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VII. Sulfate Retention in Soil

VII. Sulfate Retention in Soil

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228



N. S. PASRICHA AND R. L. FOX

Table IV



Extractable Sulfate Sulfur, Solubility of Sulfate, and Calculated Sulfate Adsorption Maxima

of Some Highly Weathered Soils of Puerto Rico'



soil

lnceptisol

(Picacho)



Ultisol

(Los Guineous)



Torres



Catalina



Pirla



Nipe



Depth

increment

(cm)



0- 18

18-35

35-60

60-85

85-130

0-8

8-25

25-45

45-65

65-90

90-150

0- 10

10-30

30-50

50-90

90- I25

0- 16

16-35

35-60

60-80

80- 120

120- I30

0-20

20-34

34-60

60-90

90- I25

0-25

25 -40

40-70

70-95

95-120



Sulfate

adsorption



SO,-Sb



SO,-S,'

0- 120 cm



Sulfate S

solubility



maxd



Saturation



(Pgg-')



(WW



(ppm)



(pg S g-' soil)



('W



4Ooo



0.5

1.5

I .o

0.8

0.8

0.4

0.4

1.2

4.2

3.2

4.0

19.0

10.0

9.4

1.6

2.8

6.0

16.0

16.0

2.0

5.0

3.0

I .7

1.7

7.0

7.5

7.0



94

280

640

710

970

98

209

670

830

1330

870

270

990

800

9 10

830

930

I180

1430

I720

1730

860



Extractable



16

16



60

340

597

20

80



4540



190



326

452

441



280

819

740

589

537



9210



444



10780



1080

1310

1267

I225

195

2

8

210

260

I30

40

255

373

46 3

792



2370



7330



0.5



1.6

1.1



4.0

7.7



17



6

10



48

62

20

40

28

25

34

41

104



10

10



262

280

235

200

360

430

770

1250



83

82

65

65

45

32

92

74

71

23

20

80

80

93

81

20

71

87

60

63



Adapted from Fox (1982) with permission.

Sulfate extracted with 500 pg ml-1 P.

Sulfate in saturation extract.

Maximum sulfate adsorbed as determined from an isotherm based on the Langmuir equation.



an abundance of sorbed sulfate. This is not always evident from published

data because most investigations of sulfate in tropical soils have been

restricted to relatively shallow depths. Also, the extraction procedures most

frequently used have not effectivelyextracted sulfate, nor has the extracted

SO, been effectively determined (Fox et al., 1987). Nevertheless, in assess-



PLANT NUTRIENT SULFUR



229



Table V

Phosphate-Extractable SO,-S in Profiles of Selected Soils from the Humid

Tropicsa

Depth increment (cm)

Location

Australia

Cape York

Columbia

Carimagua

Nigeria

Alagba

Puerto Rico

Carracas

Catalina

Hawaii

Hanipoe

Made

Akaka

Wahiawa



0- I5



15-30



30-60



60-90



90-120



5



5



9



35



31



3



18



22



5



15



25



18



56



112



55



200

I040



360

I270



1250



300

1220



17

950

5480

I60



7

960

4520

200



410

25



18



16



150

220

135



320

1210

66



830

3720

146



340



a Adapted from Fox and Blair (1986). Values in table body are in kilograms

per hectare.



ing the available SO4supply, the quantity of sorbed SO, in the subsoil, in

addition to that contained in the surface horizons, should be considered,

because adsorbed SO, is a feature of subsoils in most highly weathered soils

of the tropics. However, deep root penetration is a precondition for adsorbed SO, utilization in such soils (Probert and Jones, 1977).

Sulfate accumulates to very high levels in some soils, especially in

weathered Andepts, which contain gellike materials, and acid soils, which

contain oxides of iron and aluminum. This is illustrated in Table IV.

Phosphate-extractable S from 19 New Zealand soil profiles to a depth of

2 m was as follows:



SO,-S range (kg ha-')



20-200

200- 1000

1000-2000

2000-5000

10.000- 15,000



15,000-20,000



Number of soils



230



N. S. PASRICHA AND R. L. FOX



All soils that contained more than 10,000 kg SO4-S ha-' developed in

volcanic ash and cinders. All were from the North Island and were highly

weathered. Incipient S deficiency is encountered in the area. Soil profiles

that contained less than lo00 kg S0,-S ha-' were from the South Island,

from areas where S deficiency is a major nutritional problem for white

clover -ryegrass pasture production. Sulfate does not accumulate in large

amounts in profiles developed in volcanic ash until weathering is well

advanced.

Coarse textured sandy soils of the subtropics, especially those developed

on alluvium with neutral to alkaline reaction, adsorb little sulfate (Bahl

and Pasricha, 1984). In general, surface soil materials adsorb less sulfate

than do subsoil materials. No doubt, eluviation of clay-sized materials is

partially responsible, but organic matter and phosphate accumulations in

surface horizons, which block sulfate sorption sites, are major factors. Even

when eluviation of clay is of little consequence, the relationship is clearly

seen, as in the case of Oxisols. Sulfate from surface horizons is easily

leached to lower depths, where it accumulates in large quantities, especially

in highly weathered Oxisols and Dystrandepts (Fox, 1974). Sulfate distribution in the Nipe profile is typical of highly weathered Oxisols that have

been examined (Table IV). The fact that SO, solubility continued to

increase with increasing depth, even though saturation percentage decreased, suggests that adsorbed SO, and saturation percentage are not the

only factors controlling SO, availability. For Andepts in Hawaii and a

diverse group of Puerto Rico soils, sulfur solubility was about 5 pg ml-'

when saturation was 60-80% (Hasan et al., 1970; Fox, 1982). Sulfur

nutrition is probably borderline deficient for some crops when soil solutions contain 3 - 5 mg liter-' SO,-S. Upland soils of the Llanos of Columbia contain little adsorbed SO,. The mean is about 150pg g-I and sulfate

solubility is low (Fox, 1974).These soils require about 60 pg S g- of soil to

attain 5 pg ml-' S in solution (Fig. 5).

Sulfate is readily desorbed from soils by phosphate and hydroxide. Organic anions also compete for adsorption sites with sulfate and in this way

may account for the low quantity of sulfate found in surface soils. Desorption of SO, results in decreased concentration of SO, in solution. Repeated

desorption produces a relatively smooth desorption curve over the concentration range usually encountered in leached tropical soils (low solution

concentration). This, we believe, is evidence for sulfate (ligand) adsorption

as the principal mechanism for SO, retention by such soils. At higher

concentration, other mechanisms of SO, retention probably operate, including electrostatic adsorption associated with positive charge.

In any case, adsorption -desorption of sulfate proceeds relatively rapidly

as compared with phosphate. For example, a 24-hr equilibration time was



PLANT NUTRIENT SULFUR



23 1



I



m



I



I



I

I



I

I



I

I



1.o



10



sod-s in solution (pg m ~ ' )

Figure 5. Sulfate sorption by some diverse soils from tropical America: 0, Typic Dystrandept, Costa Rica;X, Nipe, Puerto Rico; A, Llanos, Columbia; 0, Terra Roxa, Brazil; A,

Eutropept, Costa Rica; 0, 1963-1965 ash, Costa Rica (Fox, 1974; by permission of the

publishers, Buttenvorth-Heinnemann Ltd. 0 )



more than adequate for obtaining a constant concentration of sulfate in

solution (Hasan ef al., 1970), whereas, for phosphate, up to a 6-day equilibration time was required (Fox and Kamprath, 1970). Data in Fig. 6

suggest that at levels of extractable SO,- S above 1000 mg/kg, the solubility

of SO, decreases abruptly. Each of the two points represent five soil samples (all subsoils), two from a highly weathered ash layer (60- 100 cm) and

the remainder developed in weathered basalt scoria. These data suggest one

or more of the following criteria:

1. Up to approximately 100 mg S kg-' soil, availability is controlled by

SO4- S in solution by weakly adsorbed SO4.

2. In the range of 100- lo00 mg S kg-' soil, SO, adsorption assumes an

important role and availability increases with quantity adsorbed.

3. In the range of 1000-3500 mg S kg-' soil, availability is considerably

depressed.



Perhaps decreased availability at high levels of extractable SO, is because

adsorption capacity increased more than SO, quantity, or perhaps because

of precipitated iron or aluminum sulfate compounds of low solubility

(Wolt and Adams, 1979; Wolt ef al., 1992).



N. S. PASRICHA AND R. L. FOX



232

0.200



I



I



0.180



\



r



O



0:

I

I



0)



0.140

C



-m

a



0.120 -



I



,

I



I

I

I



I



I,/I



0.100



0



I /



2



I



8



I



1



32



1



I



128



I



I



1



512



1



I



2048



/



0.080 I



Mean extractable SO., -S (mg S/kg)



Figure 6. Sulfur contents of ryegrass indicator plants in relation to SO, extracted with

phosphate from New Zealand soils (R. L. Fox, P. M. Cooper, and W. M. H. Saunders,

unpublished).



Indicated yield potential of ryegrass at 0.175% plant S is approximately

80% of the maximum attained (approximately 0.22% for maximum yield).

What do these data indicate about S solubility? Information is not sufficient for us to make a firm statement about ryegrass, but if data for maize

apply here, approximately 5 mg liter-' of S in solution is adequate for

maximum yield and only 1 to 2 mg liter-' should be adequate for 80%

yield. Thus, it appears that these soils are undersaturated with SO, (assuming that solution concentration is being controlled by adsorbed SO,).

Given proper environmental conditions and time for precipitating aluminum hydroxy sulfate, lower concentrations may be expected, thus, greater

quantities of extractable SO,.



B. SULFATE

ADSORPTION

CURVES

Adsorption curves are useful for describing, studying, and managing the

SO, status of soils. They integrate and reflect many aspects of mineralogy,

chemistry, and management history of soils. The concentration of SO, in

solution, as predicted by sulfate sorption - desorption curves (the equilibrium concentration when SO, is neither sorbed or desorbed), provides

valuable information on plant nutrition. It indicates the immediate concentration at which sulfate should be available to plants and the concen-



PLANT NUTRIENT SULFUR



233



tration of sulfate in water that drains from that horizon (Fox, 1982). It also

provides an explanation for the fact that crops may be adequately supplied

with S, which usually is 3-5 mg SO4-S liter1, even though the soil is

being leached with rainwater that contains much less S than that. An

example of this has been reported for Hawaii by Hasan et al. ( 1970). Akaka

surface soil materials equilibrated at 5 mg SO,-S liter-' and subsoil material at 3 mg SO4-S liter-' even though predicted SO,-S in rainfall at that

location is approximately 0.35 mg liter1.Annual rainfall is approximately

4500 mm and contains 16 kg S ha-'. Sugarcane and pasture vegetation

growing at this and similar locations give no obvious evidence of S deficiency.

Evapotranspiration probably does not concentrate the leachate more

than 50%. There is, therefore, a 10-fold difference in the concentration of

rainwater and predicted soil solution concentration. A probable explanation for this discrepancy is that SO, in rainwater is augmented by S leached

from vegetation, from leaves, stems, and roots, and from dry deposition in

the plant canopy and on the soil. Further reference to this effect was made

in Section IV dealing with S cycling.

Adsorption curves can also be used to follow the course of soil development. A good example is sulfate adsorption by highly weathered soils on

volcanic ash in the humid tropics, where soil materials and climatic conditions promote rapid removal of silicon and accumulation of hydrated iron

and aluminum oxide. Sulfate concentration in solution is related to the

degree of saturation of the exchange complex with sulfate or with other

specifically adsorbed anions.

Sorption maxima for highly weathered soils are usually associated with

10-2Opg SO,-S ml-I (Fox, 1982). Studies of SO, adsorption by some

West Indian soils by Haque and Walmsley (1974) indicate that higher

solution concentrations do not conform to the Langmuir equation. Therefore, SO4-S concentrations of approximately 10 pg ml-* or less should be

used for calculating adsorption maxima in such soils.

Some specifically adsorbed anions and structural anionic impurities can

give oxide surfaces a net negative charge, thus shifting the point of zero net

charge (PZNC) to lower pH values. The PZNC of synthetic hematite,

corundum, boehmite, and geothite occurs at pH values greater than 7 and

hydration can increase the pH at zero net charge (Parks, 1965). The PZNC

of surface horizons of ferruginous soils is, however, usually below pH 5.0.

Mekaru and Uehara ( 1972) demonstrated that phosphate adsorption increased the CEC of highly weathered soils, resulting in negative adsorption

of NO; and CI-. Such variable-charge soils, in which the sign and magnitude of charge on the solid phase are determined by the chemical environment, adsorb significant amounts of Sot- (Barrow, 1967; Fox, I980b).



234



N. S. PASRICHA AND R. L. FOX



Variable-charge colloids dominate the mineralogy of most soils of the

humid tropics because weathering and leaching processes dedicate soil,

forming minerals enriching them in hydrated oxides of Fe and Al. Materials such as these, together with kaolin clays and organic matter, give rise to

variable charge.

In soils in which permanent negative charge predominates, practically all

of the soil SO, is in the soil solution. But in variable-charged soils, especially if they are sufficiently acid to be near the point of zero net charge,

most of the SO, is sorbed. Thus, SO, sorption by soils requires positive

charge. Net positive charge is not required. To attain net positive charge in

whole soils requires variable-charge colloids and lower pH than is common

to highly weathered soils. The final result, however, depends on at least

three possibilities: (1) the point of zero net charge for certain components

of the soil system may be sufficiently high for net positive charge to

develop on that component at soil pH values near 7.0, (2) adsorbed SO,

may be held as ligands or by mechanisms other than electrostatic attractions, or (3) sulfate may be retained in compounds or complexes that are

relatively insoluble at low pH but are soluble in phosphate extractants.



C. MECHANISM

OF SULFATE

ADSORPTION

Soils derived from volcanic ash may adsorb large amounts of SO, owing

to hydrous oxides of Fe and Al and allophanic clays (X-ray amorphous A1

silicates) present in them (Hingston el al., 1972). Rajan (1979) observed

that SO, is adsorbed on a net positive charged surface as a bidentate

forming a six-membered ring, displacing either two aquo or hydroxyl

ligands. On neutral or negative surfaces, SO, is adsorbed as a monodentate,

displacing one aquo or one hydroxyl ligand. This makes the surface charge

more negative.

Organic ligands play an important role in determining the SO, adsorption capacity of soil and subsequent amount of OH released (Fuller ef al.,

1985). Evans (1986) noted that increases in dissolved organic carbon resulted in increased SO, transport in soil columns at pH 4.6. Similarly,

Gobran and Nilsson ( 1988) found that forest floor leachates containing

dissolved organic ligands inhibited SO, retention by a Spodosol soil at SO4

levels less than 7 mM. The ability of soils to adsorb SO, is an important

factor in determining the effect of acidic deposition on the transport of H+

and cations in terrestrial ecosystems. Inskeep ( 1989) showed that inhibition of SO, adsorption was related to the quantity of oxygen-containing

functional groups rather than soluble C. These results indicate that organic



PLANT NUTRIENT SULFUR



235



acids compete for SO, adsorption sites and that the presence of organic

acids in soil solution will influence SO, adsorption capacities.

The input of acidic deposition containing mobile anions like SO, may be

responsible for increased cation leaching from soils to ground and surface

waters in some forest ecosystems (Seip, 1980; Van Breeman ef al., 1983).

However, mobility of SO, has received considerable attention because net

SO, retention by soils can result in decreased cation leaching. Where an

equivalent displacement of OH or other anions does not occur during the

SO, retention process, cations are coadsorbed with SO, (Khanna and

Beese, 1978; Singh ef al., 1980). Highly weathered tropical soils, which

adsorb substantial amounts of SO,, are considered to be resistant to accelerated cation leaching (Huete and McColl, 1984).

The mechanism of SO, adsorption involves an exchange of OH coordinated with an Fe atom of the oxide structure, resulting in increases in pH at

higher levels of sorption. When SO, is specifically sorbed by goethite or

hematite, negative charge on the surface increases, increasing the ability of

the material to retain cations. In tropical soils, this aspect of SO, sorption

could be valuable, given the extremely low cation exchange capacities at

the low pH of many of these soils. Couto et al. (1979), however, did not

observe a constant increase in pH per unit SO., sorbed for different soil

samples. The Ap horizons sorbed small amounts of SO, and the effect on

pH was not marked, whereas B2 horizons sorbed high amounts of SO,,

and sorption clearly decreased as the pH of the equilibrium solution

increased. The differences between horizons were probably due to the

higher organic matter content of the Ap horizons, because no clear changes

in mineralogical composition were observed. Blocking of positively

charged sites may occur as a result of occupation of these sites by organic

anions or coating of oxide surfaces by organic matter. Phosphate quantity

and intensity also are almost invariably greater in surface horizons. We

may well suppose that organic matter and its decomposition products are

involved. Adams and Rawajfih ( 1977) have suggested that SO, is precipitated as insoluble basic A1 and Fe sulfates in these kinds of soils.

Acid soils that have been limed or those that have naturally high pH,

such as the Tropudalf studied by Couto ef al. (1979) and alluvial soils

studied by Bahl and Pasricha (1984), may require SO, application as a

consequence of their lowered SO, sorption capacities. In addition, liming

acid soils enhances movement of SO, out of the limed zone of soils.

Sulfate retention involves more than one mechanism. Our results (Fox,

1984) suggest that mechanisms of SO, sorption and phosphate sorption are

similar, and that both ions compete for the same sorption sites, although

sorbed SO, does not compete strongly with phosphate (Gebhart and Coleman, 1974). As a consequence of phosphate fertilization, sulfate may be



236



N. S. PASRICHA AND R. L. FOX



0.5



1



2



5



10



20



50



so4 in supernatant (vg mi-’)

Figure 7. The residual influence of phosphate fertilizers on SO, sorption by an Oxisol in

Hawaii (Fox ef a!.. 1971).



displaced from some soils. This effect may persist for several years as is

demonstrated by the shift in a SO, sorption curve that resulted from a

massive phosphate application 12 years earlier (Fig. 7).

Phosphate can extract relatively large amounts of SO, from variablecharge soils, although this may not be obvious if only surface horizons are

investigated (Neptune ef al., 1975). Even CaC1, extraction in a wide

soil :solution ratio may desorb substantial quantities of SO, from highly

buffered soils without depressing SO, concentration below an adequate

level, as indicated by vigorously stirred solution culture experiments. Thus

the external SO, requirement of plants growing in acid, variable-charge

soils and permanent-charge soils may be distinctly different.

Phosphate-extractable SO,- S in profiles of selected soils from the humid

tropics are presented in Table V. It is obvious from such diverse data that it

is unwise to generalize about the quantity of SO, in soils of the humid

tropics, except to say that somewhere in the profile of most of these soils

there is a significant quantity of SO,. Whether such sulfate is available to

plants cannot be inferred from the quantity of SO, alone. Growth of

stylosanthesis on leached sandy soils of tropical Queensland has indicated

that some species can utilize sulfate from considerable depth in such soils

(Gillman, 1973).



PLANT NUTRIENT SULFUR



237



VIII. DIAGNOSIS OF SULFUR NEEDS

A. SOILTESTS

1 . Extractable Sulfur



Estimates of plant-available S in soils have been attempted by several

scientists using a variety of methods. Phosphate-extractable S using monocalcium phosphate (Fox et al., 1964) or monopotassium orthophosphate

( KH2P04)(Ensminger and Freney, 1966) extracts soluble plus adsorbed

SO,. Repeated extraction is required for quantitative removal of sulfate

from many highly weathered soils (Hasan et al., 1970). Hot-water-soluble

sodium acetate (NaOAc, pH 4.8) and sodium bicarbonate extracts include

some organic S also. Phosphate extracts correlate best with crop yields or S

uptake by plants (Barrow, 1967; Scott, 1981) and soil tests appear to be

most valuable on sandy soils. Beaton et al. (1985) reported that soils with

the same characteristics but with soil sulfur concentrations between 7 and

15 kg ha-' would be expected to respond to applied S in dry years but not

in wet years. Brogan and Murphy (1980) did not find soil testing for S to be

promising. They observed, however, that soils with more than 50% sand

and less than 3% organic carbon would likely respond to applied S.

Determining SO4in soil extracts is fraught with difficulties. The methylene blue method of Johnson and Nishita (1952), although sensitive, has

many disadvantages (Lee ef al., 1981; Fox et al., 1987). Ion chromatographic methods are sensitive and more reliable (Kalbasi and Tabatabai,

1985) and are advantageous for simultaneous measurement of other

anions. However, equipment is expensive. Turbidimetric methods have

been widely used for SO, assay of soil extracts since Chesnin and Yien

(1950) introduced one such procedure. Hesse ( 1957) used a sodium acetate

extractant. He attributed interference to colloidal organic matter. Fox el al.

(1964) attempted to overcome this difficulty by extracting with

Ca(H,PO,), instead of NaOAc and digesting the extracts with oxidizing

agents such as nitric-perchloric acid; but even this may not eliminate the

problem of BaSO, precipitate suppression in extracts of some soils (Vander

Zaag et al., 1984). Introduction of BaSO, seed crystals as proposed by

Tabatabai (1974) and adding additional Ba as solid BaCI, in phosphate

extracts that have been evaporated and digested with nitric-perchloric

acid produced higher values. Searle ( 1979) also discovered soil extracts that

failed to yield satisfactory precipitates. Absorbance values less than the

zero standard were obtained. Additions of a SO, spike to these extracts

produced spuriously low results and several activated carbon treatments



238



N. S. PASRICHA AND R. L. FOX



did not overcome the interference. Fox et al. (1987) proposed a more

reliable, although more complicated, turbidimetric method for determining phosphate-extractable sulfate in tropical soils. This method consistently yielded more SO, than other turbidimetric procedures.

Inconsistencies between soil tests for S and crop performances have been

reported widely. These inconsistencies may result from seasonal effects on

extractable S. Castellano and Dick (1991) observed great seasonal variability in SO, levels even in control plots that have not received any S applications for at least 20 years.

Critical levels as high as 10 mg SO,-S kg-' soil have been reported for

the production of winter rape in the Pacific Northwest (Murray and Auld,

1986). One might reach different conclusions, depending on the time of

sampling. Current S soil test recommendations are usually based on samples derived from surface horizons. In environments where there are periods of significant evaporation, movement of SO, from subsurface to

surface may affect soil test results, depending on time of sampling. Additionally, plants usually develop roots below 60 cm and subsoils vary in

their SO, contents. Leaching is an even greater problem. So also is knowing

how to evaluate adsorbed sulfate and subsoil sulfate, i.e., whether it is

adsorbed or not.

2. Optimum N :S Ratio in Soil



Because S is an important component of protein, balanced N :S fertilization is important in obtaining optimal yields and protein contents. If the

N : S ratio is too great, protein synthesis may be restricted and N may

accumulate in plants in nonprotein forms (Pasricha and Randhawa, 1975).

Applications of N to soils deficient in S may lead to decreased yields

(Janzen and Bettany, 1984; Nyborg et al., 1974). The optimal fertilizer

N :S ratio vanes among soils because of differences in available soil N and

S levels. However, one estimate of a suitable available N: S ratio [(soil NO,

N fertilizer N)/(soil SO4-S fertilizer S)] is approximately 7 for upland

conditions (Janzen and Bettany, 1984).

This probably exceeds S requirements in the tropics. A suitable N:S

ratio in solution for sugarcane growth is approximately 10: 1 (Fox, 1976).

As a first approximation, this ratio could be a guide to fertilizer applications for nonlegumes in those areas of the tropics where S removal by crops

is great in comparison with soil and rainfall S (Fox and Blair, 1986).

However, because SO, is not so easily leached or so easily reduced as NO,,

and because the internal N :S ratio is greater than 10, it seems reasonable

to suppose that a 10: 1 N:S ratio in fertilizer, consistently applied, will

more than adequately meet the S requirement of nonlegumes.



+



+



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