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XI. Sulfur Fertilization and Crop Quality

XI. Sulfur Fertilization and Crop Quality

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PLANT NUTRIENT SULFUR



253



where leached and weathered soils constitute the majority of soils used for

local food production, crop quality becomes especially important. One

concern is amino acid deficits in diets. It is in the tropics, with low

anthropogenic additions of S and intensively leached soils, that S deficiency is most likely; and it is in the tropics and subtropics, generally, that

protein intake is low and primary dependence is placed on plant protein

sources, usually grain legumes and cereal grains.



A. EFFECTON PROTEIN

QUALITY

Sulfur applied to crops grown on S-deficient soils not only increases crop

yields but also favorably affects crop quality. Concerns about protein

quality have led to interest in increasing the sulfur amino acid content of

edible legumes (Luse er al., 1975; Pasricha er al., 1991).

Pasricha ef al. (1970) observed increased S-containing amino acids in

response to S fertilization of groundnut and mustard. For some lupine

varieties, S fertilization increases the S amino acid content of seeds. This

increase is associated with a change in the proportion of various proteins

with differing amino acid ratios (Blagrove ef al., 1976). Sulfur-containing

amino acid content is a better predictor of protein efficiency than is total S

(Sandhu er al., 1974), but relative ease of determination makes the N: S

ratio desirable for screening purposes. For human nutrition, legume seed

proteins are deficient in sulfo-amino acids. A possible remedy for this

deficiency is to increase the sulfo-amino acid levels in seeds by S fertilization.

Concerns about protein quality have created interest in using N : S ratios

of cowpea cultivars as a tool for screening for protein quality (Porter et al.,

1974). The protein S : protein N ratio of IVu 76 cowpea meal increased

27% over the control when cowpea was supplied with 5 mg liter-' of

SO4-S, and that of variety Sitao Pole increased 100%when cowpea was

supplied with 1.8 mg liter-' of SO,-S (Evans er al., 1977). Further details

are presented in Table XI.

Sulfur concentrations, S : N ratios, and S amino acid contents in cowpea

seeds increased with increasing levels of S fertilization. For cowpea variety

Sitao Pole, concentrations of methionine and cyst(e)ine increased approximately twofold as adequacy of S supply increased from severe deficiency to

sufficiency for maximum yields (Evans er al., 1977). For variety IVu 76,

methionine content was increased by 14%, cysteine increased 32Y0, and

S-methyl-L-cysteine increased 470%. Of the 53% increase in S percentage

associated with 5 mg liter-' SO,-S, 16% was derived from increased methionine plus cysteine and 3 I % from increased S-methyl-L-cysteine.



254



N. S. PASRICHA AND R. L. FOX

Table XI



Ratio of S in Methionine



+ Cysteine to Amino Acid N in Cowpea Varieties under Various

Sulfate S Fertilition Levels'



Recovered

Cowpea

Treatment

amino acid N Met + Cys

cultivar (SO,-S mg liter') (g/IOO g meal) (a100g meal)b %Met + Cys):N(amino acid)

IVu 76



0

0.2

0.6

I .8

5.0

15.0

45.0



3.38

3.77

3.62

3.68

3.19

3.13

3.33



0.1 10

0.104

0.1 14

0. I29

0.134

0.122

0. I52



0.033

0.028

0.03 I

0.035

0.042

0.039

0.046



Sitao Pole



0

0.2

0.6

I .8

5.0

15.0

45.0



3.90

4.36

4.22

3.6 I

3.85

3.48

4.04



0.070

0.093

0.1 I I

0.128

0.143

0.133

0.150



0.018

0.02I

0.026

0.036

0.037

0.038

0.037



'Adapted from Evans et a/.(1977).

Dry weight basis of cowpea.



B. EFFECTON OILCONTENT

Improving the S nutrition of S-deficient oil seed crops increases oil

contents in peanut (Aulakh et al., 1980b; Singh, 1968), Brussica species

(Aulakh er af.,1980a; Pasricha el af., 1988), linseed (Aulakh et af., 1989),

and soybean (Aulakh et af., 1990) (Table XII).The relative concentration

of different fatty acids in some oilseeds determines their use. Sulfur fertilization with an adequate supply of N and P resulted in a large decrease in

percentage of stearic, oleic, and linoleic acids with a concurrent increase in

the content of linolenic acid (Aulakh et af., 1989).



C. EFFECTON GLUCOSINOLATE

CONTENT

Plant S is the major factor in the glucosinolate content of oilseed rape

(Zhao et ul., 1991). Excessive S can result in unacceptability due to high

glucosinolate levels and inadequate S may substantially decrease yields.

Both situations markedly reduce the profitability of oilseed rape crops.

Therefore, the effects of S application should be quantified for both yield

and quality in order to obtain optimum benefits.



255



PLANT NUTRIENT SULFUR

Table XI1

Influence of Applied S on the Oil Content, Protein Content, and Oil Yield of

Dierent Oil Crops'



Oil content



Oil yield

(kg ha-')



(%)



Protein



Crop



No S



S



No S



S



No S



Peanut

Brassica juncea

Brassica compestris

Linseed

Soybean



39.0

36.4

41.5

41.6

21.7



48.0

42.6

47. I

43.2

23.6



659

540

300

1285

317



859

670

450

1480

412



-



-



22.5

23.3



30.8

28.1



28. I



31.9



-



S



-



Adapted from Pasricha and Aulakh (1991); by permission of The Sulphur Institute,

Washington, D. C.



D. EFFECTON NITRATE

CONTENT

Sulfur plays an important role in secondary plant metabolism, which is

related to parameters determining the nutritive quality of vegetables

(Schnug, 1990). Nitrate concentration in vegetables has become an important criterion for food quality (Corre and Breimer, 1979; Schuphan, 1976;

Vetter, 1988). A shortage of S adversely affects utilization of N during

plant metabolism. Thus S deficiency causes an accumulation of nonprotein N compounds, including NO3 (Fig. 14). Such a condition indicates

severe S deficiency and is invariably associated with S deficiency symptoms



Y = 69.35 Exp(-l.l28X)+ 0.643



0



1



2



3



4



5



6



S-Content (mg g-')



Figure 14. Nitrate concentration in the dry matter of lettuce as influenced by plant S

status of the plant (Schnug, 1990; by permission of The Sulphur Institute, Washington, D.C.)



2 56



N. S. PASRICHA AND R. L. FOX



(Schnug, 1990). Murphy (1 990) observed that S fertilization affected N : S

ratios and significantly reduced NO3 contents.

An inadequate S supply hinders protein formation and results in accumulation in forage crops of soluble N compounds such as nitrate N and

amide N (Pasricha and Randhawa, 1975).



XII. SULFUR INTERACTIONS WITH OTHER ELEMENTS

A. INTERACTION



WITH



PHOSPHORUS



The fertilizer P and S interaction may be positive or negative depending

on (1) the level of each when applied in combination and (2) soil conditions that control availability of each nutrient. If applied P induces SO,

leaching in soils in which the S level is marginal, onset of S deficiency may

be hastened. In such cases the interaction is antagonistic. On the other

hand, in highly weathered soils that may retain adsorbed SO,, added P

may mobilize the SO,, increasing its availability in the soil. In such a case,

application of S along with P may be without benefit. For example, crop

responses to applications of P and S were synergistic at fertilizer rates of

20-40 kg P and 43 kg S ha-' (Pierre et al., 1990), but others have shown

antagonistic effects (Barrow, 1969; Aulakh et al., 1990).



B. INTERACTION



WITH OTHER

ELEMENTS



Sulfur fertilization may lower the concentration of B and Mo in plants.

This antagonistic effect has been used to suppress Mo in forages growing

on Mo-toxic soils (Pasricha and Randhawa, 1971, 1972; Pasricha et al.,

1977b). On coarse-textured soils with marginal to low amounts of B and

Mo, S fertilization of Brassica species can create deficiencies for these

crops (Schnug and Haneklaus, 1990). Sulfur fertilization is a feasible technique by which to decrease plant uptake of some toxic or otherwise undesirable elements on polluted soils. In areas where Se toxicity exists, Se

uptake can be suppressed by S fertilization (Dhillon and Dhillon, 1991).

Antagonistic relationships between S and anionic trace elements such as

arsenic, bromine, and antimony have been reported.

Grill et al. ( 1990) reported that excess S fertilization may also increase

concentrations of cations such as Cu, Zn, and Cd in roots, while reducing

levels in shoots. This results from stimulated production of phytochelatines

(metallothioneins) in roots, induced by metals in the growth medium, and



PLANT NUTRIENT SULFUR



257



perhaps by enhanced S supply. By this mechanism, plants may avoid

excess uptake. Thus, S fertilization may be a feasible technique to enhance

the quality of crops grown on polluted soils.



XIII. SUMMARY AND CONCLUSIONS

Sulfur deficienciesin the tropics and subtropics have been recognized for

more than 50 years, but even today the extent and magnitude of the

problem is ill-defined. In recent years S-deficient areas of considerable

extent have been discovered and delineated, including, for example, much

of Bangladesh and South Sulawesi.

Sulfur deficiency has been slow to develop, or at least slow to be recognized, for several reasons: the atmosphere is a ubiquitous source of S; other

nutrients, especially N and P, are usually even more deficient than S; S has

been applied in irrigation water and as adjunct to other nutrients (a factor

that is rapidly decreasing in importance); SO, is more efficiently used by

plants than NO3, with which it is frequently compared; as soil organic

matter is exploited, S cycling between organic and inorganic forms is net

positive for inorganic S; adsorbed SO,, which is usually abundant at some

depth in profiles of highly weathered soils, is continually being released.

The pattern of S deficiency on a global scale leads at once to the

conclusion that areas prone to S deficiency are those that are remote from

industrial and domestic burning of fossil fuels, areas where weather patterns are controlled by air masses originating in remote regions, and areas

that have marked wet-dry seasons giving rise to savanna-type vegetation

that is burned frequently. Much of the tropics and subtropics is included in

one or more of these categories. Sulfur sources in much of the continental

tropics are meager. Long-term yields there will not exceed those that can be

supported by the incoming S supply. In some areas S yields in crops are

approximately equal to incoming S in the rainfall.

In the case of soils that do not adsorb sulfate, S supply is controlled by S

currently accruing as rainfall (wet deposition) and directly from the atmosphere (dry deposition), plus S mineralization from organic matter. Other

sources may be locally important: irrigation water, fertilizers, animal manure, and plant residues.

Adsorbed SO, and/or sparingly soluble SO,-containing minerals are

major factors in the S supply of highly weathered subtropical and tropical

soils. In most highly weathered soils, large quantities of SO, have accumulated somewhere in the profile. Usually the accumulation approaches

maximum at about a 1-m depth. Total SO,-S in some leached profiles



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