Sulfur
Importance



Vital for reactions in living cells
Important in plant and animal nutrition
Environmental pollutant
Roles of S in plants
 Constituent of amino acids [examples: cysteine; Cystine; methionine]
 Constituent of vitamins:[biotin; thiamine; B1]
 Component of protein containing enzymes eg those important in photosynthesis & BNF
 Important in association with Nitrogen in protein synthesis
 Essential component of aromatic oils eg in plants of the cabbage and onion families.
Deficiency Symptoms
 0.15% to 0.45% of dm in plant foliage. S:N is usu 1:10.
 Spindly thin stems and petioles; Slow growth and delayed maturity
 Similar to Nitrogen Deficiency symptoms: but
 Chlorosis- starting in younger leaves; Interveinal chlorosis may distinguish from N deficiency in some
plants.
 Low sugar content
 High nitrate concentration
 Sulfur deficiencies now common bcz;o Of enforcement of clean air std controlling SO2 emissions from fossil fuel combustion
o Fertilizers now more concentrated and purer with low S impurities
o More S being removed in yields- improved varieties and better yields removing more S
 S deficiencies more common in many tropical countries where :o PM is low in S-containing minerals
o Extreme weathering and leaching have led to losses of SO42o Replenishment from atm is low
o S is lost through burning of plant biomass…. Savanna grasslands lower in S than forests.
Natural S sources
Organic matter (OM). S is present in OM as
1. Carbon bonded Sulfur (C—S) compounds
2. Sulfur bonded to oxygen -Ester sulfates (C—O—S )
3. Sulfur bonded to C and also to O (C—S—O)
1
2-9kg from
air and dust
and
rain
4-12 kg
mineralized
from
SOM
2-4 kg S/Ha
from SO42adsorbed in
sub soil.
Top soil
o Sulphate SO42o
most easily solubilized
o Slowly assimilated by plants
o Most common in arid regions soils- accumulate in lower horizons of some soils (Arididsols,
Mollisols)
o Also accumulate in saline soils of arid and semiarid regions ( as neutral salts)
o Sulfides S2o Occur in humid region soils with restricted drainage
o Must be oxidized to sulfate for plant uptake
o Oxidation usually results in
extreme acidity
subsoil
o Soils high in Fe and Al oxides as well as Kaolinite adsorbs SO42- from soln (Oxisols and other highly
weathered soils of humid tropical regions)
o Sulphate may be bound with iron oxides in temperate areas in spodic horizons.
Atmospheric sulfur gases
o Include:- carbonyl sulfide (COS); Hydrogen sulfide (H2S); Sulfur dioxide (SO2 ) and S in dust particles
o Wet deposition of atmc gases causes acid rain.
o Dry deposition – is the deposition of dry gases and dust particles
o Dry and wet deposition may be directly absorbed by plants (25-30% of plant needs in some cases)
o Presents a threat to :-health of lakes and forests (acidification)
Humans- respiratory problems
Mobilization of toxic soil Aluminum and depletion of Ca (through leaching
losses).
The Sulfur Cycle
o Similar to N cycle
2
o
o
o
o
Inner cycle involves sulfates, sulfides, organic sulfur and elemental sulfur
Atm is an important source of S
Sulfur is subject to microbial oxidation and reduction
Also subject to gaeous and leaching losses like N.
Mineralization
o This is a microbial process
O2
Org S
Decay products
Proteins
& other copds
H2S and other sulfides
SO42- +
2H+
sulfates
o Examples of S containing gases produced include carbon sulfide (CS2); carbonyl sulphides (COS) and
Methyl mercaptan (CH3SH) common in anaerobic soils.
Immobilization
o Occurs when low S energy-rich organic materials are added to soils.
o Similar to N immobilization
o When C:S > 400:1 immobilization occurs
o Typical C/ N/ S ratio for soils is 100:8:1.
Sulfur Oxidation and reduction
Oxidation
o Reduced products from microbial decomposition of C bonded S copds are sulfides (S2-), Sulfur (S0),
thiosulphites (S2032-) and poly thionates (S2xO3 y 2-)
o These are subject to oxidation
i)
H2S + 2O2
H2SO4 + 2H+ + SO42ii)
2S + 3O2 +H2O
2H2SO4 + 4H+ + SO42o Sulfur oxidation in soils is biological though the oxidation of thiosulphites may be purely chemical
o The microbes involved are 5 species of bacteria belonging to the genus Thiobacillus
o Sulphur oxidation occurs under a wide range of conditions: pH <2 to pH >9.. (Unlike nitrification which
requires a narrow pH range.
Reduction
o SO42- is unstable in anaerobic environments
o It is reduced to sulfide ions
o Microbes involved: bacteria of 2 genera:
i)
Desulfovibrio (5 species)
ii)
Desulfotomaculum (3 species)
3
o Organisms obtain energy from the break down of OM.
2RCH2OH
+
Org alcohol
SO42-
2RCOOH
sulfate
org acid
+
2H2O
+
S2sulfide
o In poorly drained soils manganese and iron are present in their reduced forms as Fe2+ and Mn2+.
The S2- ion therefore quickly reacts with these reduced ions to form insoluble copds or precipitates.
i)
Fe2+ + S2-  FeS↓
ii) Mn2+ + S2-  MnS↓
Solid
Solid
o These nutrients can be toxic especially under flooded conditions that prevail in rice paddies.
o Formation of iron sulfide or manganese sulfide helps prevent toxicities of these essential elements
o Sulfide ions also hydrolyze in water forming H2S (hydrogen sulfide) which is given off as a gas that smells
like rotten eggs .. common in swampy or marshy waterlogged areas.
o Reduction of S to S2- may happen with elemental sulfur or polythionates or thiosulfates.
Acidity from Sulfur
o Sulfur oxidation is an acidifying process
→Elemental Sulfur may be applied to lower soil pH if the soil pH is higher than desired
→Sulfur containing fertilizers sometimes lower pH dramatically below desired levels
→Sulfidic soils or geological materials when drained or exposed lead to acid mine drainage (extreme
acidity from oxidation of sulfur or sulfides)
o Sulfides in these potential acid minerals are stable in the absence of oxygen
o
S0 and S2- oxidize when drained or excavated.
o
pH as low as low as 1.5 may result
o
Rocks and water may exhibit an orange colour due to some of the Fe compounds in acid water
Losses of S through leaching
o
Leaching losses of SO42- are usu. accompanied by and result in losses of Ca2+ and Mg2+ from soils
o
Leaching losses are minimal in soils capable of adsorbing sulfates (AEC).
The diagram below shows various sulfur pools important in the practical management of soil sulfur levels.
Irrigation water
fertilizers
Reidues &
manure
SOM
Sulfur bearing
minerals
Wet & dry
deposition
Available Soil
potassium
ATM
volatilization
leaching
Fixation
Plant
removal
4