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VIII. Diagnosis of Sulfur Needs

VIII. Diagnosis of Sulfur Needs

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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.





95 I


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



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

al., 1985).

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

24 1


Table VI

Yields of Irrigated Fresh Sweet Corn, and N:S Ratios, in Relation to N and S FertilizationD

SO,-S applied (kg ha-')





N applied










































8. I





'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.


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.



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

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