Study of the relevance of phosphorus saturation in Walloon
Region (Southern Belgium)
Renneson M., Vandenberghe C., Marcoen JM., Bock L. and Colinet G.
University of Liege (Belgium) - Gembloux Agro-Bio Tech – Soil Science Unit.
Passage des Déportés, 2. B-5030 Gembloux - [email protected]
INTRODUCTION AND OBJECTIVES
MATERIALS AND METHODS
Agriculture is partly responsible for the eutrophication
of surface waters due to overfertilization and
phosphorus losses. Assessment of the adequacy of
agricultural practices to the sensibility of the physical
environment should rely on indicators which can be
easily determined. The degree of phosphorus
saturation (DPS) in soils is often proposed and is
already used in some countries for environmental
purposes. The adequacy of this indicator to the
Walloon situation had to be assessed before largescale survey.
This study aims to verify the dependency of DPS to
soil type and agricultural soil fertility management.
Six reference farms were selected to represent the range of environmental conditions in
terms of geology and soil fertility management from the Belgian Loamy and Sandy-loamy
regions and the Condroz (three important crop production regions) (Fig. 1). These farms are
included in a network of reference farms which are supervised for nitrate management.
Soil samples were analyzed for oxalate extractable iron, aluminium and phosphorus content
(Feox, Alox and Pox).
Available phosphorus (Pav) was
determined by lakanen-Erviö
method (CH3COO-NH4 + EDTA,
pH 4.65). Total organic carbon,
CEC, total N and clay content
were predicted by near-infrared
spectroscopy
Fig. 1: Location of the studied farms in Wallonia
a DETERMINATION
The Degree of Phosphorus Saturation
calculated by the following equation:
DPS = 100 * Pox / a (Feox + Alox)
INFLUENCE OF SOIL PROPERTIES ON SORPTION CAPACITY
(DPS)
is
(%)
A prior assessment of a scale factor was necessary to
calibrate the a value to the regional natural
environment, even if a 0.5 value is largely used. This
parameter has been determined by the standardized
one-point short-term isotherm method (Bache &
Williams, 1971).
Our results varied from 0.43 to 0.93, with a mean value
of 0.66 and a standard deviation of 0.13. These values
are close to values from other studies. Clayey soils
showed a higher a (0.88 ± 0.06) than the other soils.
Dominant soil order is Luvisol, but some Fluvisols, Regosols and Leptosols were also observed.
Compared to regional values for cultivated soils, the mean Pav (12.8 mg P/100g) appeared
high.
The phosphorus sorption capacity (PSC = a[Feox + Alox]) ranged from 64 to 144 mmol.kg-1 and
depended on soil texture. It was higher in clayey or silty soils than in sandy soils. These latter
soils are therefore more vulnerable to soil losses.
Drainage had also an influence on DPS, PSC and Pox, which were smaller in poor drainage
situations.
High PSC values explained the low DPS values observed in some parcels, even if P content
was high. Indeed, in these soils, P fixation capacity is important and P is well linked.
Various parameters were correlated with DPS, like pHwater (-0.478***), CEC (-0.514***) or clay
content (-0.644***). The use of multiple regression with these edaphic factors didn’t really
improve the evaluation of DPS or other associated parameters. So, simple regressions can
be used.
AGRICULTURAL DEGREE OF PHOSPHORUS SATURATION
50
a
a
a
a
20
ab
ab a
b
b
a
40
a
b
15
30
10
20
5
10
0
0
1
2
3
4
5
Degree of phosphorus saturation (%)
DPS were compared to following parameters to verify the dependency of DPS to agricultural soil fertility management:
• A 5 years P balance for each parcel: no clear relationship could be found with DPS.
• Pav values: a significant relationship (0.595***) between DPS and Pav has shown that DPS was relevant to illustrate soil
fertility and growth potential for crops. This relationship can be improved when the correlation analysis take the
texture into account.
• Fertility classes etablished according to Pav and pH thresholds used in Walloon Region: a good relationship (-0.689***)
between DPS and fertility classes showed that the P saturation gives similar agronomic advice than it is made
routinely (Fig. 3).
25
Pox (mmol/kg)
DPS values ranged from 12.9 to 63.8%, with a mean of 32% (Fig. 2). So, 77% of our samples are above the Dutch
environmental threshold of 25% proposed by Breeuwsma et al. (1995) and 20% should be classified as saturated
according to Flemish legislation (40%, Chardon & Schoumans, 2007). However, until now, no study has been made in
Walloon Region to rely DPS with measurements of P transfers from soil to surface waters.
6
Farms
Fig. 2: Means and standard deviations of oxalate phosphorus
(orange) and degree of phosphorus saturation (green) per
farm.
Boxplot of DPS (%) and alpha parameter
0.7
60
intensive farming of pigs
cultivation
y = 4.4837e-0.052x
R² = 0.9
50
mixed farming and breeding
0.5
DPS (%)
The ratio Pav/Pox evaluates the fraction of sorbed P which is more susceptible to
desorb and migrate to plants or in the soil. The higher the PSC, the smallest the
proportion of available P to sorbed P for the plant growth but also for the
environment. This suggests that the binding strength depends upon the number
of sites and that availability of P is improved with soil saturation. The figure
below also allows one to distinguish intensive pig farming from other cultural
systems (Fig. 4).
Available P / oxalate P
0.6
0.4
0.3
y = -0.0034x + 0.3959
R² = 0.3805
0.2
0.1
40
30
20
y = 0.0006x2 - 0.0627x + 1.7082
R² = 0.4982
10
very_high
0.0
30
40
50
60
70
80
high
good
low
very_low
Fertility classes
Phosphorus Sorption Capacity (mmol/kg)
Fig. 4: Relationship between Pav/Pox and phosphorus sorption
capacity according to cultural system.
Fig. 3: Boxplot of the degree of phosphorus saturation
according to fertility classes.
CONCLUSIONS
Used in many countries, the Degree of Phosphorus Saturation had to be adapted to be useful in Southern Belgium. An a value of 0.66 appears more
relevant to Walloon situation than 0.5 used in many studies. DPS is an environmental indicator which is hardly interpretable because it depends both on soil
properties (mainly texture, drainage and pH) and P fertilization management. Although DPS is well correlated with Pav, it is not very agronomically
transparent. Indeed, to explain DPS behaviour, it has to be divided into Pox and PSC.
So, the use of DPS in Walloon Region is possible but requires other studies to understand better the DPS fate and establishe agronomical and
environmental thresholds, eventually according to regions.
Bache BW, Williams EG (1971). A phosphate sorption index for soils. Journal of Soil Science 22, 289-301.
Breeuwsma A, Reijerink JGA, Schoumans OF (1995). Impact of manure on accumulation and leaching of phosphate in areas of intensive livestock farming. p. 239-251. In ‘Animal waste and the land water interface’. (Ed. Steele K) pp. 239-251. (Lewis
Publishing: New York).
Chardon WJ, Schoumans OF (2007). Soil texture effects on the transport of phosphorus from agricultural land in river deltas of Northern Belgium, The Netherlands and North-West Germany. Soil Use and Management 23 (Supplement 1), 16-24.