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VIII. Diagnosis of Sulfur Needs
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.
PLANT NUTRIENT SULFUR
Sulfur content in
younger leaves (mg S g-’ )
Figure 8. Utilization of fertilizer N by oliseed rape based on the S status of plants (net
utilization by seeds) (Schnug, 1991; by permission of The Sulphur Institute, Washington,
Leaching of underutilized NO3 can create serious environmental problems. Nitrogen and sulfur are both involved in protein synthesis, thus a
shortage of S in relation to N leads to poor N fertilizer efficiency (Fig. 8)
(Schnug, I99 1).
1. Plant Sulfur
Sulfur deficiency in plants results in pale yellow-green leaves. Unlike N
deficiency, S deficiency symptoms first appear in young leaves. Older
leaves may accumulate S and, if mobile nutrients are adequate, may
develop normal color. These symptoms persist even after adequate N
application. Sulfur-deficient plants are often spindly with short and slender
Four criteria of assessing the S status of plants have been used: ( 1) total S,
(2) SO,-& (3) N: S ratio, and (4) SO,-S: total S ratio. The concentration
of S in plants is a direct consequence of S supply; thus total plant S may be
the first choice for evaluating the adequacy of the S supply. Ulrich and
Hylton (1968) concluded that SO,-S content of blades or stems is useful
for diagnosing the S status of rye grass (Loliurn mulfij7orurnLam.). Critical
N. S. PASRICHA AND R. L. FOX
values of S in plants are affected by stage of plant growth, plant part
analyzed, and amount of defoliation previous to sampling, as in the case of
rapeseed mustard and grazed or mowed pasture crops (Jones et al., 1975).
Sulfate, N:S ratio, and SO,-S:total S ratios may be less influenced by
these side issues than total S. However, plant sampling for diagnostic
purposes is usually restricted to a relatively short period of time and to
specific tissues so that the problems enumerated above are not as serious as
is sometimes assumed. The usefulness of foliar analysis is greatly enhanced
if nutrients other than S are adequate. Because this is often not the case, the
use of ratios of nutrients is helpful in diagnosing problems and making
recommendations. Sumner ( 1981) has vigorously advocated the “Diagnosis and Recommendations Integrated System” (DRIS) for that purpose.
The system appears to have merit when a large data base of yields and
chemical compositions is available. But even for major crops, S data are
scarce and for many crops in the tropics and subtropics data are almost
totally lacking. However, pasture crop yield data from thousands of sites
are not available and would be difficult and expensive to obtain (Beaton et
Data for foliar diagnosis cannot be transferred with confidence among
various sampling systems. Therefore, standard procedures should be
agreed on that would serve as reference against which other procedures
could be calibrated. One such procedure has been suggested for banana
and calibration work on S has been published (Fox et al., 1979).
2. Optimum N :S Ratio in Plants
The N :S ratio in plant material has been used as an index to determine
the probability of crop response to N, and more particularly, S fertilization.
Although total N :total S ratios have been used successfully to diagnose S
needs over a wide range of S nutrition, considerable fluctuations in the
ratio have been reported as S levels in plants approach adequacy
(Pumphrey and Moore, 1965). One of the problems of using N :S ratios is
that S is a relatively immobile nutrient in plants. Older leaves tend to be
higher in S than young leaves. Research with sugarcane, banana, and
macadamia demonstrate that very well (Fox et al., 1976). Nitrogen is
mobile; young leaves tend to have higher N concentrations than old leaves.
So it is not correct to assume that the N :S ratio is more stable with age, or
stage of maturity, or plant tissue sampled, than total S or SO,-S is. If the
critical level of S in the ear leaf or maize is about 0.24%,a critical N : S ratio
of about 12 is indicated. Daigger and Fox ( 1971) observed that most of the
N : S ratios of nonS-fertilized sweet corn were little greater than 12 (Table
VI). It is evident from their data that yield versus N :S ratios do not show
PLANT NUTRIENT SULFUR
Yields of Irrigated Fresh Sweet Corn, and N:S Ratios, in Relation to N and S FertilizationD
SO,-S applied (kg ha-')
'Adapted from Daigger and Fox (197 1).
any notable trend. The correlation coefficient for the relationship was only
0.277. For apparently adequate nutrition (yields 13.0 to 13.7 tons ha-'),
ratios ranged from 8.2 to 14.9. There are other problems. When no S was
applied, 225 kg N ha-I was required for maximum yield (13.5 tons), but
when 66 kg S ha-' was applied also, yield was maximum ( 1 3.4 tons ha-')
at 135 kg N ha-'. Thus, a case for the N : S ratio could not be made from
the data examined. Perhaps a better system, at least it seems so for banana,
is to compare the S in young leaves to the S in old leaves. If the ratio
approaches unity, S nutrition is adequate. This system is consistent with
the general observation that S deficiency produces a chlorosis of young
leaves whereas a N deficiency produces a chlorosis of old leaves.
IX. CRITICAL SOIL SOLUTION CONCENTRATION
Data on the external SO4 concentration required by plants have been
accumulating for several years and have now become sufficiently numerous to provide a basis for predicting adequacy of the soil S supply and S
fertilizer requirements. Spencer ( 1975) cites data showing 3- 5 pg ml-' S
adequate for growth of many species, although rape and lucern were
somewhat higher in their requirements at 8 p g ml-I S. From the work done
with several crops under a variety of conditions (Hasan et al., 1970;
Daigger and Fox, 1971; Fox et al., 1976, 1977, 1979), it appears that the
external S@- S requirement for several crops is approximately 5 pg m1-I.
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
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