962) molybdenum blue procedure (Hunter, 1974). Potassium is determined by atomic absorption on a 5 mL aliquot of the filtrate after dilution with 20 mL de-ionised water. Zinc and Mn are determined by atomic absorption on the remaining undiluted filtrate.

Effective CEC (ECEC) and Acid saturation

Effective CEC is calculated as the sum of KCl-extractable Ca, Mg, and acidity and Ambic-2 extractable K. Percent acid saturation of the ECEC is calculated as "extractable acidity" x 100 / (Ca + Mg + K + "extractable acidity").

References

Farina, M.P.W. 1981. The Hunter system of soil analysis. Fertilizer Society of South Africa Journal, 1, 39-41.
Hunter, A. 1974. Tentative ISFEI soil extraction procedure. International Soil Fertility and Improvement Project. N.C. State University, Raleigh, NC.
Hunter, A. 1975. New techniques and equipment for routine soil/plant analytical procedures. In: Soil Management in Tropical America. (eds E. Borremiza & A. Alvarado). N.C. State University, Raleigh, NC.
Manson, A.D. and Roberts, V.G., 2011. Analytical methods used by the soil fertility and analytical services section. KwaZulu-Natal Department of Agriculture and Rural Development, Pietermaritzburg, South Africa.
Murphy, J. and Riley, J.R. 1962. A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta, 27, 31-36.

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