SOME POTENTIAL FIELD METHODS TO ASSIST WITH THE CORRECT
IDENTIFICATION OF SOIL MATERIAL AND DIAGNOSTIC HORIZONS
IN FUTURE SOIL CLASSIFICATION PROCEDURES
F Ellis1 and JJN Lambrechts1
Department of Soil Science, University of Stellenbosch, Private Bag X1, Matieland 7602
The Soil Classification Working Group (SCWG) is currently in the process of updating Soil
Classification: A Taxonomic System for South Africa (1991). One of the reasons why the “Blue
Book” is the most successful and practical system in use in South Africa is that it largely makes
use of properties that can be identified in the field. This paper gives a review of that field
indicators/test presently in use but also of a few very promising ones that may help with correct
identification of soil materials and diagnostic horizons that are currently difficult to identify (e.g.
high carbon topsoils under different hydromorphic conditions) in the field.
MATERIAL AND METHODS
Soil properties (e.g. colour, texture, consistence and structure) that form part of a normal soil
profile description were revisited. Simple soil chemical field tests such as the application of
diluted HCl for test of free carbonates were extended by field tests for the identification of podzol
B horizons [e.g. NaF test with pH paper of Brydon & Day (1970) and measuring pH in the
laboratory as a control] on high carbon topsoils collected from a variety of wetlands, melanic A
horizons, “classic” humic A horizons and other high carbon topsoils in lower lying positions in
traditional humic landscapes that was difficult to classify in the field. The methyl orange field
test was used to test for presence of sepiolite and petrosepiolite in certain arid soils. Strong
adherence of a fragment to the wetted tongue was also used to test for sepiolite. By heating a
small sample on a metal spoon with a gas blow lamp the colour change of high carbon topsoils
were determined in the field.
RESULTS AND DISCUSSION
A review is given of those field tests that are currently applied in the definitions of diagnostic
horizons/materials used in the Blue Book. According to Francis et al, (2007) the pinkish colour
that develops after wetting with methyl orange to a sample that contains sepiolite is a diagnostic
test. Strong adherence of a fragment to a moist tongue is further evidence of sepiolite. The NaF
pH test (medium to strong purple colour development) is an effective field test to separate high
carbon topsoils with high concentrations of reactive aluminium (e.g. humic A horizon).
The blow test seems to be an effective way in separating high Fe containing topsoils (e.g.
“classic” humic A) from topsoils containing lower Fe (e.g. wetland topsoils). A positive NaF on
pH test paper and a reddening or yellowing of soil material using the blow lamp is suggested to
separate “classic” humic topsoils from other high carbon topsoils.
Keith Snyman for selection and supplying humic topsoil samples. ISCW for supplying some
selected melanic topsoil samples. Mico Stander for NaF analysis on samples.
BRYDON & DAY 1970. Can J Soil Sci 50, 35 – 41
FRANCIS et al. 2007 J Arid Environments 70, 588 – 603
Keywords: Field tests, soil classification, high carbon topsoil, sepiolite.