THE M I N E R A L O G Y OF S O M E S O I L S
FROM CENTRAL ITALY
By C. L1PPI-BONCAMBI,* R. C. MACKENZIE,~and W. A. MITCHELLt
* Universita di Perugia, Italy
t Macaulay Institute, Aberdeen
[Read 15th April, 1955]
ABSTRACT
A set of seven soils representative of types of considerable p~dological
importance in the Umbrian region of Italy have been examined by
petrological and clay mineralogical methods in order to supplement
previous data upon field and chemical characteristics (LippiBoncambi, 1950). Some basic data upon colour, locality of the sample
examined, parent material, rainfall (P), temperature (T), rain factor
(P/T--see Lang, 1915) and pH are summarised in Table 1. It will be
noted that (a) the soils form an interesting colour group ranging from
red through intermediate colours to yellow and then through brown to
black, (b) all except one are derived from calcareous rocks, and (c) all
except one are neutral to alkaline in reaction.
EXPERIMENTAL
The clay fraction of the soils (< 2 t~ equivalent spherical diameter)
was separated by the usual sedimentation method after dispersion
with ammonium hydroxide, coagulated with neutral normal magnesium chloride, washed free from salts, treated with hydrogen peroxide
to remove organic matter, dried, and sieved through 100-mesh. The
silt and sand fractions were separated from the residue. The sand
fractions were examined microscopically but the presence of large
amounts of weathered materials made identification of particular
minerals very difficult, or, at times, even impossible. The clay
fraction was examined by X-ray and differential thermal methods
using normal techniques. Amounts of the various minerals detected
are given to the nearest 5~
RESULTS AND DISCUSSION
M e d i t e r r a n e a n t e t r a rossa. This soil, which has a high clay content
and is almost plastic, is composed of the residue left in situ after the
dissolution of limestone together with some wind-borne materials
and, in Umbria, some volcanic detritus from the neighbouring
Quaternary volcanic areas of Latium and Toscana. It is found in
Mediterranean climatic areas where the summers are hot and dry
and the winters mild and moist. Climatic and parent material data
for occurrences in Umbria are given in Table 1. Usually associated
with karst-land topography it gives an A-C profile of very variable
depth. It is frequently "bag-shaped" into the parent rock and may
form typical "organ-pipes" of one-half to one metre in length. There
is only a trace of calcite, organic matter is low, and, as might be
281
282
c . LIPPI-BONCAMBI, R. C. MACKENZIE AND W. A. MITCHELL
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SOILS I~'ROM CENTRAL ITALY
283
expected from the colour, the sesquioxide content is high. The soil
is usually approximately neutral in reaction and the silica-sesquioxide
ratio is in the neighbourhood of 2.
Under the microscope, the following minerals were observed:
quartz grains with ferruginous coatings, partly-decomposed felspars,
iron oxides, olivine, zircon, glaucophane, biotite, augite, and
hornblende.
In the.clay fraction, d.t.a, and X-ray examination both showed the
presence of 65-70% of a kaolin mineral (probably kaolinite) and about
5% goethite.
X-ray examination also showed the presence of 10~
illite, 5% haematite and, possibly, 5% montmorillonoid (these, either
because of their nature or because of the small amounts present, were
not observable upon the d.t.a, curve). The predominantly kaolinitic
nature of this terra rossa is particularly interesting and it is the first
observation of this type made here. All samples examined by
Mufioz Taboadela (1953) were essentially illitic as were samples from
Cap d'Antibes and Algeria (unpublished da~fa). Cecconi (1953) has
however, recently observed one example of a kaolinitic terra rossa in
Apulia; the others, including the Umbrian samples, examined by
him were illitic. It would seem therefore, that this type is only of
limited occurrence. The factors which might lead to development of
such a type are not clear; they might be climatic, or, if the clay is
present in the original rock, they might be a reflection of the conditions under which the rock itself was formed.
A c l i m a t i c terra rossa.
This soil type is usually developed upon a
massive grey cavernous limestone and only at higher altitudes; at
lower levels brown soils are more common on the same rock type.
The soil is generally of limited depth (about 20 cm.), varies in colour
from liver-red to brown, and is generally found in small depressions.
On slopes, or where stratification of the limestone is inclined, occasional outcrops give the landscape a rocky aspect; this topography
may, in part, be a consequence of the felling of tree cover. The
soil is deficient in organic matter but rich in sesquioxides, alumina
predominating as in karst-land soils. The silica:sesquioxide ratio
is in the range 0.8-1-0. Certain characteristics indicate that this soil
should be classified as a terra rossa, but others (e.g., the rain factor of
Lang) suggest it occurs in what should be the Brown Earth region
(see Lippi-Boncambi, 1950). It has consequently been classified as
an "aclimatic terra r o s s a . "
In the coarse sand fraction, which constituted the major portion of
the soil, there were few heavy minerals; felspars and other minerals
present in the parent limestone predominated but there were also
smaller amounts of wind-borne materials, such as olivine, zircon,
glaucophane, epidote, hornblende, haematite, and ilmenite. The
fine sand fraction contained about 60% quartz, 10% calcite, and
felspars.
The composition of the clay fraction was 70% illite, 20-25%
halloysite and 5~ quartz--not dissimilar to that observed by Mu~oz
284
C. LIPPI-BONCAMBI, R. C. MACKENZIE AND W. A. MITCHELL
Taboadela (1953) for dry calcareous soils of the Mediterranean
region. The definite identification of halloysite by X-ray diffraction
is interesting, and agrees with d.t.a, evidence. The slope ratio
(Bramao et al., 1952), for example, was 1.9, compared with 0.6-1.3
for kaolinite, 1.1-1-7 for the partially-disordered lattice mineral, and
about 2.0 for halloysite; the peak temperature (Robertson, Brindley
and Mackenzie, 1954) was 537~ compared to 557~ for approximately the same reacting weight of kaolinite and 532~ for halloysite.
Such evidence would indicate that the mineral was either halloysite
or very close to it, and it is interesting to note that the large amount of
illite does not apparently distort the halloysite peak sufficiently to
give a very erroneous value for the slope ratio. This and particlesize effects are the two factors most likely to upset identifications
using these criteria.
Podzolized Red Loam. The surface layer of this aclimatic soil type
of limited occurrence is characterized by an intensely red or yellow
coloration with brown spots and streaks of iron and manganese oxides.
The colours, and the ochreous layers which cover decalcified stones,
are reminiscent of the older Quaternary moraines and alluvium in the
Alps, and it appears that the complete decalcification of the Pliocene
conglomerates of the Pievese plateau led to oxidation, or conversion,
of iron compounds to limonite and to an earthy clay-oxide complex.
The soils, therefore, contain little or no limestone, are base-unsaturated and are rich in silica and sesquioxides (Lippi-Boncambi,
1950). In the uncultivated facies there is a tendency to podzolization
which is promoted by the presence of acid humus from the forest
cover and by low base status, but is limited by the low rainfall.
Where a clay band in the conglomerate reduces percolation,
decalcification is reduced and the soil passes into the "Brown Soil"
type widespread in Umbria.
In the sand fractions were found quartz (85% in the fine sand fraction, somewhat less in the coarse), felspars (plagioclase and perhaps
also sanidine) haematite, mica, magnetite, zircon and other silicates
referred to epidote and glaucophane. The presence of a thick
ochreous layer made identification of many grains difficult.
The clay fraction of a cultivated soil of this type contained 45%
illite, 30~o halloysite, 10% vermiculite or chlorite, 10% quartz, 5%
haematite, and about 2% goethite. Usual criteria applied to the
d.t.a, curve again indicated a mineral close to, or at, the halloysite
end of the kaolinite-halloysite series; the X-ray diffraction pattern
confirmed the presence of halloysite. This soil differs from the last
in containing less illite, more quartz (as might be expected in view of
the parent material), more crystalline iron oxides, and some vermiculite or chlorite (definite identification was not possible). The
general composition is quite in accord with observations made upon
podzolic soils from other localities (e.g., Scotland).
Climatic terra gialla. This soil type, developed under a moist and
temperate climate, does not form a continuous belt but occurs
irregularly. Its yellow colour is due to iron oxides and a deficiency
SOILS FROM CENTRAL ITALY
285
of organic matter. The silica content is high and, owing to leaching,
the concentrations of carbonates and bases are low. According to
the rain factor (Table 1) it should fall into the Brown Earth group,
but its characteristics are not in accord with this classification.
When the climate is colder and wetter, organic matter accumulates
in greater quantity, the soil reaction becomes more acid and the soil
may develop podzolic character. On the other hand, where marl
intercalations are more frequent, or where the rocks are more calcareous, there is a gradual change into brown soils. This soil type,
therefore, appears to be intermediate between the Podzol and the
Brown Earth group. The soils are covered with woods of chestnuts
and acacias, with, in the undergrowth and on heathland, Erica,
Calluna, and Equisetum. The profile shows A, B and C horizons: the
A horizon is yellow in colour and slightly brown-stained by the very
small amount of organic matter present; the B horizon is reddishyellow owing to accumulation of small amounts of sesquioxides; the
C horizon is composed of highly siliceous material.
The soil stones are mainly greenish flints and small pieces of marly
decalcified limestone, while' the coarser fractions of the soil also
contain flints, some calcite and felspars. The fine sand fraction was
composed of 75% quartz together with calcite and felspars.
The clay fraction was, again, highly illitic, containing 60-70~ of
this mineral. A kaolin mineral (probably the partially-disordered
lattice type) occurs to the extent of 20-25% and accessories were
5-10% quartz, about 2% hydrated ferric oxide gel (Mackenzie, 1949,
1952) and, perhaps about 1% goethite.
Lithochromic terra gialla. The intensely yellow colour of these very
calcareous sandy soils is due to the breakdown of calcareous sandstone first into cemented sands and then into individual grains which
form the predominant part of the soil. They are, therefore, not
climatic but lithochromic. When the parent material is richer in
clay these soils lose their characteristics and they appear to be a
transitional type between yellow and brown soils. They occur in the
western parts of the Umbrian region and are particularly widespread
to the south-west of Lake Trasimeno. On account of the instability
of the soil on the slopes it is difficult to find a mature profile.
Because of the moisture retention in the substrate the rain factor
classification cannot be validly applied; lithological rather than
climatic factors are preponderant. There appears to be a decrease
in carbonate content towards the surface but there is no sign of
the leaching and deposition phenomena characteristic of podzolization. The almost complete absence of organic matter does not
seem in accord with the flourishing vegetation, but it must be that
organic residues are rapidly decomposed. The lower layers are very
compact and may be considered the parent material.
The soil has a low stone content and a very low clay content. The
stones are mainly small quartzite fragments and residues of sandstones, gravels and Pliocene sands. The sand fraction of a cultivated soil contained essentially the same minerals as the parent rock ;
286
c . LIPPI-BONCAMBI, R. C. MACKENZIE AND W. A. MITCHELL
quartz, iron oxides, calcite, mica and ferromagnesian minerals were
detected. The fine sand fraction of the sample examined contained
20% quartz, 60% calcite, and felspar.
In the clay, illite again predominated, forming 70% of this fraction;
other minerals found were: <5% kaolin mineral, 15% quartz and
10% montmorillonoid. From d.t.a, evidence a trace of gibbsite
(1-2%) might be present, and the montmorillonoid may be of the
high-iron (i.e., nontronite) type, as there is a suggestion of a peak at
450-475~
Neither of these observations have, however, been
proved. The very low concentration of kaolin mineral is interesting
and may be connected with the suggestion of Millot (1949) that illite
is stable in highly calcareous media while kaolin minerals are not
(see Grim 1953, p. 355).
Rendzina. This soil type is widespread in Umbria being developed
on the marly calcareous rocks of the Umbrian Appenines at heights
above 1,000 m.; the conditions leading to its formation have been
previously described (Lippi-Boncambi and Warimpietri, 1949). In
the A-C profile the black soil is composed of only partially decomposed organic residues with characteristic sub-angular calcareous
fragments dispersed through it; these fragments are in situ and
demonstrate its sedentary nature. The soil is neutral to alkaline in
reaction (pH 6-8-7.8) and the total nitrogen and organic carbon
contents are high, the C:N ratio varying from 10-2 to 13.6. The
organic matter is base-saturated, as in Polish black-coloured soils.
Analysis after the m~thod of van Bemmelen (1888) gives a SiO2 :R203
ratio of 2-5-3-0, which, if this method may be trusted to give some
indication of the clay-fraction composition, would suggest the
presence of minerals of the mica or montmorillonoid types.
X-ray examination showed the presence of 70% illite, 5% kaolin
mineral and 10% quartz. The d.t.a, curve was difficult to interpret;
the presence of 5% kaolin mineral was quite in accord with the curve,
but the large hygroscopic moisture peak suggested the presence of a
montmorillonoid rather than illite. X-ray evidence, however,
showed quite definitely that montmorillonoid is absent, and furthermore that the illite was of a well-crystallized type, which would not
be expected to retain more hygroscopic moisture than normal. It is
interesting to note that a rendzina from Algeria recently examined
(unpublished results) gave very similar results. It seems pertinent
to ask whether the peculiar nature of the curve may not be associated
with allophane, which, of course, would not be observable on the
X-ray pattern. Tests using piperidine-treated samples (Sudo, 1954)
were not conclusive but results were rather vitiated by the small
amount of material available. It seems, however, that the clay
mineralogy of rendzinas might repay further study by a combination
of d.t.a, and X-ray methods.
Brown Soils. This, the major soil type of Umbria, is usually under
cultivation and does not occur at great heights. The natural vegetation is deciduous forest, e.g., oak, chestnut, beech. Because of the
variety of rocks upon which they occur, these soils are rather variable
SOILS FROM CENTRAL ITALY
287
and a variety of azonal sub-types develop, but these have in Umbria
lost many of their original characteristics through cultivation and
manuring. However, as lithological factors have the main influence,
it is possible to distinguish the brown soils on the mountains, those
on the hills of Villafranchian lacustrine deposits, and those on
alluvial deposits in the plain.
The product of limited leaching in a temperate climate with
moderate evaporation and not too high a mean annual temperature,
they are very sensitive to change in climatic factors. Where colder
conditions and higher rainfalls occur they undergo podzolization,
while tinder warmer conditions and lower rainfalls they are replaced
by Red and Yellow Soils. They are characterized by a low content
of base-saturated organic matter and the presence of iron and lime in
fair amounts. They are in essence transitional between Red and
Yellow Soils and Brown Earths, the latter being very widespread in
Northern Italy. Although the typical profile shows A, B and C
layers, the B horizon, which normally occurs in brown earths, may
sometimes be almost or completely absent. The A horizon has a
good crumb structure, arising from base-saturation of the organic
matter, and is of a uniform brown colour owing to the presence of
well-dispersed organic and iron compounds.
The fine sand fraction of the soil examined contained 753/0 felspar
and 20% quartz. The clay fraction was once again predominantly
illitic containing 60-80% illite, 5-10% kaolin mineral, 10~ quartz,
and, probably 5~ goethite + gibbsite. As no brown soils f r o m
similar localities have been examined no comparisons can be made.
It may be mentioned, however, that in distinction to the rendzina the
hygroscopic moisture peak on the d.t.a, curve was somewhat smaller
than would be expected from such a high illite concentration; it
may be therefore that the illite tended towards the mica end of the
mica-illite-montmorillonoid series.
CONCLUSIONS
With the exception of the Mediterranean t e r r a r o s s a the soils examined are all essentially illitic in the clay fraction. Kaolin minerals
are always present and seem to cover the entire range from kaolinite
to halloysite. The iron minerals seldom exceed 5% of the clay but
are of importance in giving the soils their characteristic red and yellow
colours. The absence of iron minerals gives the rendzina soils their
typical black and white profile.
REFERENCES
Vaa Bemmelen, J. M. 1888 Landw. Vers. Sto., 35, 67-136.
Bramao, L., Cady, J. G., Hendricks, S. B. and Swerdlow, M. 1952 Soil Sci.,
"/3, 273-287.
Cecconi, S. 1953 Rie. Sci., 23, 1645.
Grim, R. E. 1953 "Clay Mineralogy," McGraw Hill, New York.
Lang, R. 1915 Int. Mitt. Bodenk., 5, 312-346.
288
c. LIPPI-BONCAMBI, R. C. MACKENZIE AND W. A. MITCHELL
Lippi-Boncambi, C.- 1950 "1 Terreni Agrari della Provincia di Perugia." Tipografia Porziuncola, Perugia, Italy.
Lippi-Boncambi, C. and Warimpietri, L. 1949 Ric. Sic., 19, 476-488.
Mackenzie, R. C. 1949 Nature, 164, 244.
1952 Symposium on "Problems of Clay and Laterite Genesis,"
A.I.M.E., New York, pp. 65-75.
Millot, G. 1949 G6ol. appl., 2, Nos. 2, 3, 4.
Mufioz Taboadela, M. 1953.J. Soil Sci., 4, 48-55.
Robertson, R. H. S., Brindley, G. W. and Mackenzie, R. C. 1954 Amer. Min.,
39, 118-139.
Sudo, T. 1954 Clay Min. Bull., 2, 96-106.