extractable sulphate and organic sulphur in soils and their availability to plants

8
Plant andSoil 164: 243-250, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands. Extractable sulphate and organic sulphur in soils and their availability to plants F. Zhao and S.P. McGrath 1 Soil Science Department, AFRC lnstitute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, UK. l Corresponding author Received 14 March 1994. Accepted in revised form 14 June 1994 Key words: organic sulphur, soil testing, sulphate, wheat Abstract Ten soils collected from the major arable areas in Britain were used to assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extractable inorganic SO4-S and total soluble S(SO4-S plus a fraction of organic S) were determined using ion chromatography (IC) or inductively-coupled plasma atomic emission spectrometry (ICP-AES), respectively. Water, 0.016 M KH2PO4, 0.01 M CaC12 and 0.01 M Ca(H2PO4)2 extracted similar amounts of SO4-S, as measured by IC, which were consistently smaller than the total extractable S as measured by ICP-AES. The amounts of organic S extracted varied widely between different extractants, with 0.5 M NaHCO3 (pH 8.5) giving the largest amounts and 0.01 M CaC12 the least. Organic S accounted for approximately 30-60% of total S extracted with 0.016 M KH2PO4 and the organic C:S ratios in this extract varied typically between 50 and 70. The concentrations of this S fraction decreased in all soils without added S after two months growth of spring wheat, indicating a release of organic S through mineralisation. All methods tested except 0.5 M NaHCO3 - ICP-AES produced satisfactory results in the regression with plant dry matter response and S uptake in the pot experiment. In general, 0.016 M KH2PO4 appeared to be the best extractant and this extraction followed by ICP-AES determination was considered to be a good method to standardise on. Introduction Sulphur (S) deficiency has become increasingly widespread in grassland and arable crops in the U.K. (McGrath et al., 1993; Syers et al., 1987; Zhao et al., 1993) and other European countries (Schnug, 1991). This is caused mainly by decreased S inputs from the atmosphere and fertilisers. Sulphur dioxide emissions in the U.K. have decreased by about 40% since the early 1970's (Syers et al., 1987), due to the adoption of pollution control measures. Similar trends have been observed in other Western European and North Amer- ican countries (Whelpdale, 1992). Meanwhile, tradi- tional fertilisers which contain considerable amounts of S, such as ammonium sulphate and single super- phosphate, have been replaced progressively by high- analysis fertilisers containing little or no S. Increased S deficiency has led to a greater need for soil testing and plant analysis to diagnose whether applications of fertiliser S are necessary. Plant analysis is usually a reliable tool in the diagnosis of S deficiency (Randall et al., 1981), although the results are often obtained too late for corrective action to be taken on the current crop. Numerous methods have been proposed to evaluate the amounts of soil S available for plant uptake, and results ranging from highly positive to totally unsuc- cessful have been obtained (see reviews by Syers et al., 1987 and Anderson et al., 1992). Most of the methods of soil S testing involve extraction of soil with a weak salt solution, and the extracted S is then determined by a reduction-colorimetric procedure (Johnson and Nishita, 1952) or by a turbidimetric method (e.g. Sin- clair, 1973), both of which are tedious and suffer from the effects of serious chemical interferences, resulting in poor precision and lack of accuracy (Pasricha and Fox, 1993). In addition, these methods do not evaluate the labile pool of organic S, which can become avail-

Upload: f-zhao

Post on 06-Jul-2016

215 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Extractable sulphate and organic sulphur in soils and their availability to plants

Plant andSoil 164: 243-250, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands.

Extractable sulphate and organic sulphur in soils and their availability to plants

F. Zhao and S.P. McGrath 1 Soil Science Department, AFRC lnstitute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ, UK. l Corresponding author

Received 14 March 1994. Accepted in revised form 14 June 1994

Key words: organic sulphur, soil testing, sulphate, wheat

Abstract

Ten soils collected from the major arable areas in Britain were used to assess the availability of soil sulphur (S) to spring wheat in a pot experiment. Soils were extracted with various reagents and the extractable inorganic SO4-S and total soluble S(SO4-S plus a fraction of organic S) were determined using ion chromatography (IC) or inductively-coupled plasma atomic emission spectrometry (ICP-AES), respectively. Water, 0.016 M KH2PO4, 0.01 M CaC12 and 0.01 M Ca(H2PO4)2 extracted similar amounts of SO4-S, as measured by IC, which were consistently smaller than the total extractable S as measured by ICP-AES. The amounts of organic S extracted varied widely between different extractants, with 0.5 M NaHCO3 (pH 8.5) giving the largest amounts and 0.01 M CaC12 the least. Organic S accounted for approximately 30-60% of total S extracted with 0.016 M KH2PO4 and the organic C:S ratios in this extract varied typically between 50 and 70. The concentrations of this S fraction decreased in all soils without added S after two months growth of spring wheat, indicating a release of organic S through mineralisation. All methods tested except 0.5 M NaHCO3 - ICP-AES produced satisfactory results in the regression with plant dry matter response and S uptake in the pot experiment. In general, 0.016 M KH2PO4 appeared to be the best extractant and this extraction followed by ICP-AES determination was considered to be a good method to standardise on.

Introduction

Sulphur (S) deficiency has become increasingly widespread in grassland and arable crops in the U.K. (McGrath et al., 1993; Syers et al., 1987; Zhao et al., 1993) and other European countries (Schnug, 1991). This is caused mainly by decreased S inputs from the atmosphere and fertilisers. Sulphur dioxide emissions in the U.K. have decreased by about 40% since the early 1970's (Syers et al., 1987), due to the adoption of pollution control measures. Similar trends have been observed in other Western European and North Amer- ican countries (Whelpdale, 1992). Meanwhile, tradi- tional fertilisers which contain considerable amounts of S, such as ammonium sulphate and single super- phosphate, have been replaced progressively by high- analysis fertilisers containing little or no S.

Increased S deficiency has led to a greater need for soil testing and plant analysis to diagnose whether

applications of fertiliser S are necessary. Plant analysis is usually a reliable tool in the diagnosis of S deficiency (Randall et al., 1981), although the results are often obtained too late for corrective action to be taken on the current crop.

Numerous methods have been proposed to evaluate the amounts of soil S available for plant uptake, and results ranging from highly positive to totally unsuc- cessful have been obtained (see reviews by Syers et al., 1987 and Anderson et al., 1992). Most of the methods of soil S testing involve extraction of soil with a weak salt solution, and the extracted S is then determined by a reduction-colorimetric procedure (Johnson and Nishita, 1952) or by a turbidimetric method (e.g. Sin- clair, 1973), both of which are tedious and suffer from the effects of serious chemical interferences, resulting in poor precision and lack of accuracy (Pasricha and Fox, 1993). In addition, these methods do not evaluate the labile pool of organic S, which can become avail-

Page 2: Extractable sulphate and organic sulphur in soils and their availability to plants

244

able to plants through mineralisation (Freney, 1986). On the other hand, the traditional method of frac- tionation of organic S into ester-S (HI-reducible) and carbon-bound S does not produce much useful infor- mation about the short-term availability of organic S (Bettany et al., 1974; Freney et al., 1975; Haynes and Williams, 1992; Lee and Speir, 1979).

Recently, instrumental methods of S determina- tion, such as ion chromatography (IC) and inductively- coupled plasma atomic emission spectrometry (ICP- AES), have become more widely used. Ion chromatog- raphy is sensitive and specific to inorganic SO4-S (Tabatabai, 1992). The determination of total S using ICP-AES has been found to be rapid, precise and accu- rate for both soil and plant materials (Anderson et al., 1992; Zhao etal., 1994), One advantage of using ICP- AES is that it allows simultaneous measurement of SO4-S and dissolved organic S (Maynard et al., 1987; Vendrell et al., 1990). A few recent studies with grass- land soils have shown that soil testing methods which include a fraction of organic S along with extractable sulphate correlated best with the availability of S to herbage (Blair et al., 1991), and the organic S extract- ed was directly related to the mineralisable organic S (Watkinson etal . , 1991). Use of these instrumen- tal methods has not been fully examined in S testing for arable soils, so the objective of this study was to investigate how the amounts of SO4-S and organic S, extracted by various extractants from arable soils, related to the response of plant growth to S addition and to the uptake of S by wheat.

Materials and methods

Soils Top soils (0-20 cm) were collected from ten sites locat- ed in the major arable areas in Britain, air-dried and ground to pass a 5 mm sieve for use in a pot experi- ment. Subsamples of each soil were ground further to pass 2 mm and 0.15 mm sieves for chemical analyses. Locations, soil series and selected properties of these soils are presented in Table 1.

Pot experiment Moisture contents of the air-dried soils and their water holding capacities were measured prior to the exper- iment. The equivalent of 1 kg oven-dried soil was placed into eighty I litre plastic pots, each with a saucer. Two treatments were applied to each soil,

i.e. 0 and 50 mg S pot-l (denoted as So and $50, respectively) and each was replicated fourfold. Sul- phur was applied as K2SO4. All pots received a basal dressing of 200 mg N (NH4NO3), 100 mg P (KH2PO4), 248 mg K (KH2PO4, K2SO4 or KC1), 50 mg Mg (MgC12.6H20), 10 mg Mn (MnCI2), 1 mg Cu (CuCIz.2H20), 2 mg Zn (ZnC12) and 1 mg B (H3BO3). These were added to the soils as solutions and mixed thoroughly. The moisture content of each soil was then raised to 60% of the water holding capacity by adding appropriate volumes of deionised water, and was main- tained approximately at this level by frequently weigh- ing and adding water during the experiment. Spring wheat (Triticum aestivum, variety Alexandra) seeds were allowed to germinate on filter paper soaked with deionised water. Ten germinated seeds were then sown in each pot 3 days after nutrient addition. All pots were arranged randomly on trolley benches and the posi- tions were rearranged every fortnight. The experiment was conducted inside a greenhouse, with the following controlled environmental conditions: day/night dura- tion 14h/10h, day/night temperatures 20°C/16°C and day/night humidities 70%/90%. Plants were sprayed with approximately 200 mL of a solution containing 2 g L -x Triadimefon (CI4HI6C1N302) and 0.3 g L -I Pirimicarb (CllHlsN402) to prevent mildew disease and aphids. The plants were harvested at anthesis, 2 months after sowing, by cutting at the soil surface, rinsing with deionised water and drying at 80°C for 24 h before measuring their dry weights. A subsample of fresh soil was taken from each pot for the measurement of extractable S and moisture content.

Chemical analyses The following extractants were chosen for the extrac- tion of soil S : water, 0.016 M KH2PO4, 0.01 M CaClz, 0.01 M Ca(HzPO4)z, 0.25 M KCI and 0.5 M NaHCO3 (pH 8.5). These extractants are commonly used by various workers (Anderson et al., 1992). All extrac- tions, except 0.25 M KCI, were carried out at the same conditions, i.e. 1:5 soil to solution ratio, shaking for 1 h at room temperature (20 + 2°C), centrifuging at 8000 g for 10 rain and filtering through Whatman No. 42 filter paper. The extraction with 0.25 M KCI was carried out at 40°C for 3 h (Blair et al., 1991), using a temperature-controlled shaker. All extractions were performed in duplicate.

Extracted S was determined by ICP-AES (ARL- Fisons simultaneous multichannel model 34000) using a wavelength of 180.73 nm. Corrections for spectral

Page 3: Extractable sulphate and organic sulphur in soils and their availability to plants

Table 1. Some properties of the soils used

No. Location Soil series Texture pH Organic C Total N Total S C:N:S

(inwater) (mgg -1) (mgg - l ) ( m g g - I )

245

1 Elrick, Scotland Dess Sandy loam

2 Garblies, Scotland Forres Loamy sand

3 Cockle Park, Northumberland Dankeswick Clay loam

4 Langston, Devon Denbigh Clay loam

5 Raynham, Norfolk Barrow Sandy loam

6 Sessay, N Yorkshire Sessay Loamy sand

7 Rothamsted, Hefts Batcombe Silty clay loam

8 Flodden, Scottish Borders Wick I Sandy loam

9 Wobum, Beds Cottenham Loamy sand

10 Bridgets, Hants Andover Silty clay loam

5.6 49.1 3.50 0.63 77.9 : 5.6 : 1

5.8 16.7 1.60 0.31 53.9:5.2 : 1

7.0 23.3 1.82 0.37 63.0 : 4.9 : 1

6.2 23.2 2.99 0.41 56.6 : 7.3 : 1

7.8 13.9 1.25 0.23 60.4 : 5.4 : 1

6.7 9.0 0.95 0.19 47.4 : 5.0 : 1

6.2 20.4 1.91 0.33 61.8 : 5.8 : 1

7.2 28.8 2.70 0.54 53.3 : 5.0:1

6.7 8.9 1.12 0.18 49.4 : 6.2 : 1

7.8 36.0 2.42 1.17 30.8 : 2.1 : 1

interference effects, in particular that caused by Ca, were applied on-line by the instrument's computer. In the solutions extracted with water, 0.016 M KH2PO4, 0.01 M CaCI2 or 0.01 M Ca(H2PO4)2, SO4-S was also determined by IC (Dionex 2000i/sp), using an Ion- Pac AS4A separation column fitted with an IonPac AG4A guard column. The eluent solution was 1.8 mM NazCO3 + 1.7 mM NaHCO3 at a flow rate of 2 mL min -I and 12.5 mM H2SO4 was used as a regener- ant. The sample solution was filtered through a 0.2/1m membrane filter (Whatman Anotop 10) prior to injec- tion into the Dionex. Close agreement between ICP- AES and IC was obtained using pure sulphate solu- tions. Dissolved organic C was determined using the UV-persulphate oxidation method (Wu et al., 1990). Results were corrected to oven-dried soil basis.

Plant samples were digested with HCIO4/HNO3 using a Carbolite heating block connected to a tem- perature Controller/Programmer (Zhao et al., 1994). Concentrations of S in the digested solutions were determined by ICP-AES. This procedure gave a mean recovery of 99.1% for S in five certified plant materials (Zhao et al., 1994).

All chemicals used were of analytical reagent grade.

Statistics Analysis of variance and curve fitting were performed using Genstat statistical software (Genstat 5 Commit- tee, 1987).

Results

Extractable sulphate and organic S in soils

Recovery rates of the sulphate-S (Na2SO4) added to soil No. 7 were similar for the extractions with water, KH2PO4, Ca(H2PO4)2 and CaCI2, ranging from 99.7 to 101.9%. The extractions with NaHCO3 and KCI recovered on average 97.2 and 93.3% of the added sulphate-S, respectively.

The ranges of extractable S in the ten soils are shown in Figure 1 for various extractants. Water, KH2PO4, Ca(H2PO4)2 and CaCle extracted similar amounts of sulphate, as measured by IC, which were consistently smaller than the total extractable S as mea- sured by ICP-AES. The concentrations of S extracted with NaHCO3 were 3 to 5 times greater than those with other extractants. Among the other extractants, KH2PO4 extracted the largest amounts of total dis- solved S (measured by ICP-AES), whereas CaCI2 extracted the least.

Since ICP-AES measures total dissolved S, while IC measures only SO4-S, the difference between the two methods can be taken as extractable organic S (Table 2). Direct determination of S04-S by IC was impossible in the KC1 and NaHCO3 extractions, because of the large background anion concentrations. The average concentrations of SO4-S extracted with the other three reagents were used to estimate the con- centrations of organic S extracted by these two meth- ods. It is clear that NaHCO3 (pH 8.5) extracted far more organic S than the other extractants. KH2PO4 also extracted a significant amount of organic S, account-

Page 4: Extractable sulphate and organic sulphur in soils and their availability to plants

246

Table 2. Concentrations (mg kg - t ) of extractable organic C and S and C:S ratios

So0 No, KH2PO 4 Ca(H2PO4) 2 CaCI 2 C S C:S C S C:S C S C:S

Water KCI NaHCO 3 C S C:S C S C:S C S C:S

1 336.3 5.5 60.9 222,1 5.5 40.3 102.0 1.1 89,7

2 146.4 2.5 59,5 91.7 1.9 47.2 43.5 0,8 52.0

3 267.7 4.9 55.1 170.5 3.9 44.2 78.5 1.2 62.9

4 363.7 6.0 60,9 234.8 5,2 45.2 92.0 0,9 103.4

5 161.1 2.6 62.5 90.2 1,8 511 62.4 0.4 174.9

6 212.5 3,5 60.1 111.2 3.0 37.1 49.0 0,3 147.0

7 266.5 5.7 46.5 177.4 5.4 33.0 57.2 1.2 47.4

8 207.1 3.8 54.0 114.9 3.6 32.1 54.9 1.7 32.3

9 202.4 3.2 63,4 76.7 2.8 26.9 35.5 1.1 30,6

10 155.2 2.3 68.0 82.8 2.3 36,6 49.4 0,6 85.4

210,0 3.6 58.6 273.7 4.8 57.5 1936.0 48.8 42.2

103.7 1.4 73.3 187.0 2.7 69,2 814.5 25.8 32.6

183.6 2,9 63.8 141,1 1,6 89.7 1077,5 25.2 44.2

269.3 4,1 66.2 204.1 1.9 108.9 1651,5 47.2 36.2

145.0 1.8 80.3 1333 2.0 65.3 670.3 12.9 53.2

187.6 2.7 69.1 121.7 3.0 40.6 827.8 11,1 75,0

152.7 2.6 57.8 128.3 1.2 109.8 1124.3 28.2 41.5

219.8 4.1 53.3 158.9 2,9 55.3 999.3 24.4 42.7

171.3 3.1 55.0 78.9 1,7 46.5 711.0 12.0 60,5

118.3 2.4 49,8 130.3 1.4 90.6 1046,0 19.9 543

60

5O

40

30

UJ 20 i 0 ~ i i I i i

KH2PO4 Ca(H2pO~)~ CaCI 2 Waler KC~ NaHCQ~

&C ICP-AES IC ICP-AES ~C ICP-AES &C ICP-AES ICP-AES tCP-AES

Fig. 1. Ranges of extractable S in ten arable soils by various extractants. In the box plot, 50 and 80% of data points are within the ranges of the rectangular box and vertical bars, respectively, and any data points outside these ranges are indicated by open circles. Solid and dotted lines inside the box represent median and mean, respectively.

ing for approximately 30--60% of the total S extract- ed with this reagent. In contrast, CaCI2 extracted lit- tle organic S. Other reagents (water, Ca(H2PO4)2 and KCI) extracted intermediate amounts.

Similar patterns occurred for the extractable organ- ic C (Table 2), with NaHCO3 extracting the largest and CaCI2 extracting the least. For KH2PO4, the typical percentage of total organic C extracted was 1%. Ratios of C:S in the extractable organic substances are also shown in Table 2. In the extractions with KH2PO 4 and water, the organic C:S ratios varied typically between 50--70, which were similar to the soil organic C:S ratios (Table 1), except in soil 10 (a calcareous soil) which had a much narrower C:N:S ratio, probably because of the presence of precipitated CaSt4. The organic C:S ratios in the Ca(H2PO4)2 and NaHCO3 extracts were generally narrower. The large variations in the organic

C:S ratios in CaCI2 and KC1 extracts were probably due to the fact that only small amounts of organic C and S were extracted, resulting in relatively large errors in the calculation of C:S ratios.

Relationship between soil extractable S and plant growth and S uptake

Symptoms of S deficiency started to appear three weeks after sowing in plants grown on the So treatment of some soils, with young leaves in particular showing chlorosis. Decreases in the chlorophyll concentrations in the S-deficient plants may be the major reason for the reduced plant growth, as there was a close linear relationship between the chlorophyll concentrations in flag leaves, measured at the stage of flag leaf emer- gence using a chlorophyll meter (Minolta SPAD), and the shoot dry matter at the end of the experiment (anthe- sis stage) (Fig. 2). Decreases in the concentration of chlorophyll due to S deficiency have been observed by many other workers. For example, Burke et al. (1986) found that S deficiency in wheat did not affect the num- ber of chloroplasts per mesophyll cell, but decreased chlorophyll content per chloroplast dramatically, par- ticularly in the younger leaves. Addition of S resulted in significant (p < 0.01) increases in shoot dry matter in all soils.

Relative dry matter yields of shoots (DM of So/DM of Ss0 × 100) were related to the initial concentra- tions of S extracted by the different methods, using an exponential regression model (Table 3). Judging from the percentage variance accounted for by the model, K}-IePO4 appeared to be the best at extracting plant- available S, whereas NaHCO3 gave the poorest fit as this was the worst measure of plant-available S. Other extractants (water, Ca(H2PO4)2, CaCI2 and KC1) also gave satisfactory results. Except for the extraction with

Page 5: Extractable sulphate and organic sulphur in soils and their availability to plants

22 t t L I

Y=2.31 +0.33X, R 2:0.90 • •

• •e •

Y X 3 •

lo

6 ~ . . r - . i f

10 20 30 40 50 60

Chlorophyll meter reading Fig. 2. Relationship between chlorophyll content in flag leaves and shoot dry matter for both So and $50 treatments.

CaC12, determination of SO4-S by IC resulted in a bet- ter fit than total extractable S by ICP-AES, although the difference was small for the KHzPO4 and water extractions.

The relationship between S uptake in the absence of fertiliser S addition and the initial concentrations of extractable S in soils was better described by a linear model (Table 4). Except for the NaHCO3 extraction, which gave a very poor fit, all other extractions pro- duced satisfactory results, with the percentage variance accounted for by the model ranging from 78 to 92%. The best method was KH2PO4-IC, followed by KC1 - ICP-AES, KH2PO4 - ICP-AES and Ca(H2PO4)2 - ICP- AES, The relationships between KHzPO4 extractable S measured by the two methods and relative dry matter yield and S uptake at So are shown in Figure 3.

Changes in KH2P04 extractable S

At the end of the pot experiment, the concentrations of KH2PO4 extractable SO4-S decreased to approxi- mately 1 mg kg-~ in all soils with the So treatment. The concentrations of total extractable S also decreased sharply to 2.5 - 5.0 mg kg - l , so that the differences between soils had largely disappeared. By compar- ing the final with the initial concentrations, the net changes of KH2PO4 extractable organic S were calcu- lated. Figure 4 shows that the decreases in K H 2 P O 4

extractable organic S ranged from 0.5 to 4.3 mg kg- l

247

Table 3. Parameters for the regressions between the amounts of extractable S (X) and relative dry matter yields (Y) using an exponential model: Y = A + BR x

Methods of extraction A B R % of variance

and determination accounted for

KH2PO4- ICP-AES 82.5 -188.0 0.721 85.7

KH2PO4- IC 84.0 -95.8 0.661 87.0

Ca(H2PO4)2-1CP-AES 84 ,5 -124,6 0,752 77.3

Ca(H2PO4)2- IC 81.5 -154.2 0.511 87.1

CaCI2- ICP-AES 79.0 -6927.0 0.202 81.0

CaCI2 - IC 81.2 -175.0 0.500 74,4

Water- ICP-AES 93.9 -120.9 0.815 83.6

Water- IC 83.5 -143,7 0.595 86.0

KCI- ICP-AES 93.1 -126.9 0.787 79.2

NaHCO3- ICP-AES 76.7 -124.0 0.895 39.6

Table 4. Parameters for the regressions between the amounts of extractable S (X) and S uptake in the So treatment (Y) using a linear model: Y = A + BX

Methods of extraction A B % of variance

and determination accounted for

KH2PO4 - 1CP-AES 1.50 0.58 85.3

KH2PO4 - IC 2.40 0.86 92.3

Ca(H2PO4 )2 - ICP-AES 1.77 0,63 81,4

Ca(H2PO4)2 - IC 2.46 0.97 85.3

CaCI2 - ICP-AES 2.12 0.85 82,4

CaCI2- IC 2.58 0.92 77,9

Water - ICP-AES 0.68 0.77 84.1

Water - IC 2.46 0.86 81.6

KCI - ICP-AES 0.12 0,94 89.5

NaHCO3 - ICP-AES 4.02 0.09 27.9

in the absence of applied S. In contrast, there were considerable increases in 0.016 M KI-I2PO4 extractable organic S when fertiliser S was added.

Discussion

Since plants grown on all the soils finally developed S deficiency in the absence of applied S in the present pot experiment, it is difficult to determine the critical values of soil extractable S. It is clear, however, that the critical values would be different depending on the methods of extraction and S determination. Unifying the procedure of soil S testing is thus desirable.

Page 6: Extractable sulphate and organic sulphur in soils and their availability to plants

248

x

r f

(a) lO0

80

60

40

20

o • 2 0 •

/ y , ,.0.80

k ?" o IC

• ICP-AES

2 4 6 8 10

Extractable S (mg kg'~)

, i

12 14 16

m

== o_ #)

(b) 12

10

8

6

4

2

0

R 2 = 0.92 R 2 = 0.85

o • • oO y o •

i I i I i i ,i . _

2 4 6 8 10 12 14 16

Extractable S (rag kg -1)

Fig. 3. Relationships between 0.016 M KH2PO4 extractable S and relative dry matter yield (a) and S uptake in the So treatment (b).

8

~ 6 co

4 3

2

°

z ~5

I I S o

S~o

•/'g. 4 .

extractable organic S after 2 months growth of spring wheat.

__J____.J J _ t - - . . - - ~ _ _ _ ~ 1 2 3 4 5 6 7 8 9 10

Soils

Net changes in the concentrations of 0.016 M KH2PO4

Most of the existing methods of soil S testing have been criticised for their inability to estimate the pool of mineralisable organic S (Freney et al., 1975; Hoque et al., 1987; Syers et al., 1987). Since organic S general- ly represents more than 90% of total S content in soils (Freney, 1986), the mineralisation of organic S is likely to make an important contribution to plant available S. Results from this study showed apparent decreases in the concentrations of organic S in soils extracted with KH2PO4 after 2 months growth of spring wheat. In fact, the net changes of extractable sulphate-S accounted for 22-85% (mostly 45-55%, mean 52%) of total S uptake into plant shoots in the So treatment, suggesting that a proportion of plant S must have come from other sources. Other researchers have also shown a greater S uptake than the amount of extractable sulphate-S in pot experiments (Cowling and Jones, 1970; Lee and Spelt, 1979; Scott, 1981). This extra S can be derived from both dry deposition of S from the air and miner- alisation of organic S in soil. Continuous monitoring showed that the present mean SO2 concentration in the air was approximately 10/~g m -3 at the site of the pot experiment (McGrath, unpubl, data). Assuming depo- sition velocities of SO2 to a wheat crop of 0.7 cm s- l

during the day and 0.3 cm s-1 at night, which are typi- cal values for a dry canopy of wheat in the May to July period (Fowler and Unsworth, 1979), it is estimated that S inputs from the atmosphere amount to 1.9 mg pot-i during the entire experimental period. This is probably just enough to account for the amount of S contained in the roots, which were not recovered in the pot experiment. Therefore, approximately 45% of shoot S was estimated to be derived from soil organic S in the absence of applied S.

When fertiliser S was added, however, there were apparent increases in the extractable organic S in all soils. This suggests that when S is present in sufficient quantities, the organic fraction extracted, for example with KH2PO4, forms an immediate sink for inorganic sulphate.

Mineralisation of soil organic S is likely to become more important if the growth period is prolonged. In the present pot study with a relatively short period of growth, the concentrations of extractable sulphate in soils still correlated slightly better with relative dry matter yields and S uptake than the concentrations of total extractable S (except in the extraction with CaCI2). Other investigations have shown that in grass- land soils the extractable organic S is an important indicator of S availability (Watkinson et al., 1991).

Page 7: Extractable sulphate and organic sulphur in soils and their availability to plants

Use of the instrumental methods, such as IC and ICP-AES, in the determination of S overcomes many of the drawbacks of the traditional chemical methods (e.g. turbidimetric and reduction-colorimetric), such as the lack of precision and accuracy and the uncertainty about the chemical forms of S measured (Anderson et al., 1992; Pasricha and Fox, 1993). The ICP-AES method is particularly suitable as a routine method because it is precise and very rapid (approximately 10 times faster than IC). The measurement of all the soluble organic S and sulphate by ICP-AES could also be an advantage in assessing soil S availability.

Ensminger (1954) first proposed 0.016 M KH2PO 4

to extract soil sulphate and it is one of the most com- monly used extractants for soil S testing. This solution extracted considerable amounts of organic S, rang- ing from 30 to 60% of the total S extracted from the ten arable soils, with well defined C:S ratios, and the extracted S correlated highly significantly with relative yield and S uptake. In comparison, NaHCO3 extracted too much organic S, resulting in poor correlations with relative yield and S uptake. In contrast, the extractions with Ca-based reagents tended to give small amounts of organic S due to the flocculation of organic colloids. This agrees with the results of other authors (Maynard et al., 1987; Vendrell et al., 1990). In addition, these extractants are not preferred in the determination of S by ICP-AES, because of the spectral interference of Ca on S. The method of Blair et al. (1991), i.e. 0.25 M KC1 at 40°C, performed reasonably well in the present study. However, the necessity to maintain the temper- ature at 40°C during extraction and a rather lengthy extraction (3 h) make it less attractive.

Most U.K. surface soils have limited capacity of sulphate adsorption (Curtin and Syers, 1990). This is also confirmed by the present work, as there were only small differences in the concentrations of sulphate extracted by the phosphate extractants and by the non- phosphate extractants or water. Subsoils, however, can have much larger amounts of adsorbed sulphate (Syers et al., 1987) and the contribution of subsoils to plant S uptake cannot be ignored (Hue and Cope, 1987; Zhao and McGrath, 1994). For this reason, an extractant containing phosphate which can extract adsorbed sul- phate, such as KH2PO4, is preferred.

Conclusions

Although both dry matter response of spring wheat to S addition and S uptake correlated highly significantly

249

with the concentrations of soil S extracted with various reagents except 0.5 M NaHCO3 in the pot experiment, we believe that extraction with 0.016 M KH2PO4 and measurement of both sulphate and extractable organic S by ICP-AES is a good method to standardise on. This procedure needs to be evaluated further in field conditions. However, it should be remembered that other factors also need to be considered to predict S deficiency, in particular S inputs from the atmosphere and potential losses of sulphate from soils by leaching and the significance of S uptake from subsoil (Zhao and McGrath, 1993, 1994),

Acknowledgements

This work was funded by the Home-Grown Cereals Authority, London, UK. We would like to thank Mr P J A Withers, Dr P E Bilsborrow and Dr A Sin- clair for collecting some soils used in this study. Mr L Woods was involved in the pot experiment and Mr A R Crosland helped with analytical work.

References

Anderson G, Lefroy R, Chinoim N and Blair G 1992 Soil sulphur testing. Sulphur Agric. 16, 6-14.

Bettany J R, Stewart J W B and Halstead E H 1974 Assessment of available soil sulphur in an 35 S growth chamber experiment. Can. J. Soil Sci. 54, 309-315.

Blair G J, Chinoim N, Lefroy R B, Anderson G C and Crocker G J 1991 A soil sulfur test for pasture and crops. Aust. J. Soil Res. 29. 619-626.

Burke J J, Holloway P and Dalling M J 1986 The effect of sulfur deficiency on the organisation and photosynthetic capability of wheat leaves. J. Plant Physiol. 125, 371-375.

Cowling D W and Jones L H P 1970 A deficiency in soil sulfur supplies for perennial ryegrass in England. Soil Sci. 110, 346- 354.

Curtin D and Syers J K 1990 Extractability and adsorption of sulphate in soils. J. Soil Sci. 41,305-312.

Ensminger L E 1954 Some factors affecting the adsorption of sulfate by Alabama soils. Soil Sci. Soc. Am. Proc. 18, 259-264.

Fowler D and Unsworth M H 1979 Turbulent transfer of sulphur dioxide to a wheat crop. Q. J. R. Meteorol. Soc. 105, 767-783.

Freney J R, Melville G E and Williams C H 1975 Soil organic matter fractions as sources of plant-available sulphur. Soil Biol. Biochem. 7, 217-221.

Freney J R 1986 Forms and reactions of organic sulfur compounds in soils. Ira Sulfur in Agriculture, Agronomy Monograph no 27. Ed. M A Tabatabai. pp 207-217. American Society of Agrono- my, Crop Science Society of America, Soil Science Society of America, Madison, USA.

Genstat 5 Committee 1987 Genstat 5 reference manual. Clarendon Press, Oxford.

Page 8: Extractable sulphate and organic sulphur in soils and their availability to plants

250

Haynes R J and Williams P H 1992 Accumulation of soil organic mat- ter and the forms, mineralization potential and plant-availability of accumulated organic sulphur: effects of pasture improvement and intensive cultivation. Soil Biol. Biochem. 24, 209-217.

Hoque S, Heath S B and Killham K 1987 Evaluation of methods to assess adequacy of potential soil sulphur supply to crops. Plant and Soil 101, 3-8.

Hue N V and Cope J T Jr 1987 Use of soil-profile data for predicting crop response to sulfur. Soil Sci. Soc. Am. J. 51,658-664.

Johnson C M and Nishita H 1952 Microestimation of sulphur in plant materials, soils and irrigation waters. Anal. Chem. 24, 736-742.

Lee R and Speir T W 1979 Sulphur uptake by ryegrass and its rela- tionship to inorganic and organic sulphur levels and sulphatase activity in soil. Plant and Soil 53,407--425.

Maynard D G, Kalra Y P and Radford F G 1987 Extraction and determination of sulfur in organic horizons of forest soils. Soil Sci. Soc. Am. J. 51,801-805.

McGrath S P, Zhao E Crosland A R and Salmon S E 1993 Sulphur status in British wheat grain and its relationship with quality parameters. Asp. Appl. Biol. 36, 317-326.

Pasricha N S and Fox R L 1993 Plant nutrient sulfur in the tropics and subtropics. Adv. Agron. 50, 209-269.

Randall P J, Spencer K and Freney J R 1981 Sulfur and nitrogen fertilizer effects on wheat. I. Concentrations of sulfur and nitrogen and the nitrogen to sulfur ratio in grain, in relation to the yield response. Aust. J. Agric. Res. 32, 203-212.

Schnug E 1991 Sulphur nutritional status of European crops and consequences for agriculture. Sulphur Agric. 15, 7-12.

Scott N M 1981 Evaluation of sulphate status of soils by plant and soil tests. J. Sci. Food Agric. 32, 193-199.

Sinclair A G 1973 An autoanalyzer method for determination of extractable sulphate in soil. N. Z. J. Agric. Res. 16, 287-292.

Syers J K, Skinner R J and Curtin D 1987 Soil and fertiliser sulphur in UK agriculture. Proceedings of The Fertiliser Society. No. 264. Fert. Soc., London.

Tabatabai M A 1992 Methods of measurement of sulphur in soils, plant materials and waters. In Sulphur Cycling on the Continents, SCOPE 48. Eds. R W Howarth, J W B Stewart and M V Ivanov. pp 307-344. John Wiley and Sons, Chichester, England.

Vendrell P E Frank K and Denning J 1990 Determination of soil sulfur by inductively coupled plasma spectroscopy. Comm. Soil Sci. Plant Anal. 21, 1695-1703.

Watkinson J H, Perrott K W and Thorrold B S 1991 Relation- ship between the MAF pasture development index of soil and extractable organic sulphur. In Soil and Plant Testing for Nutrient Deficiencies and Toxicities. Eds. R E White and L D Currie. pp 66-71. Fertilizer and Lime Research Centre, Massey University, Palmerston North, New Zealand.

Whelpdale D M 1992 An overview of the atmospheric sulphur. In Sulphur Cycling on the Continents, SCOPE 48. Eds. R W Howarth, J W B Stewart and M V Ivanov. pp 5-26. John Wiley and Sons, Chichester, England.

Wu J, Joergensen R G, Pommerening B, Chaussod R and Brookes P C 1990 Measurement of soil microbial biomass by fumigation- extraction - an automated procedure. Soil Biol. Biochem. 22, 1167-1169.

Zhao F and McGrath S P 1993 Assessing the risk of sulphur defi- ciency in cereals. J. Sci. Food Agric. 63, 119.

Zhao E Evans E J, Bilsborrow P E and Syers J K 1993 Influence of sulphur and nitrogen on seed yield and quality of low glucosi- nolate oilseed rape (Brassica napus L.). J, Sci. Food Agric. 63, 29-37.

Zhao E McGrath S P and Crosland A R 1994 Comparison of three wet digestion methods for the determination of plant sulphur by inductively coupled plasma atomic emission spectrometry. Comm. Soil Sci. Plant Anal. 25,407-418.

Zhao F and McGrath S P 1994 Comparison of sulphur uptake by oilseed rape and the soil sulphur status of two adjacent fields with different soil series. Soil Use Manage. 10, 47-50.

Section editor: H Marschner