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XI. Sulfur Fertilization and Crop Quality
PLANT NUTRIENT SULFUR
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
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.
N. S. PASRICHA AND R. L. FOX
Ratio of S in Methionine
+ Cysteine to Amino Acid N in Cowpea Varieties under Various
Sulfate S Fertilition Levels'
amino acid N Met + Cys
cultivar (SO,-S mg liter') (g/IOO g meal) (a100g meal)b %Met + Cys):N(amino acid)
0.1 I I
'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
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.
PLANT NUTRIENT SULFUR
Influence of Applied S on the Oil Content, Protein Content, and Oil Yield of
Dierent Oil Crops'
Adapted from Pasricha and Aulakh (1991); by permission of The Sulphur Institute,
Washington, D. C.
D. EFFECTON NITRATE
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
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.)
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
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).
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
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