1
Sulphate leaching in an organic crop rotation on sandy soil in
Denmark
2
3
J. Eriksen*, M. Askegaard
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Department of Crop Physiology and Soil Science, Danish Institute of Agricultural
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Sciences, PO Box 50, DK-8830 Tjele, Denmark.
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* Corresponding author. Tel: +45 89 99 18 70; Fax: +45 89 99 17 19; E-mail:
Jorgen.Eriksen@agrsci.dk
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Abstract
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Sulphate leaching losses may reduce the long-term possibility of maintaining the S
14
supply of crops in low input farming systems. Sulphate leaching and S balances were
15
investigated in an organic dairy/crop rotation (barley [Hordeum vulgare]  grass-clover
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[Lolium perenne/Trifolium repens]  grass-clover  barley/pea [Pisum arvénse] 
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winter wheat [Tritucum aestivum]  fodder beet [Beta vulgaris]) from 1994 until 1998.
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The importance of climatic conditions and use of different organic manure types at
19
different application rates for sulphate leaching were investigated and related to the
20
concurrent nitrate leaching. As an average of years, sulphate leaching from the crop
21
rotation was 20 kg S ha-1, which was equivalent to 60% of the total input to the rotation.
22
Sulphate leaching was very variable over the years (4-45 kg S ha-1 for the same crop)
23
and closely related to the annual drainage volume (r2=0.99; P<0.01). The same
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relationship between drainage and nitrate leaching was not significant. No differences
25
were observed in sulphate leaching between the organic manure types or the application
26
rates, but significant differences were found in sulphate leaching between the different
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crops in the rotation. In the year with the largest drainage volume, there was a
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significant correlation between the S input in irrigation and sulphate leaching (r2=0.69;
29
P<0.05). The S balance was slightly positive when averaged over the four years, as
30
more S was imported than removed. Thus, immediate S deficiency may not occur, but
31
in the longer term a negative S balance must be expected in this crop rotation. However,
32
even with a positive overall S balance, the S input is not necessarily synchronised with
1
1
plant needs. In order to maintain a sufficient S supply in the future when further
2
reductions in the atmospheric deposition are expected, it is important to reduce leaching
3
losses of sulphate.
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Keywords: Leaching; Nitrate; Organic farming; Sulphate; Sulphur balances
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1. Introduction
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During the last decade sulphur (S) deficiency has become widespread in Northern
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Europe. This is mainly caused by the desulphurisation of industrial flue gas as a result
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of national policies to reduce concentrations of primary air pollutants and by changes in
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the composition of commercial fertilisers towards lower S contents. In Denmark, S
14
containing fertilisers are now commonly used in all crops to avoid S deficiency. Where
15
commercial fertilisers cannot be used (e.g., organic farming systems) there is concern
16
whether the S requirement of plants will be met in the longer term (Nguyen et al.,
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1995).
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Plant and soil tests have been used for assessing the S requirement of crops.
19
However, it was suggested by Nguyen and Goh (1994a) that under steady state
20
conditions, mass-balance S models are more reliable than traditional soil or plant S tests
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for assessing long-term annual requirements. If a balance approach is used for
22
estimating the S demand of a crop or an agricultural system, precise predictions must be
23
obtained for S input (atmospheric deposition and S in fertiliser and irrigation water) and
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S output (harvested crops and leaching losses). Precise estimates are easily obtained for
25
all items in the balance except leaching losses, which is unfortunate since leaching
26
losses in temperate regions may be the major S output (McGrath and Zhao, 1995).
27
The retention of sulphate in soils is dependent on the nature of the colloidal
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system, pH, concentration of sulphate and the concentration of other ions in the solution
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(Harward and Reisenauer, 1966). The amount of sulphate adsorbed depends on the
30
surface area of the clay, and the surface charge (Bohn et al., 1986). The effect of pH on
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sulphate adsorption is related to the net charge of Fe and Al oxides. If pH in the soil is
32
lower than zero point of charge (ZPC) it will lead to a positive surface because of
2
1
hydration of the metal oxides, and sulphate will be absorbed in the soil. In agricultural
2
soils, however, pH is often higher than ZPC leading to low retention of sulphate in the
3
soils. Thus, Curtin and Syers (1990) found that virtually all sulphate in soils with pH>6
4
was in solution. Even for soil with a marked capacity to retain sulphate, the strength of
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the retention seems weak. Chao et al. (1962) found that adsorbed sulphate could be
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removed by repeated extraction with water. Finally, sulphate adsorption is influenced by
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the presence of other anions. The order of adsorption strength of anions in soils is:
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hydroxyl>phosphate>sulphate>nitrate/chloride (Tisdale et al., 1984). As a consequence,
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sulphate leaching may be subject to considerable variations caused by differences in soil
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or climatic conditions and by the differences in agricultural management.
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Some workers have used field lysimeters to study sulphate leaching from arable
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land (Shepherd and Bennett, 1998; Tveitnes et al., 1996), pastures (Sakadevan et al.,
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1993a; 1993b) or forest soils (e.g., Prietzel et al., 1995; Sogn and Abrahamsen, 1998). A
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less costly alternative to lysimeters is the suction cup technique, which has been widely
15
used for nitrate determination in soil water (e.g., Lord and Shepherd, 1993; Francis et
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al., 1992). In structured clay soils this method has serious limitations because water
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from macro pores by-pass the cups by preferential flow in fissures (Grossman and
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Udluft, 1991; Hatch et al., 1997). On sandy soils, however, the suction cup technique
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has proved useful for estimating the concentration of the anions nitrate (Djurhuus and
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Jacobsen, 1995), chloride (Barbee and Brown, 1986) and sulphate (Alewell et al.,
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1997).
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The objective of this work was to determine sulphate leaching from an organic
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six-course crop rotation during a four-year period. The importance for sulphate leaching
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of climatic conditions (precipitation) and use of different organic manure types at
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different application rates was investigated and related to the simultaneous nitrate
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leaching. Sulphur balances were determined in order to evaluate the long-term
27
possibility of an adequate S supply for the crops in the rotation.
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2. Materials and methods
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2.1. Experimental site
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The dairy crop rotation is located at Research Centre Foulum, in the central part of
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Jutland (9°34’E, 56°29’N). The soil is classified as an Orthic Alisol with 77 g clay kg-1
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soil, 16 g C kg-1 soil and 0.21 g total S kg-1 soil. The fields were converted to organic
6
farming in 1987, where a six-year rotation (barley – grass-clover – grass-clover –
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barley/pea/ryegrass – winter wheat – fodder beets) was introduced as replacement for a
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conventional cereal rotation.
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2.2. Field trial
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All S data were collected in the rotation in an experiment designed for studying nutrient
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flows, especially nitrogen, in organic farming systems. The experiment has been
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described in detail by Eriksen et al. (1999). In summary, four organic fertiliser
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treatments were established in four replicates in autumn 1993 and spring 1994. The
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treatments represented two systems of cattle housing based on either slurry alone or a
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combination of slurry and deep litter at two livestock densities. The livestock densities
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were 0.9 and 1.4 livestock units (LU) per hectare. One LU is equivalent to 1 milking
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cow of a large breed. In slurry, dry matter and total-S contents in dry matter were 6-8%
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and 0.40-0.55%, respectively. In deep litter, dry matter and total-S contents in dry
21
matter were 27-31% and 0.30-0.97%, respectively.
22
Each of the six fields in the rotation was divided into four blocks with treatments
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in plots of 15x18 m. Following one cut of herbage, the grass-clover fields were grazed
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by cattle from early June. The treatments with 0.9 and 1.4 LU ha-1 were grazed by two
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separate groups of heifers. On average the grass-clover fields were subject to 455
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animal days ha-1 yr-1. After spring-ploughing the 2nd year grass-clover, barley was sown
27
in a mixture with pea and ryegrass, which in July was harvested green for silage
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production. The stubble field was ploughed in late September and winter wheat was
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sown. After harvest of winter wheat, the field was left undisturbed, except for 2-3
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passes with a harrow for weed control, until the following spring when fodder beet was
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sown. The beet field was harvested in October-November and left undisturbed until
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spring when barley was sown with a mixture of white clover and ryegrass to establish
4
1
the following grass-clover field. Occasionally, irrigation was used especially to
2
maintain the grass-clover fields in summer (average 94 mm yr-1). Cereals were irrigated
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less intensively (average 44 mm yr-1).
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Harvest yields were obtained from an area of 36 to 108 m2 depending on the crop.
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A plot combine was used for cereals and grass-clover, whereas fodder beets was
6
removed by hand. Dry matter content of subsamples was determined after drying at
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80°C and total-S was determined by turbidimetry after wet-ashing with magnesium
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nitrate and perchloric acid (Nes, 1979).
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2.3. Sulphate leaching
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Prior to installation the ceramic suction cups were washed in 0.1 M HCl and rinsed in
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tap water. The ceramic cups were tested by applying vacuum to the cups while inserted
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in a nutrient solution (20 µg SO4-S l-1). For this test 16 cups were used, eight had
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previously been installed in the field for at least one year and eight were new. Sulphate
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concentrations were determined in the nutrient solution and in the solution extracted by
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the ceramic cups.
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In each of the 96 plots three ceramic suction cups were installed before
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experiment start at a depth of 1 m and with an internal distance of 2 m. Every two or
20
four weeks, depending on precipitation, a suction of approximately 80 kPa was applied
21
three days prior to sampling. During this period, the suction decreased as a result of
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water sampling. The samples were either analysed separately or bulked for the three
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replicates per plot before analysis for sulphate concentrations. Before November 1995
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sulphate concentrations in water were determined using barium perchlorate/thorin
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titration (Haartz et al., 1979). From this date onwards extracts were filtered through a
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0.45 m filter and sulphate was measured using suppressed anion chromatography. The
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HPLC was equipped with a guard column (IonPac AG4A-SC 4 mm, Dionex, CA), an
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analytical column (AS4A-SC 4mm, Dionex, CA), an Anion Suppressor (ASRS-I 4 mm,
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Dionex, CA) and a Pulsed Eletrochemical Detector (Dionex, CA). The mobile phase
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consisted of 1.8 mM Na2CO3/1.7 NaHCO3 and was passed through the system at 2.0
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ml/min.
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The water balance was calculated using the model Evacrop (Olesen and
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Heidmann, 1990) where inputs were daily measurements of meteorological data
3
(precipition, temperature and evaporation), type of crop, time of sowing, cutting and
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irrigation and soil physical parameters. Sulphate leaching was estimated using the
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trapezoidal rule (Lord and Shepherd, 1993), assuming that sulphate concentrations in
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the extracted soil water represented average flux concentrations. The accumulated
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leaching was calculated from 1 April to 31 March. In the four experimental years
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variations in climatic conditions resulted in differences in drainage of more than 500
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mm (Table 1).
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3. Results
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3.1. Sulphate concentrations in soil water
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It has been observed that new ceramic suction cups can release sulphate to the sample
17
creating a difference between concentrations in soil water and in the sample
18
(Grossmann et al., 1987). When analysing samples extracted from a nutrient solution
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using old and new ceramic suction cups, no differences were found between
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concentrations in the nutrient solution and any of the samples extracted by the cups.
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Thus, this type of ceramic probe did not release sulphate.
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Sulphate concentrations were determined throughout the experimental period, on
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average 13 times per year. It cannot be ruled out that peaks of high sulphate
24
concentrations occurred between two sampling dates. The sulphate concentrations in the
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six fields in the rotation are shown in Fig. 1. Sulphate concentrations were generally
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below 20 mg SO4-S l-1.
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3.2 Sulphate leaching
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The estimated annual leaching losses of sulphate are shown in Table 2. Differences in
31
sulphate leaching between years were caused by differences in drainage volume. An
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extremely good relationship (r2=0.99; P<0.01) existed between the average annual
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1
drainage and the average annual sulphate leaching (Fig. 2). The correlation between
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drainage and nitrate leaching was not significant (Eriksen et al., 1999).
3
No significant differences in sulphate leaching were observed between the organic
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fertiliser treatments or the livestock densities, but differences in sulphate leaching from
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the different crops in the rotation were significant (Table 2).
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3.3. Yield and S content of harvested materials
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Generally, increased livestock density (organic fertiliser application) increased yields of
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cereals in the rotation by 10-15% (Eriksen et al., 1999). The main cause was probably
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the nitrogen content of the fertiliser. At least no significant differences were found
12
between treatments in the S content of plant material (Table 3). The experiment did not
13
include any conventionally treated plots so statistical comparison with conventional
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farming was not possible. However, when comparing S concentrations with Danish
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standard values for S content in plant material (Strudsholm et al., 1997) it seems likely
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that the organically grown crops had lower S contents than conventionally grown crops,
17
as has been found by Nguyen et al. (1995).
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3.4. S balance
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The S balance for the crop rotation is shown in Table 4. Volatilisation from crops or soil
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was ignored as the amount of S emitted from these sources was considered insignificant
22
(Janzen and Ellert, 1998). Inputs through irrigation water was 9.5 kg S ha-1, which was
23
44% of the total-S input. Compared with the S removal in plant material of 9.8 kg S ha-1
24
the input through irrigation was significant. When including the contribution from
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atmospheric deposition estimated from Hovmand et al. (1994) the overall S balance
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becomes positive. Considering that inputs through irrigation and atmospheric deposition
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clearly exceed the S uptake by plants, it is not surprising that sulphate leaching is a
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major S output (on average 66% of total S output).
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4. Discussion
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1
As an average of the years 1994-97 sulphate leaching from the crop rotation was 20 kg
2
S ha-1, which was equivalent to 60% of the total input to the rotation. Sulphate leaching
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was very variable, ranging from 4 to 45 kg S ha-1 for the same crop in different years.
4
Thus, it is very important to have an estimate for sulphate leaching when using a mass
5
balance approach for determining the S status of a crop rotation or a farm, especially for
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soils in temperate regions with high winter rainfall and with a low sulphate retaining
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capacity.
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Sulphate leaching was closely related to drainage volume as also found by
9
Shepherd and Bennett (1998). Differences in S uptake by plants and the application of S
10
in animal manure did not explain the differences in sulphate leaching from the different
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crops. In 1994, when the largest drainage volume was found, there was a significant
12
correlation between the S input in irrigation and sulphate leaching (r2=0.69; P<0.05). If
13
this was caused by the precipitation being well above average (Table 1) then the
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relationship between S input in irrigation and sulphate leaching must be expected to be
15
insignificant in most years. The S content of ground water used for irrigation was 18 mg
16
SO4-S l-1, which is typical for Danish conditions (Simmelsgaard, 1996). Considering
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that at present 18% of Danish farm land can be irrigated (Anonymous, 1997) locally the
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S contribution from this source is considerable.
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Grazing animals in the grass-clover fields did not affect the pattern of sulphate
20
leaching as was the case with the simultaneously determined nitrate leaching. Every
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year high peaks of nitrate concentrations were observed in 2nd year grass-clover in at
22
least one of the four replicates (Eriksen et al., 1999) due to urination by the grazing
23
cattle. Similar behaviour of sulphate concentrations could be expected since urinary S
24
can contribute significantly to S leaching (Nguyen and Goh, 1994b). But in our
25
experiment sulphate concentrations did not show any random variations in the grass-
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clover fields as did nitrate. The coefficient of variation (CV) for sulphate leaching in
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grass-clover (5-48%, average 23%) was no different from that of the arable crops in the
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rotation (5-64%, average 26%). When investigating sheep dung and urine on pasture
29
Sakadevan et al. (1993b) found no increase in sulphate leaching in agreement with our
30
results. This was explained by S in dung primarily being in an organic form. A slightly
31
higher adsorption strength of sulphate compared with nitrate could also cause a faster
8
1
movement of nitrate to 1 m depth following urination. This is supported by Uhlen
2
(1994), who found that leaching of sulphate was delayed relative to that of nitrate.
3
The overall average S balance was slightly positive, as more S was imported than
4
removed. The question remaining is what will happen if the S input decreases either
5
because of reductions in atmospheric deposition or reductions in the use of irrigation?
6
Initially, lower inputs will reduce sulphate leaching losses (Bristow and Garwood,
7
1984), which will only partly compensate for the reductions in input. Therefore,
8
immediate S deficiency may not occur but in the longer term a negative S balance must
9
be expected in this crop rotation. However, even with a positive overall S balance, the S
10
input is not necessarily synchronised with plant needs. Monoghan et al. (1999) found
11
that 50% of grain S in wheat had been accumulated by the plant before anthesis. At this
12
time soil S needs to be available or otherwise the well-known negative effects on yield
13
and quality of the grain may occur (eg. Randall and Wrigley, 1986). In organic farming
14
systems where animal manure is the sole S source, the S supply is limited at this time as
15
the plant-availability of S in animal manure is low for crops with a short growing season
16
(Eriksen et al., 1995).
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5. Conclusions
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Considerable sulphate leaching occurred in the organic crop rotation equivalent to 60%
22
of the total input. Although immediate S deficiency may not occur a negative S balance
23
must be expected in the longer term.
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In order to maintain a sufficient S supply in the future when further reductions in
25
the atmospheric deposition are expected, it is important to reduce leaching losses of
26
sulphate. It has been demonstrated that a catch crop succeeding the main crop can
27
absorb nitrate from the root zone during autumn and winter and thereby reduce nitrate
28
leaching (Thorup-Kristensen and Nielsen, 1998). Similar beneficial effects of catch
29
crops on sulphate leaching may be expected, and we are currently investigating this
30
possibility.
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1
Acknowledgements
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3
Financial support was provided by the Ministry of Food, Agriculture and Fisheries in
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the research programme “Organic Farming 1993-97”. The technical assistance of
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Holger Bak and the staff at Foulumgaard is gratefully acknowledged.
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7
8
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6
7
Tisdale, S.L., Nelson, W.L., Beaton, J.D., 1984. Soil fertility and fertilizers. Macmilan
Publishing Company, New York.
8
Tveitnes, S., Fugleberg, O., Øpstad, S.L., 1996. Leaching of plant nutrients in drainage
9
water as influenced by cropping system, fertilization and climatic factors.
10
Norwegian J. Agric. Sci. 10, 555-582.
11
Uhlen, G., 1994. The leaching behaviour and balances of nitrogen and other elements
12
under spring wheat in lysimeter experiment 1985-92. Acta Agric. Scand. Sect. B,
13
Soil and Plant Sci. 44, 201-207.
13
Table 1
Precipitation and drainage (mm) in the experimental years as average of all crops
Year
Precipitation
Calculated drainage at 1 m´s depth
April to Sept.
Oct. to March
1 April to 31 March
1994-95
456
478
578
1995-96
281
121
64
1996-97
233
363
208
1997-98
402
406
338
Normal
334
292
not calculated
14
Table 2
Sulphate leaching. Average of fertiliser treatments (kg SO4-S ha-1). Values with the same
letter are not significantly different within the column (P<0.05)
1994-95
1995-96
1996-97
1997-98
Average
Barley
44a
3c
15b
37a
25a
1st year grass-clover
45a
4b
20a
29b
24a
2nd year grass-clover
41ab
4b
15b
22c
21b
Barley/pea wholecrop
35b
2d
10cd
14d
15c
Winter wheat
44a
3cd
7d
26bc
20b
Fodder beets
22c
7a
12c
23c
16c
Average
38
4
13
25
20
15
Table 3
Yield and S content in plant material from the crop rotation as average of treatments
Plant fraction
1994
Av.
1995
SD
Av.
1996
SD
Av.
1997
SD
Av.
SD
Yield (Mg ha-1 year-1)
Barley straw
2.9
0.2
5.6
0.4
3.7
0.1
4.1
0.2
Barley grain
3.7
0.3
4.0
0.4
4.2
0.2
4.1
0.5
1st year grass-clover*)
3.5
0.1
4.1
0.1
3.3
0.1
4.2
0.2
2nd year grass-clover*)
3.2
0.4
4.7
0.4
3.6
0.4
5.6
0.3
Barley/pea wholecrop
8.8
0.7
7.8
0.6
8.7
0.4
8.7
0.6
Winter wheat straw
4.0
0.3
4.6
0.3
5.9
0.5
5.1
0.2
Winter wheat grain
4.6
0.3
4.5
0.3
6.0
0.4
5.2
0.1
Fodder beet roots
8.5
0.2
9.0
0.2
12.9
0.7
14.5
0.3
Fodder beet tops
3.0
0.2
2.7
0.1
3.7
0.2
3.7
0.2
S content (g kg-1)
Barley straw
1.2
<0.1
1.0
0.1
1.5
<0.1
1.1
0.1
Barley grain
0.9
<0.1
1.0
0.1
1.2
0.1
1.2
<0.1
1st year grass-clover*)
1.3
<0.1
1.5
<0.1
1.7
<0.1
1.4
0.1
2nd year grass-clover*)
1.5
0.1
1.5
0.1
1.8
0.1
1.4
<0.1
Barley/pea wholecrop
1.6
0.1
1.2
0.1
1.5
<0.1
1.2
<0.1
Winter wheat straw
1.0
<0.1
1.1
<0.1
1.5
0.1
0.8
<0.1
Winter wheat grain
1.2
0.1
1.1
<0.1
1.3
<0.1
1.3
<0.1
Fodder beet roots
0.4
<0.1
0.5
<0.1
0.5
0.1
0.5
<0.1
Fodder beet tops
2.2
0.1
2.7
0.1
2.8
0.1
2.8
0.1
*)
Results in grass-clover are from first cut, after which the plots were grazed by cattle
16
Table 4
S balance. Average of the growth seasons 1994-97 and of organic fertiliser treatments (kg S ha-1). SE
in brackets
Barley
1st yr.
2nd yr.
Barley/
Winter
Fodder
grass-cl.
grass-cl.
pea
wheat
beets
Average
Input
Atmospheric dep.*)
11.0
11.0
11.0
11.0
11.0
11.0
11.0
Animal manure
17.3 (2.8)
0.0
5.5 (1.6)
4.8 (1.6)
18.6 (1.7)
27.1 (1.9)
12.2
Irrigation
10.2 (0.9)
17.1 (1.4)
16.7 (1.4)
3.3 (1.5)
10.0 (0.7)
0.0
9.5
Plant material
9.1 (0.4)
6.2 (0.1)
6.6 (0.3)
11.8 (0.5)
10.4 (1.0)
14.7 (0.9)
9.8
Weight gain cattle
0.1 (<0.1) 0.7 (<0.1) 0.8 (<0.1)
0.0
0.0
0.0
0.3
Leaching
24.6 (4.3)
24.3 (3.9)
20.6 (3.6)
15.3 (3.2)
19.8 (4.3)
15.7 (1.8)
20.0
4.7 (4.9)
-3.1 (3.7)
5.2 (3.6)
-8.0 (2.7)
9.4 (3.0)
7.7 (2.3)
2.6
Output
Balance
*)
Estimated from Hovmand et al. (1994)
17
Figure captions
Fig. 1. Sulphate concentrations in drainage extracted by ceramic suction cups at 1 m depth in the
six-course crop rotation. Error bars: SE
Fig. 2. Correlations between annual drainage and annual leaching of sulphate and nitrate in the
organic crop rotation. Average of treatments and crops
18