This multi-element test (Mehlich 1984) covers extractable P, the basic exchangeable cations (Ca2+, Mg2+, Na+, K+), extractable S, selected trace elements (Cu, Mn, Zn, B), extractable Fe and extractable Al. Environmental soil P tests (Bloesch and Rayment 2006) that can be derived from components obtained from Mehlich 3, specifically Mehlich 3–P saturation ratios, are documented in Chapter 9 as Method 9K1a [Mehlich 3–P Saturation Ratio (M3PSRcolourP)] and as Method 9K1b [Mehlich 3–P Saturation Ratio (M3PSRICPAES)].
The main function of the extractant’s components are (Anon 2007): CH3COOH – buffers the extracting solution to pH 2.5 when all reagents are added and mixed, thus preventing Ca2+ from being precipitated as calcium fluoride; NH4NO3 – facilitates extraction of the exchangeable cations Ca2+, Mg2+, Na+ and K+, since it reacts with the CH3COOH to form ammonium acetate; HNO3 – extracts a portion of calcium phosphates, while its H+ component aids the extraction of basic and micronutrient cations; NH4F – the F– component helps to extract Fe and Al phosphates, while NH4+ complements NH4NO3 in extracting basic cations; and EDTA – chelates micronutrients (particularly Cu) and prevents precipitation of calcium fluoride.
The Mehlich 3 extractant has applicability for diagnostic purposes across a wide assortment of soils ranging in reaction from acidic to basic (Jones Jr. 1990). It is used widely in North America and increasingly in other countries, including the Czech Republic (Zbíral and N
mec 2000) and Australia (Bolland et al. 2002; Rayment 2004). Rayment et al. (2009) have summarised the analytical performance of Australasian soil testing laboratories relative to North American laboratories for a suite of Mehlich 3 soil tests on four soils from North America.
In practice, the Mehlich 3 test utilises a 1:10 v/v soil-solution ratio or a 1:10 w/v ratio, inclusive of vigorous shaking for 5 min at 200 × 4 cm reciprocations/min (or equivalent), followed by measurement of a range of elements in the subsequent soil-free extract. It is a rapid, empirical method that offers low soil testing costs by replacing at least three ‘traditional’ soil tests.
Across Australasia, method comparisons and field calibration studies with Mehlich 3 mostly involve a 1:10 w/v soil-solution ratio, as in this procedure. This reflects local practice for many empirical soil tests. One reason is that the particle size distribution of a wide selection of dry, ground (or crushed) Australian soils of <2 mm can vary considerably, contributing to variability in weights of equal volumes of sub-samples. The US Department of Agriculture (Burt 2004) and Canadians’ Sen Tran and Simard (1993) also specify a w/v soil/solution ratio of 1:10. Approximate results for a v/v soil/solution ratio can be calculated using a weight-volume conversion calculation for each sample.