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IX. Critical Soil Solution Concentration

IX. Critical Soil Solution Concentration

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242



N. S. PASRICHA AND R. L. FOX



Several attempts have been made to identify sulfur-deficient soils on the

basis of total and/or extractable sulfate, and some of these attempts have

been successful in the temperate zone. This approach may be less than

satisfactory in the tropics. One reason for this is the large quantity of

“sorbed” sulfate in many soils of the tropics (Hue et al., 1990). For

example 58% of the readily available S in a group of soils from Brazil was

sorbed sulfate but only 2% of the readily available S in Iowa was due to

sorbed sulfate (Neptune ez al., 1975). A problem arises because of uncertainty about the nature and availability of extractable sulfate (Adams and

Rawajfih, 1977); although it is clear that plants can utilize adsorbed sulfate

to some degree (Barrow, 1969) and availability of basaluminite has been

demonstrated (Wolt and Adams, 1979), it has been observed that S deficiency may develop in crops growing on tropical soils that contain more

than 1000 kg ha-’ of SO,-S within the root zone. Sulfate sorption by soils

is concentration dependent, and the reciprocal is also true, so that sulfate

in soil solution, the availability of which plants must depend on, can be less

than 1 p g ml-’, even if adsorbed S is high, but sorption capacity is even

greater (Table VII).

Sulfate in some tropical soils is held so strongly that it is sometimes

considered virtually insoluble. The fact that it persists in these soils regardless of leaching is evidence of low solubility. But it is by no means insoluble. Concentrations of SO, usually exceed those of PO, by one or two

orders of magnitude. Subsoil S frequently is less soluble than S in the

surface soil (Lund and Murdock, 1978). If the hypothesis that plants derive

sulfate from soil solution is valid, and if an adequate concentration of S in

solution is in the range of 2 - 5 pg ml-’, then it is clear from Table VII that

some subsoils, even though they may be rich in adsorbed SO,, will not

support sulfate concentration in the soil solution sufficient for adequate

plant nutrition.

However, the importance of subsoils as a S source has been recognized

for a long time. Probert and Jones ( 1977)accurately distinguished fertilizer

S-responsive sites from nonresponsive sites by using weighted profile

means of extractable S to a depth of 1 m or more (Fig. 9). That this sulfate

is also positionally available is evident from data on the uptake of subsoil

SO,-S using a radioisotope dilution technique (Goh et al., 1977; Gregg et

al., 1977). This research indicated uptake of S by grass and clover roots to

depths of at least 1 m in one soil and 50 cm in another.

A 1982 study, as yet unpublished by R. L. Fox, P. M. Cooper, and W. M.

H. Saunders, evaluated the availability of soil sulfate in a set of samples

taken at 20-cm increments to a depth of 200 cm from 19 New Zealand

soils. Some of the samples contained > 1000 mg SO,-S kg-I (Fig. 6).

These soils were well leached and acidic. Sulfur was extracted with



24 3



PLANT NUTRIENT SULFUR

Table VII

Status of Sulfate S in Some Tropical Soil Profilesa

Sulfate as

sampled



Saturation

at 5 pg l i t e r '

in solution



s g-9



(%I



(%I



32

27

145

220



78

67

39

51



herto Rico, Dagney (Orthoxic Tropohumult)

0- 10

10

0.6

10-30

I70

3.0

250

I .5

30 - 60

60-95

373

I .5



51

435

548

67 I



20

39

46

56



herto Rico, Catalina (Tropeptic Haplothox)

0- 16

414

6.0

16-35

1080

16.0

35-60

1310

16.0

60 - 80

I267

2.0

80- 120

1225

5.0

120-130

195

3.0



930

I I80

1430

I720

1730

860



45

92

92

74

71

23



Hawaii, Hanipoe (Typic Dystmndept)

0- I 5

25

0.4

15-30

18

16

30 - 60

17

60-90



220

-



11

-



-



-



-



-



Depth

increment

(cm)



Adsorbed

sulfate

(pg g-')



Sulfate in

solution

(pg s m1-9



Nigeria, alagba (Oxic Paleustalf)

0- 15

25

15-30

18

30 - 60

56

60 - 90

I12



8.0

4.5

I .2

0.7



Hawaii, Akaka (Typic Hydmndept)

0-15

220

15-30

1210

30 - 60

3730

60 - 90

5480

-



Sulfate

adsorption

maxima

h 3



Adapted from Fox ( I980a); by permission of The Sulphur Institute, Washington, D. C.



Ca( H2P04)2solution and by short-term growth of ryegrass seedlings. Plant

S percentage increased linearly with increasing log SO,-S until soil S

reached 250 mg kg-'. Maximum uptake was attained at 1000 mg kg-I,

after which uptake decreased. Apparently, the low-concentration mechanism of SO4 retention (presumably adsorption) began to phase out after

soil SO4 reached 250 mg kg-' and a second mechanism (presumably a

compound of low solubility) controlled availability (solubility) as soil



N. S. PASRICHA AND R. L. FOX



244

1 .o



0.8



0.6

9

al



.-x

al

>

.-c



04



m

-



2

0.2



0

0



2

4

6

8

Phosphate-extractable S (pprn)



10



Figure 9. Relationship between relative yield of legumes and weighted profile mean of

extractable sulfer. Fitted curves are as follows: A = I - Y d Y , = exp(-0.7228 S ) , R = 0.43.

19 D F B = 1 - Y d Y , = exp(-0.7262 S ) , R = 0.58, 18 DF. 0, Siratro; 0, various Stylosanthes spp. (Probert and Jones, 1977).



SO,-S content approached 1000 mg kg-'. Further studies of this type are

needed, but it is probable that both adsorbed and precipitated SO, are

major factors in the S economy of highly weathered soils. That probability

offers a plausible explanation for why S deficiencies have been slow to

develop in such soils.

In negatively charged soils that may have no sulfate buffering capacity,

the magnitude of external S is such that movement of S to roots by mass

flow in the transpiration stream is suggested (SO, concentration in soil

solution X transpiration ratio = plant S). In variable-charge soils, the concentration of SO,-S may be lower than necessary for adequate nutrition,

even though deficiency of S may not be obvious (Fox, 1984). Yield response curves that relate plant growth to the external SO,-S concentration

demonstrate that substantial yields can be made at solution concentrations

that require SO4 movement to roots along concentration gradients. These

observations suggest that in some well-buffered soils, adsorbed SO, is

moving to plant roots by diffusion.

A constant but low SO, concentration in solution in relation to large

amounts of P-extractable SO, from chemically similar soils suggests that

S-bearing minerals, such as basaluminite, are controlling soil solution

concentrations (Fig. 10). However, a smooth plot of concentration versus



PLANT NUTRIENT SULFUR



245



22



B+

Q



20



s

-J



Qa



18



-



Jurbanite



I



1



1



I



I



I



I



quantity suggests adsorption. There are reasons to believe that both mechanisms are involved in controlling sulfate solubility in highly weathered

soils. Sulfate adsorption isotherms of volcanic ash soils generally show

biphasic properties and suggest that 40-8Ofig SO4-S g-' is required to

maintain 3 - 6 mg SO,-S liter-' in soil solution, a concentration range

considered adequate for growth of most crops (Hue et al., 1990).

A rational approach to S nutrition is to detem'ine the required sulfate

concentration in the soil solution (the external S requirement) and the

amount of S fertilizer needed to produce that concentration. Investigations

to define specific external S requirements of plants have been conducted

using four lines of approach:

1. The minimum S content of imgation water associated with near

maximum production (Blair et a/., 1979; Wang, 1978; Yoshida and

Chaudhry, 1972).

2. Yield curves as a function of SO4-S in solution based on sulfate

sorption curves. This approach is appropriate for soils that have a high

capacity for sulfate sorption (Hasan et al., 1970).

3. Solution or sand culture experiments with SO, concentration as a

variable (Fox, 1976).

4. Frequent leaching of soils with dilute sulfate solutions to establish and



246



N. S. PASRICHA AND R. L. FOX



maintain a range of SO, concentrations in “soil solutions,” in which plants

are grown (Fox ef al., 1976, 1977, 1979).

From results of these investigations, using several crops, it seems reasonable to generalize that the external S requirements of crops in the subtropics and tropics are approximately 2 - 5 mg liter-’ S in solution.



X. CROP RESPONSES

Numerous papers have been published on responses to S by crops

growing on highly weathered or intensely leached soils. An extensive listing

has been prepared by Blair (1979). That S deficiency is a problem in the

tropics and subtropics, and that the deficiency has a potential for becoming

worse, is abundantly clear from papers on sugar, fiber, and oil crops

(Aulakh and Pasricha, 1988; Pasricha el al.. 1987, 1988, 1991; Pasricha

and Aulakh, 199 1 ; Braud, 1969; Stanford and Jordan, 1966; Fox, I976),

legume forages (Metson, 1973; Pasricha and Randhawa, 197 1, 1975), grain

legumes (Fox ef al., 1987; Aulakh and Pasricha, 1986; Aulakh et al., 1990;

Pasricha ef af.,1987; 1991), rice (Wang, 1978; Mazid, 1986), corn (Kang

and Osiname, 1976; Pasricha et al., 1977a), coffee (de Freitas et al., 1972),

and banana (Fox ef al., 1979). Sulfur deficiencies in tropical, subtropical,

and warm temperate areas are being reported with increasing frequency

(Blair, 1974; Jones ef af., 1975; Tandon, 1991).

Sulfur probably is the fourth most limiting nutrient in highly weathered

soils, and if only the tropics are considered, it probably ranks third. Furthermore, if effectively nodulated legumes are being grown, S moves up

one place in the ranking to third or second. The relative adequacy of 12

nutrients for clover on seven mountain soils from Equador is summarized

in Table VIII, which clearly shows that S is second only to P for legume

growth in these soils.

Fox et al. ( I 977) observed that the S content of the seeds of cowpea

increased with increasing S fertilization of soil. The levels of S adequate for

seed yield were also sufficient for near maximum S content in the seed.

Sulfur percentage associated with 95% of maximum yield was 0.26%. Seed

yield increased 15-fold as soil solution concentration increased from near

zero to 1.8 mg m1-I. The S: N values were in the range 0.03-0.04 at S

concentrations less than 2 mg liter-’, to 0.07-0.08 at > 5 mg liter-’. These

results imply that to obtain maximum yield of cowpeas in the tropics, S

fertilization will be required in many areas. The S concentration in rainwater in northern Nigeria during high-rainfall months is about 0.2 mg



247



PLANT N U T R I E N T SULFUR

Table VIII

Mean Relative Yields of Trifolium mpens as Intluenced by Withholding

Nutrients from Plants Grown in Seven Mountain Soils of EquadoP



Relative yield (%)

Nutrient(s) withheld

None (all added)

All (none added)

P

S

K

Ca

B



General responseb



Mean



-



Range



100



-



Deficient, 7 soils



16



Deficient, 6 soils

NS'



41

95

92

96



0-45

0-46

17-84

80- I15

67- I17

66-134



NS



Deficient, I soil

Toxic, 1 soil



17



-



-



Calculated from data of Poultney (1975).

'

No statistical evidence that Mg, Zn, Fe, Mn, Mo, or Cu increased

yield.



'NS, Not statistically significant.



liter-' (Bromfield, 1974), a level far below adequacy for either good yield

or high seed S content. However, S contents of rainwater that infiltrates the

soil are augmented by S leached from the standing crop and crop residues

and are further concentrated by water evaporation and transpiration. The

final concentration may be more favorable than is indicated by rainwater

composition.

Laurence et a/. (1976) observed that in Malawi, S applied either as a

foliar dust or soil treatment increased yields of groundnut, although the

effects of complementary treatments were not fully additive. Besides improving yield, S application also produced kernels of better size and quality

than untreated samples.

There is great pressure to achieve breakthrough in human nutrition by

introducing new foods into diets or by developing new cultivars of staple

crops that have high protein contents and at the same time produce high

yields. The improbability of accomplishing either goal (much less a happy

combination) on a sustained basis given the S nutrition constraints of vast

areas in the tropics is evident from research in West Africa (Bromfield,

1974; Fox ef al., 1977). On the other hand, it is possible to increase the S

amino acid content of cowpea, groundnut, and mustard and at the same

time achieve greater yields of grain by increasing the S supply (Pasricha et

al., 1970; Evans ef al., 1977; Fox ef al., 1977). Likewise, S fertilization



N. S. PASRICHA AND R. L. FOX



248



Table IX

Elemental Composition of 11 Cultivars of Cowpea Grain Produced at IITA,

Ibadnn, Nigeria and Calculated Quantities in Harvested Grain for

Two Levels of Production'



Elemental yield (kg ha-')

Concentration (%)

Element



Mean



Range



N

P

K

Ca

Mg

S

Zn



3.97

0.47

I .63

0.10

0.23

0.25

0.0044



3.64-4.36

0.44-0.54

1.50- 1.80

0.10-0.1 I

0.21 -0.23

0.2 I -0.28

0.0038-0.0051



Typical

yield'



Agronomicall

possible yield



8.9



59.7

1.4

24.4

I .5

3.4

3.8

0.07



1.1



3.7

0.2

0.5

0.6

0.01



i



Adapted from Fox et al. ( 1977).

A typical yield is 224 kg ha-I; 1500 kg ha-l is believed to be agronomically

feasible.



improves the quality of rice (Jones el al., 1975; Yoshida and Chaudhry,

1972).

Summerfield et a/. (1974) placed cowpea (Vigna unguiculata) among the

grain legumes, with immediate potential for alleviating human malnutrition in the tropics. Cowpea is relatively rich in protein. The leading area of

production is in the West African Savanna. Per season yield in Nigeria was

224 kg ha-', but it is feasible to produce much higher yields. It is obvious

from Table IX that such agronomically feasible yields will require much

higher outlays of nutrients. Fox ef al. (1977) examined what these numbers

mean with respect to S supply. Assuming typical yields of 224 kg ha-', a

mean sulfur percentage of 0.2596, and 50%partition of plant sulfur into the

grain, a projected value of 1.12 kg S ha-' in the cowpea is obtained. Mean

annual sulfur in the rainfall for northern Nigeria is about 1.14 kg ha-'

(Bromfield, 1974). These estimates suggest that cowpea production in the

dry savanna is already limited by a nutritional (S) constraint. To attain

near-maximum yield, the indicated requirement for sulfur will exceed

natural inputs by an order of magnitude. Obviously, production cannot be

sustained with such a deficit.

Few studies have been reported on responses of millets to S. Relatively

large difference between yields of millet fertilized with single superphosphate and triple superphosphate after 7 years of cultivation (Fig. 1 1 ) are

evidence of the need for S in the soils of West Africa. The implications for



PLANT N U T R I E N T SULFUR



1400

1200



1

-



249



Single superphosphate



1000 800

.-C



s



600

400

L



200



I



I



I



I



I



4.4



80



130



175



P-applied (kg P/ha)



Figure 11. Effect of P sources and rates of application on pearl millet grain yield at

Sadore, Niger. Rainy season, I988 (Bationo and Mokwunye, I99 I ; Reprinted by permission

of Kluwer Academic Pub1ishers.l



S fertilization are discussed by Friesen ( I99 I ). Single superphosphate,

rather than more concentrated fertilizers, may be a preferred source for

crops that require both P and S to increase crop yields (Aulakh and

Pasricha, 1988; Aulakh et al., 1980b; Pasricha et al., 1987, 1991).

Table X presents a summary of yield responses to sulfur applications in

Bangladesh. In most cases, 500 ppm P extracted approximately 10 p g g-'

soil. Yield increases in various crops attributable to S were 5-95% where S

was applied in conjunction with N, P, and K.

Yield responses to applied S were observed in corn grown in the Dominican Republic (Pierre et al., 1990). Sulfur deficiency is intensified by

burning, which volatilizes up to 75% of S contained in residues (Sanchez,

1976). Because much of the total S in soils of subhumid regions is in

organic forms (Tabatabai, I982), total and available S are expected to be

low in conventionally tilled soils where residues are burned. In Thailand, S

applied along with NPK fertilizers significantly increased yields of cassava,

corn, and sesame growing on coarse-textured soil with less than 8 - 12 mg

kg-I phosphate-extractable SO,-S (Parkpian el al., 1991).

Intensive cropping and use of S-free fertilizers has caused 60 - 70% of rice

in South Sulawesi, Indonesia, to become S deficient; pastures respond to S



Table X

Average Yield Increase of Crops Due to S Application in Bangladesh'



Yield

(tons ha-l)b

Crop



Increase in

yield (%)



Response

ratio (kg

extra yield

per kg S)



Trials



Locations



NPK



NPKS



Rice



50



8



4.26



4.53



6.34



9



Wheat

Maize

Mungbean

Chickpea



39

10



10



3.2I

4.62

0.59



1 .oo



9.93

40.85

9.26

9.64



45

2

6



10



1



1



3



1



2.92

3.28

0.54

0.84



Mustard



4



3



0.81



1.27



56.79



15



Groundnut

Potato



3

3



3



I



1.69

22.I3



1.85

22.80



9.41

3.03



5

22



Sugarcane



6



3



16.39



81.31



6.44



164



Conon



5



1



1.67



1.83



9.58



5



18



7



2.31



2.42



2.1 1



18



4



1



1.91



2.00



4.7I



4



Jute

Tobacco



a



2



Reference(s)

Bangladesh Agricultural Research Council

(BARC)(1981,1982, 1983)

-do-doAhmed er al. ( 1 984)

Talukder er 01. (1984)and Bangladesh Agricultural Research Council (BARC)

(1981,1982)

Bangladesh Agricultural Research Institute (BARI) (1981,1982)

Noor and Islam (1983)

Bangladesh Agricultural Research Council

(BARC) (1981,1982)

Bangladesh Agricultural Research Council

(BARC) (1981,1982, 1983)

Bangladesh Agricultural Research Council

(BARC)(1981, 1982)

Bangladesh Agricultural Research Council

(BARC)(1981, 1982, 1983)

Bangladesh Agricultural Research Council

(BARC) (1981,1982)



Adapted from Hussain (1990).

NPK, Addition of nitrogen, phosphorus, and potassium; NPKS, addition of nitrogen, phosphorus, potassium, and sulfur.



PLANT NUTRIENT SULFUR



2s 1



4



z

Figure 12. Response in Centrosema pubescence to sulfur applications in South Sulawesi,

Indonesia (Blair ef a/.. 1978).



application (Blair et al., 1978), and responses to these applications were

greater than to P applications (Fig. 12).

In the Sahelian and Sudanian zones of West Africa, soil organic matter is

being exploited to supply S to crops (Bationo and Mokwunye, 1991).

Friesen ( 1991) reported on the fate and efficiency of S fertilizer applied to

food crops in West Africa. Sulfur fertilizer increased grain yields from 10 to

65% in 14 out of 20 site-years in semiarid and subhumid West Africa.

Substantial leaching losses resulted in low crop recovery of fertilizer S. Low

S fertilizer rates were required, which suggests that S deficiencies in the

region can be corrected at relatively low cost.

Leaching losses probably explain the poor residual value of sulfate fertilizers on highly permeable soils of West Africa. The low organic matter

content of soils provides a very small sink for S immobilization. Most of

the residual S (about 71%) remained as SO, in the profile (Fig. 13) and is

again subject to leaching at the onset of the next rain. The high mobility of

SO, in this region is a result of sandy soil texture.

Considerable interest has developed in acid subsoil amelioration by



252



N. S. PASRICHA AND R. L. FOX

Fertilizer-S(% applied)



0



10



20



30



15



30



5



v



45



%.a

n



60



75



Figure 13. Soil profile distribution at harvest of S derived from phosphogypsum applied

to millet at Sadore, Niger (Friesen, 1991; Reprinted by permission of Kluwer Academic

Publishers.)



applying large quantities of gypsum on the surface of highly weathered soils

and letting natural leaching move Ca and SO4 into the subsoil. The primary purpose, A1 inactivation, deals only indirectly with S as a nutrient,

but S in solution and S nutrition of crops will be influenced, irrespective of

the primary purpose. The physical chemistry and mineralogy of what

happens in the subsoil is complicated (Fig. 10) and crop performance may

be influenced in ways that are unexpected (Farina and Channon, 1988). In

this regard, it will be appropriate to remember that SO4-S concentrations

in solution greater than 15 mg liter-' have depressed the growth of banana

(Fox d al., 1979) and cowpea (Fox et al., 1977).



XI. SULFUR FERTILIZATION AND CROP QUALITY

The contribution of soil fertility to crop quality should not be overlooked. In areas of highly weathered soils, but especially in tropical areas



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