A2525

Understanding
Plant
Nutrients

Soil and Applied Sulfur

E.E. Schulte and K.A. Kelling

A lthough sulfur is described as a

secondary plant nutrient, largely
because it is not deficient as often as are
Figure 1, organic sulfur and reduced
sulfide sulfur (S=) combine with oxygen
to form available sulfate sulfur (SO4=)
soil warms or as aeration improves, this
unavailable sulfide sulfur combines
with oxygen to re-form available sulfate
nitrogen, phosphorus, and potassium, it in warm, well-aerated soils. This sulfur.
is as important as any of the major process is very similar to the conversion Harvesting and leaching remove
nutrients. In fact, many crops contain of organic nitrogen into available sulfur from the sulfur cycle. Crop
approximately equal amounts of sulfur ammonium (NH4+) and nitrate removal varies from less than 10 lb/a of
and phosphorus. nitrogen (NO3–). sulfur for grain crops to more than 20 lb/a
Available sulfate sulfur is tied up for legumes and corn silage (Table 1).
SULFUR REACTIONS by bacteria during the decomposition of Sulfate sulfur is not readily held by soil
IN SOILS crop residues rich in carbon. Available particles, except for acid clays, so in
sulfur can also be changed into most soils it can be leached below the
S ulfur transformations are very
similar to those of nitrogen. Most
sulfur in the soil is unavailable as a part
unavailable sulfide sulfur in water-
logged soils. Fortunately, this
root zone. However, sulfate sulfur does
not leach as readily as nitrate nitrogen,
immobilized or reduced sulfur is usually and some acid, clayey subsoils contain
of the soil organic matter. As shown in
only temporarily unavailable. As the sizeable reserves of available sulfate.

Figure 1. The sulfur cycle.

Understanding
Plant
Nutrients

Table 1. Sulfur removed in the harvested portion

of crops.

CROP PORTION YIELD PER SULFUR

HARVESTED ACRE REMOVED
lb/a
Alfalfa Hay 4 tons 23
Corn Grain 150 bu 10
Silage 15 tons 25
Oat Grain 80 bu 5 ■ 10 lb/a/yr
Straw 2 tons 9 ■ 20 lb/a/yr
Potato Tubers 400 cwt 10

Figure 2. Sulfate sulfur in precipitation.

Sources of Sulfur substantial amounts of plant-available All sulfate forms of sulfur fertilizer
Soils commonly contain 200–600 sulfate sulfur. Annual sulfur-supplying are equally effective when surface-
lb/a of total sulfur. Nearly all is in the capacities of subsoils are considered low applied or incorporated (see Table 3).
unavailable organic form. As organic at 10 lb/a, medium at 20 lb/a, or high at Elemental sulfur, however, is insoluble
matter decomposes, a small portion of 40 lb/a. and must be transformed into sulfate-
this sulfur is converted into available Sandy soils may require annual sulfur by soil bacteria before plants can
sulfate sulfur. Approximately 2.8 lb/a of applications of sulfate forms of sulfur use it. The rate of this transformation
sulfur are released annually from each because the sulfate leaches through depends on particle size and degree of
1% organic matter in Wisconsin soils. these soils relatively rapidly. Irrigation mixing with the soil. To be effective,
Another source of un-applied sulfur is water, however, may contain sufficient elemental sulfur should be worked into
atmospheric contamination, which sulfate-sulfur for the crop. In these the soil well in advance of the time the
results from burning coal and, to a cases, response to fertilizer sulfur is crop needs it. Without mechanical
lesser extent, oil and gas. This likely only in years with above-average incorporation, elemental sulfur is
atmospheric sulfur is washed from the rainfall, when little irrigation water is incorporated to some extent by falling
air and deposited on the land in applied. into cracks when the soil dries or by the
rainwater. Precipitation typically
deposits 5–30 lb/a of sulfate-sulfur per
Table 2. Estimates of available sulfur from manure as affected by
year in Wisconsin. Western and
animal and manure type.
northern Wisconsin receive about 10
lb/a of sulfate sulfur annually in ——————— SULFUR CONTENT ———————
precipitation. The remainder of the
state gets twice as much (Figure 2). SOLID (lb/ton) LIQUID (lb/1000 gal)
Many Wisconsin farmers use KIND OF TOTAL AVAILABLE TOTAL AVAILABLE
manure to apply sulfur. The sulfur ANIMAL
contribution from manure varies among
animal species and with the rate of Beef 1.7 0.9 4.8 2.6
application (Table 2). About 55% of
Dairy 1.5 0.8 4.2 2.3
total manure sulfur becomes available
to crops in the year of application. The Poultry 3.2 1.8 9.0 5.0
final source of sulfur is the subsoil.
Clayey, acidic subsoils may contain Swine 2.7 1.5 7.6 4.2
 Understanding
Plant
Nutrients

activity of earthworms and burrowing Sulfur, as a constituent of nitrate Soil Analysis

insects. If the soil is known to be reductase, is involved in the conversion Soil tests for available sulfur are
deficient in sulfur, include some sulfate of nitrate into organic nitrogen. Sulfur helpful but in general are not as precise
sulfur in topdress applications for deficiency consequently interferes with as tests for phosphorus and potassium.
immediate sulfur availability. nitrogen metabolism, which explains Sources of sulfur not directly measured
Row crops and small grains require why sulfur deficiency resembles by the soil test may contribute
about 10 lb/a of sulfur in sulfur- nitrogen deficiency in many crops. significantly to crop requirements. The
deficient areas. Alfalfa requires 25–50 However, the symptoms usually are not Wisconsin soil-testing labs calculate a
lb/a if applied during the seeding year as dramatic and are not localized on the sulfur availability index (SAI) by
or 15–25 lb/a when topdressed on older leaves. Lack of sulfur appears as a estimating sulfur released from organic
established alfalfa. These treatments light green coloring of the whole plant. matter, sulfur in precipitation, subsoil
generally will supply enough sulfur to Legumes, especially alfalfa, have a high sulfur, and sulfur in manure if applied,
last 2–3 years. Sulfur may be applied as sulfur requirement, so deficiencies which they add to the soil-test sulfur-
row fertilizer, but thiosulfate should not usually appear on these crops first. sulfate to determine the total available
be seed-placed. Corn, small grains, and other grasses sulfur. If these contributions add up to
show sulfur deficiencies less frequently. 40 units or more, response to added
DIAGNOSTIC TECHNIQUES Sulfur deficiency in corn sometimes sulfur is unlikely. If the index is
mimics other deficiencies such as between 30 and 40, the sulfur need
Deficiency Symptoms manganese or magnesium in that it should be confirmed by plant analysis,
S ulfur is a component of the amino
acids cysteine, cystine, and
methionine. These amino acids are
causes interveinal chlorosis: the upper
leaves tend to be striped, with the veins
and if the index is less than 30 sulfur
should be added. See Extension
remaining a darker green than the area publication A2809, Soil Test
among the “building blocks” of protein, between the veins. Recommendations for Field, Vegetable and
and shortage of sulfur retards protein
Fruit Crops, for additional information
synthesis.
on the sulfur availability index.

Table 3. Fertilizer sources of sulfur.

NAME OF FERTILIZER CHEMICAL ANALYSIS PERCENT

FORMULA (N-P2O5-K2O) SULFUR
VERY SOLUBLE
Ammonium sulfate (NH4)2SO4 21-0-0 24
Ammonium thiosulfate
(60% aqueous solution) (NH4)2S2O3 + H2O 12-0-0 26
Magnesium sulfate MgSO4•7H2O (Epsom salts) 0-0-0 14
Ordinary superphosphate Ca(H2PO4)2 + CaSO4 0-20-0 14
Potassium magnesium sulfate K2SO4•2MgSO4 0-0-22 23
(Sulpomag)
Potassium sulfate K2SO4 0-0-50 18
SLIGHTLY SOLUBLE
Calcium sulfate (gypsum) CaSO4•2H2O 0-0-0 17
INSOLUBLE
Elemental sulfur S 0-0-0 88–98
Understanding
Plant
Nutrients

Plant Analysis ADDITIONAL INFORMATION

Some laboratories analyze sulfur
These publications in the
routinely as part of their plant analysis
Understanding Plant Nutrients series are
program, while others analyze this
available from your county Extension
element only upon request. When a soil
office:
tests low or moderate in available sulfur
(SAI less than 40), it is a good idea to Soil and Applied Boron (A2522)
verify the suspected deficiency through Soil and Applied Calcium (A2523)
plant analysis. Instructions for sampling Soil and Applied Chlorine (A3556)
various crops and directions for sending Soil and Applied Copper (A2527)
samples to the laboratory are available
from county Extension offices and the Soil and Applied Iron (A3554)
Soil & Plant Analysis Laboratory, 5711 Soil and Applied Magnesium (A2524)
Mineral Point Road, Madison, WI Soil and Applied Manganese (A2526)
53705. Soil and Applied Molybdenum (A3555)
Guidelines for the required levels
Soil and Applied Nitrogen (A2519)
of sulfur in plant tissue for some of the
major agronomic crops are given in Soil and Applied Phosphorus (A2520)
Table 4. Soil and Applied Potassium (A2521)
Soil and Applied Sulfur (A2525)
Soil and Applied Zinc (A2528)

Table 4. Sulfur plant-analysis interpretations for common Wisconsin field crops.

———————— INTERPRETATION ————————

CROP PLANT PART TIME OF DEFICIENT LOW SUFFICIENT HIGH
SAMPLED SAMPLING
————————————- % ————————————-
Alfalfa Top 6 inches Early bud <0.20 0.20–0.25 0.26–0.50 >0.50
Corn Earleaf Silking <0.10 0.10–0.20 0.21–0.50 >0.50
Oat Top leaves Boot stage <0.15 0.15–0.20 0.21–0.40 >0.40
Soybean First trifoliate Early flower <0.15 0.15–0.20 0.21–0.40 >0.40

Authors: E.E. Schulte and K.A. Kelling are professors of soil science, College of Agricultural and Life Sciences, University of Wisconsin-
Madison and University of Wisconsin-Extension, Cooperative Extension. The authors wish to thank L.M. Walsh, professor of soil
science, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension, for contributions from an
earlier edition of this publication and P.D. Ehrhardt for editorial assistance. Produced by Cooperative Extension Publications, University
of Wisconsin-Extension.
University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin
counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress. An Equal Employment
Opportunity/Affirmative Action employer, University of Wisconsin-Extension provides equal opportunities in employment and
programming, including Title IX requirements.
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A2525 Soil and Applied Sulfur R-09-92