Summary
This report shows the results
of a systematic mapping of Swedish arable land with
regard to humus content and the most important soil
chemical properties. The study includes 3 100
plough-layer samples (0-20 cm) and 1 700 subsoil
samples (40-60 cm) from sampling sites randomly
distributed throughout the agricultural areas in
Sweden. Plough-layer samples were collected from 1988
through 1995, although most were collected during
1994-1995. Subsoil samples were taken in 1995. The
results of the mapping are presented in the form of
statistics that show the average levels at the
country and county levels, and maps that show the
geographic variation.
The following variables were
analyzed in the plough-layer and subsoil samples:
pH-H2O and nitric acid-soluble arsenic
(As), cesium (Cs), cadmium (Cd), chromium (Cr),
cobolt (Co), copper (Cu), lead (Pb), manganese (Mn),
mercury (Hg), molybdenum (Mo), nickel (Ni), strontium
(Sr), vanadium (V) and zinc (Zn).
The following variables were
only analyzed in the plough-layer samples: water-
soluble boron (B), aqua regia-soluble selenium
(Se), ammonium lactate-acetate-soluble phosphorus
(P-AL), hydrochloric acid-soluble phosphorus (P-HCl),
exchangeable calcium (Ca), exchangeable magnesium
(Mg), exchangeable potassium (K), and exchangeable
sodium (Na), exchangeable acidity, total carbon
content (tot-C), carbon content after acid treatment
(organic C) and total nitrogen and sulfur contents
(tot-N and tot-S).
From the basic variables, the
humus content, carbonate content and various ratios
were calculated.
The most important results are
as follows: The organic matter content was relatively
high and averaging 6.2 %, organic soils
included. The median value was 4.1 %. For soils
with an organic matter content of less than 12 %
(mineral soils) the corresponding values were 4.3 and
3.9 % respectively. Less than 5 % of the
soils were poor (< 2 %) in organic
matter. The carbon/nitrogen ratio was lowest in the
plain regions in Skåne and around Lake Mälaren,
where values around 9 were common. The median value
for the country was 11.
The average pH value in the
plough layer was 6.3. Of the soils analyzed, ca
14 % had a pH of 7.0 or higher, while 29 %
had a pH of 6.0 or lower, and just 6 % had a pH
of 5.5 or lower. A pH lower than 5.5 generally
indicates that the soil needs to be limed. The
highest pH values were registered for soils
influenced by calcareous parent material, the lowest
ones for some organic soils.
The content of exchangeable
magnesium in agricultural soils was found to be
strongly negatively correlated with the level of
exchangeable calcium. Magnesium contents were lowest
in southern Sweden and parts of Norrland. Some of the
areas with the lowest magnesium levels also had a
high K/Mg ratio which could make it difficult for the
crops to obtain an adequate magnesium supply. The
highest potassium levels were recorded in areas with
clay soils in Östergötland and eastern Svealand.
The phosphorus level in the
plough layer was generally high. Only 13.5 % of
the soils had P-AL levels corresponding to the lowest
soil fertility classes, I and II (< 4 mg 100 g-1).
Of P-HCl, only 3.3 % of the soils had levels
corresponding to P-HCl classes I (only one sampling
site) and II together (< 40 mg 100 g-1).
Forty-one percent of the soils were of P-HCl class V
(> 80 mg 100 g-1).
Above-average concentrations of
As, Cd, Cu, Hg, Mn, Pb, Se and Zn can often be traced
to the influence of alum shale rich parent material.
This is probably a factor contributing to the high
concentrations of many of these elements in
Österlen, around the plateau mountains in
Västergötland and in Jämtland. Concentrations of
many trace elements, especially Cd, Cr, Cu, Pb and
Zn, were also high in clay soils around Lake
Mälaren. In addition, concentrations of most trace
elements showed a positive relation to the soil
organic matter content. This was most pronounced for
B, Cd, Hg, and Se, the concentrations of which were
generally higher in areas with organic soils.
Trace element concentrations in
agricultural soils are, in other words, determined to
a large degree by the parent material, which is also
illustrated by the fact that concentrations in the
plough layer are highly correlated with those in the
underlying subsoil. The latter is not influenced much
by supplies from external sources. However, for Hg,
Cd and Pb, concentrations were 253, 61 and 24 %
higher in the plough layer as compared with the
subsoil, presumably owing to external supplies in the
form of atmospheric deposition, seed disinfectant
(only Hg) and phosphorous fertilizer (main source for
Cd). Supplies in the form of atmospheric deposition
could be of natural (especially for Hg) as well as
anthropogenic origin.
In 37 % of the arable land
area one or more of the studied heavy metals Cd, Cr,
Cu, Hg, Ni, Pb, and Zn, occurred at concentrations
above the upper limit allowed for sewage sludge
disposal sites. In most cases it is Zn (27 %)
and Cr (22 %) that restrict possibilities for
using sewage sludge. For cadmium, 8.5 % of the
land area has levels above the limit value. In the
area around Lake Mälaren, the proportion of soil
area with concentrations of Cr and Zn above the
respective limit values was substantially higher than
the mean for the country as a whole. Close to
25 % of the soils had Cu concentrations below or
at the limit value assumed to indicate Cu-deficiency.
With the help of data from the
Agricultural Register on farming conditions during
the sampling years any possible effects of farm type
on the soil could be roughly estimated. Analysis of
data from Skåne, Halland and Skaraborg counties
indicates that on farms concentrating on cattle
production the plough-layer pH was lower and the
organic matter content higher compared with farms
concentrating on pig or crop production. Plough-layer
concentrations of P-AL, Zn and Cu were higher on pig
farms than on other types of farms, probably owing to
the relatively large amounts of these elements in pig
feed.