Effects of parent material and land use on soil phosphorus
forms in Southern Belgium
Renneson1 M., Dufey2 J., Bock1 L. and Colinet1 G.
1
University of Liege (Belgium) - Gembloux Agro-Bio Tech – Soil Science Unit.
Passage des Déportés, 2. B-5030 Gembloux - Malorie.Renneson@ulg.ac.be
2
University of Louvain-la-Neuve (Belgium) – Soil Sciences Unit.
INTRODUCTION AND OBJECTIVES
Soil phosphorus faces both environmental and agronomic issues because it is responsible for
eutrophication of surface waters and, at the same time, it is an essential element for plants growth. It
is therefore important to deepen our knowledge about the quantities and forms of P in Walloon soils
and the part of P content due to the background characteristics.
MATERIALS AND METHODS
Twelve types of parent materials have been selected in Walloon Region. For each one, 10 fields
have been chosen (Fig. 1). Soil types were observed by augering and soil samples were taken in
surface horizon (0-15 cm) and in the deep 100-120 cm horizon. They were located in 76 crop fields,
15 temporary grasslands and 29 pastures, chosen according to regional land use distribution.
Different parameters were determined to characterize edaphic conditions : pHwater and pHKCl, total
organic carbon (TOC), cation exchange capacity (CEC) and particle size distribution.
Available phosphorus (Pav) (Lakanen and Erviö, 1971) and total phosphorus (Ptot) (NF X 31-147, 1996)
were determined. Inorganic phosphorus (Pinorg) was extracted by H2SO4 (1:40, w:v) while organic
phosphorus (Porg) was determined by difference of Ptot by Pinorg.
Fig. 1: Location of the parent materials in Walloon Region.
The influence of parent materials can be seen in deeper horizon but also in surface horizon.
Surface P content is also influenced by land use.
There is a large range of variation for studied
soil properties
pH water
4.62 - 8.4
Clay content
6.2 - 67.2%
TOC
0.42 - 9.71%
CEC
2.5 - 71.2 cmol+ kg-1
Pav
15.5 - 241.5 mgP kg-1
P total
167 - 2294 mgP kg-1
Al total
7.5 - 77.7 g kg-1
Fe total
12 - 101.2 g kg-1
The ratio Porg/Pinorg is relatively variable (from 7.9 to
74%), with a mean of 30% close to values from
Conesa & Fardeau (1994).
Influence of land use and parental material in
surface horizon
Fig. 2: Particle size distribution.
Influence of parent material on deeper horizon
Statistical analyses have shown differences between parent materials
for Ptot, Pinorg and Porg contents, Pinorg percentage but also for edaphic
properties. But, despite the diversity of geological contexts , the Ptot
differences are relatively limited (Fig. 3), excepted for sandy loamy
soils which presented low Ptot and Porg contents.
The difference between surface and
deep P content can be attributed
to effects of biogeochemical cycle
and fertilization inputs.
To determine the geochemical
effect, regression models for P
content in deep horizon can be
applied to surface. The difference or
the ratio between observed and
predicted P contents permits to
estimate
the
importance
of
fertilization and other inputs.
a
a
a
ab
ab
ab
ab
ab
ab
ab
b
ab
The land use effect adds to parent
material influence on surface P content. P availability, however, depends
mainly upon parent material (Fig. 4). In
sandy-loamy and loamy soils, the
higher P availability is attributed to
weaker sorption capacities. On average, Pav represented 9% of Ptot.
The statistical analysis of land use
effect on P content should integer the
influence of parent materials. Otherwise, conclusions might differ as can
be seen in Fig. 5 and Fig. 6.
a
b
b
Fig. 3: Difference of total P content according to parent
materials.
Fig. 4: P availability
according to parent
materials.
Significant relationships have been found between total P and total Fe and Al
contents, which are indicators of the quantity of P-adsorption sites. But, on
contrary of other studies, no relationship has been observed with clay content
or Ca. Multiple linear regressions can be usefull to predict soil P content from
more easily measurable or more available soil properties.
bc
cd
d
d
d
d
d
d
a
1200
a
P content (mg.kg-1)
Range of values
1000
b
800
a
ab
b
600
Total P
a
a
400
Inorganic P
Organic P
b
200
0
Crop
Temporary
grassland
Pasture
Fig. 5: Difference of total, inorganic and organic P
according to land use without consideration of
parent materials.
250
Differences of means of total,
inorganic and organic P (mg P.kg-1)
Parameters
On average, deep soil P amounts to half of
surface P.
a
150
a
a
50
Total P
ab ab
b
Inorganic P
-50
-150
Crops
Temporary grassland Pasture
b
Organic P
b
b
-250
Land use
d
Fig. 6: Differences of Ptot, Pinorg and Porg means
between land use modalities and crops. ANOVA.
Ptot = 453 + 25 Fetot - 20.7 clay + 124 TOC (R² = 64.6%)
Pinorg = -174 + 0.657 Ptot + 40.6 pHwater - 5.68 CEC (R² = 92.6%)
Porg = 250 + 0.379 Ptot + 2.28 clay - 48.9 pHwater (R² = 80.8%)
CONCLUSIONS
The fate of P in the soil is under the dependence of soil characteristics and agronomic practices. Parent materials have an influence on both surface and
deep horizons, explained by Fe and Al contents mainly. In surface, land use also influences P content and availability.
So, the management of phosphorus resources in cultivated soils has to take into account the sub-regional specificities of soil parent materials and land
use. Indeed, taking into account parent material modifies conclusion concerning difference of P content according to land use.
Finally, comparison between surface and deep P content should allow to distinguish between P management and geochemical influences.
Fardeau JC, Conesa AP (1994). Le phosphore In ‘Pédologie Tome 2. Constituants et propriétés du sol’. (Eds B Bonneau, M Souchier) pp. 649-658. (Masson publishing: Paris).
Lakanen E, Erviö R (1971). A comparison of eight extractants for the determination of plant available micronutrients in soils. Acta Agralia Fennica 123, 223-232.