THE HORTUS MEHLICH 3 SOIL TEST

 There is growing international interest1 - 5 in the Mehlich 3 (‘M3’) test, primarily because it is a

multi-element extraction. Since many elements can be determined from the one chemical

extraction, the resulting laboratory efficiencies can then be passed on to farmers and growers

through lower soil test charges.

The M3 test was originally developed in 1984 by Dr Adolph Mehlich6. It is very similar to the

Bray P-1 test7 (a dilute acid and fluoride extraction for assessing phosphate), but also has a

chelating agent (EDTA) to enhance the extraction of trace metals. This test is popular in the

United States8, and is used to assess phosphorus, potassium, calcium, magnesium, sodium,

manganese, zinc, copper, aluminium and boron.

Hortus Technical Services is pleased to offer the M3 soil test to complement our existing soil tests, by

providing useful additional information for little extra cost.

M3 Enhances Basic Soil Test

Many overseas laboratories offer only the M3 test, with pH, as their basic soil test profile.

However we have decided that cations (K, Ca, Mg and Na) and CEC obtained from the

ammonium acetate extraction must remain as part of our Basic Soil test, as many of our clients

find the CEC results and derived % Base Saturation data extremely useful.

Our clients also have comprehensive fertility histories of their properties based on the Colwell P

test, and so for the sake of continuity we are retaining the Colwell P test in our Complete Soil test.

Consequently we are offering the M3 test as an additional option with our current Complete Soil

test, to provide M3 determinations of P, Mn, Fe, Zn, Cu, and B.

Hortus Technical Services also offers an Extended M3 test (the above elements plus K, Ca, Mg, Na determined by the M3 method) for specific projects such as fertility mapping, where large

numbers of samples may be involved and the omission of the CEC and Colwell P data may be

acceptable.

Soil Phosphorus: M3 vs Colwell P

Soil testing services in Australia have been relatively unchanged for some 25 years. The Colwell test has been the dominant soil P test, with only a few alternative tests (Resin P, Bray) being performed.

While the Colwell test is internationally recognised as being a very good soil P test, some shortcomings have been identified with it:

• It does not include reactive phosphate rock (RPR) residues, and will therefore underestimate soil P status for properties with a history of RPR use.

• The pH of the soil affects the amount of P extracted by the Colwell method. High pH soils give lower Colwell P levels, whereas low pH soils will yield higher Colwell P levels,often over-estimating the soil P status.

• The Colwell test is also not “universal” in that the response curves vary with soil type, and this must be taken into account when interpreting the result.

The M 3-P test is very similar to the Bray P1 test, and gives essentially an identical P result. As an acid extractant, it will include RPR residues, in contrast to the alkaline Colwell test. But as for all acid extractants, the test can over-estimate P on recently formed soils (e.g. some sedimentary soils), due to dissolution of free apatite. Caution is therefore necessary when interpreting the M3-P levels for these soils.

Both tests have advantages, and limitations, and so we suggest that the M3-P result be regarded as being complementary to the Colwell P test, providing a second perspective on this important element.

Cations (K, Ca, Mg, Na)

 These are included in our M3 test. The M3 extractable cations showed very good correlations with ammonium acetate extractable cations, and normal range levels have been derived from these.

M3-Trace Metals (Manganese, Zinc, Copper and Iron)

 As with all soil trace element tests, the relationship between soil extractable levels and plant uptake is not strong. Interpretation should be made with caution, without undue weight being placed on the soil trace element levels.

In-house investigations have shown good correlations between the M3 test with the standard DTPA  test that is currently being used in Australia 10. Normal range levels

for the M3 trace metals have been cross-calibrated from this existing test.

We have also added cobalt into the standard M3 test, because of the importance

of this element in agriculture.

M3-Boron

The hot water soluble boron (HWSB) test is regarded as the standard soil test for assessing B status. The M3-B test correlates well with the HWSB test at elevated levels. Like trace element tests in general, both the HWSB and M3-B tests are not particularly reliable. This is especially so at low levels, when B deficiency may be suspected. In contrast, both tests are more reliable at higher levels, to identify possible B toxicity, or to monitor soil B levels for B-loving crops (e.g. avocados, olives).

We believe the M3-B test can replace the HWSB test when the B levels are elevated (>1-2 ug/g). For lower levels, the relationship with the HWSB levels is poor, and we are not yet confident that these low-level M3-B results have any diagnostic value.

Method Outline

The Mehlich3 extractant is a mixture of ammonium fluoride, ammonium nitrate, EDTA and acetic and nitric acids

Soil is shaken in a volume of Mehlich3 extract (pH 2.6) and elements are analysed by AAS-ContrAA.

Expression of Results

The Mehlich3 results are expressed as mg/l of soil (i.e. ppm on a volume basis). The interpretive criteria used to

generate our bar graphs have primarily been derived by cross-calibration of the M3 test with currently offered tests.

Hortus Technical Services believes that this test will be particularly useful for the following situations:

• as a cost-effective soil test for fertility mapping;

• where soil trace element tests are required, e.g. for crops such as avocados

• as an additional soil P test, to complement the standard Colwell P test.

• to provide an indication of the sulphate and phosphate retention properties of the soil.

 References

1 Burt, R.; Mays, M.D.; Benham, E.C. and Wilson, M.A. Phosphorus Characterisation and Correlation with Properties of Selected Benchmark Soils of the United States. Commun. Soil Sci. Plant Anal. 2002, 33, 117-141.

2 Buondonno, A.; Coppola, E.; Felleca, D. and Violante, P. comparing Tests for Soil Fertility: 1. Conversion Equations between Colwell and Mehlich 3 as Phosphorus Extractants for 120 Soils of South Italy. Commun. Soil Sci. Plant Anal. 1992, 23, 699-716.

3 Garcia, A.; de Iorio, A.F.; Barros, M.; Bargiela, M. and Rendina, A. Comparison of Soil Tests to Determine Micronutrient Status in Argentina Soils. Commun. Soil Sci. Plant Anal. 1997, 28, 1777-1792.

4 Zbiral, J. and Nemec, P. Integrating of Mehlich 3 Extractant into the Czech Soil Testing Scheme. Commun. Soil Sci. Plant Anal. 2000, 31, 2171-2182.

5 Fernandez Marcos, M.L.; Alvarez,, E. and Monterroso, C. Aluminium and Iron Estimated by Mehlich-3 Extractant in Mine Soils in Galicia, Northwest Spain. Commun. Soil Sci. Plant Anal. 1998, 29, 599-612.

6 Mehlich, A. Mehlich 3 Soil Test Extractant: A Modification of Mehlich 2 Extractant. Commun. Soil Sci. Plant Anal. 1984, 15,277-294.

7 Bray-1 P Extraction, Soil Analysis. Handbook of Reference Method,s, Soil and Plant Analysis Council, CRC Press. 1999, 70-73. 8 Mehlich No. 3 Extraction, Soil Analysis. Handbook of Reference Method,s, Soil and Plant Analysis Council, CRC Press. 1999, 100-104.

9 Colwell’s Sodium Bicarbonate Extraction, Soil Analysis. Handbook of Reference Method,s, Soil and Plant Analysis Council, CRC Press. 1999, 73-76.

10 Forbes, E.A. , Cobalt, copper and zinc in yellow-brown pumice soils under grazed, permanent pasture. Australia Journal of Agricultural Research. 1976, 153-164.

 Contact Details

For further information about any of the above tests please contact our office on 07 4132 5000 or [email protected]