The Importance Of Sulphur As A Plant Nutrient
Sulphur is an important nutrient for optimal plant growth: it is one of the key
macroelements essential for plant growth. Sulphur is taken up from the soil
solution by the plant in the sulphate form (SO42-).
In the plant sulphur is a component of methionine, cysteine and cystine, three of
the 21 amino acids which are the essential building blocks of proteins.
Animals need to consume methionine in their diet as they cannot manufacture it
themselves; methionine is essential for dairy cattle in particular.
Sulphur is also a component of key enzymes and vitamins in the plant and is
necessary for the formation of chlorophyll. In legumes sulphur is necessary for
the efficient fixation of nitrogen by the plant. This makes sulphur of fundamental
importance in the establishment and maintenance of legume-based improved
pastures. It is also essential for flowering and seed set in canola.
Plants which are deficient in sulphur show a pale green colouration of younger
leaves first as sulphur is not very mobile in the plant. In severe cases of sulphur
deficiency the entire plant can be stunted and pale green. Affected plants may
be thin-stemmed and spindly; brassica and canola crops may develop a reddish
colouration on the underside of leaves and on stems, and flowers may be pale to
greyish in colour.
Sulphur has become more important as a key plant nutrient in recent times due
to a number of factors:
Increased use of high analysis fertilisers such as monoammonium phosphate
(MAP) and diammonium phosphate (DAP). These products contain much less
sulphur that the traditionally used single superphosphate (SSP);
Decreased sulphur addition to the soil from atmospheric sources as industrial
pollution has been reduced;
Introduction of higher yielding crop varieties which remove more sulphur
from the soil in harvested commodities.
the sulphur cycle
Sulphur is present in various forms in the environment. Up to 95% of the total sulphur in the soil is associated with organic matter.
Other sources of sulphur in soils are animal manure, irrigation water and, close to the coast or industrial areas, the atmosphere, where
sulphurous gases such as sulphur dioxide and sulphur trioxide are dissolved in rainwater and washed into soil. The availability of
sulphur from industrial emissions is relatively low in countries like Australia with legislation aimed at reducing industrial pollution.
In the soil sulphur is present as organic sulphur compounds, sulphides (S-), elemental sulphur (S0), and sulphate (SO42-). Plants cannot
absorb organic or elemental sulphur. For plants to be able to utilise sulphur from the soil it must be in the sulphate form. Organic
sulphur and elemental sulphur are converted to the sulphate form in the soil.
Sulphur can be removed from soil through uptake by plants, leaching through and out of the root zone by rainfall or irrigation, and by
volatilisation. Sulphur can be transformed from one form to another in the soil through various biological and physical processes. This
movement in and out of the soil between different chemical forms in the soil is known as the sulphur cycle. The sulphur cycle can be
represented as shown below:
How sulphur is lost from the soil
The main ways sulphur is removed from pastures and soils
is through leaching out of the root zone of plants and by
ingestion of pasture by grazing animals. Leaching is the
process whereby water, in the form of rainfall, flood waters
or irrigation, is flushed through the root zone. This flushing
process takes with it dissolved nutrients so that they become
unavailable to plants through normal root uptake.
Because sulphur in the sulphate form is very soluble it is easily
leached out of the root zone under conditions of heavy rainfall
or irrigation, and under conditions of moderate rainfall or
irrigation in light soils. Studies in New Zealand have shown
that 5-40kg S per hectare per year can be lost from grazed
pastures through leaching.
The advantage of adding elemental sulphur to the soil is that
it acts as a ‘slow release’ source of sulphur to the plant as
elemental sulphur is resistant to leaching and is oxidised slowly
to sulphate as the plant requires it.
Sulphur is also lost from the soil in plant and animal products.
The following table provides a guide to the amount of sulphur
removed by various crops and by dairy production:
CROP & COMMODITY
SULPHUR REMOVAL
(kg/tonne of product)
Barley (grain)
Cattle – live
Cattle – live wt gain
Canola (grain)
Cotton (lint and seed)
Greasy wool
Hay – cereal
Hay – legume
Hay – lucerne
Maize (grain)
Milk
Oats (grain)
Potato (tuber)
Sheep – live
Sheep – live wt gain
1.1
3.9
1.5
9.8
4.0
28.5
1.5
2.0
3.2
3.8
0.3
1.5
4.5
4.0
1.7
Sorghum (grain)
Wheat – Prime Hard (grain)
Wheat – Aust. Hard (grain)
Wheat – ASW (grain)
Wheat – Soft (grain)
Wheat – Durum (grain)
2.8
1.64
1.40
1.35
1.19
1.52
(Australian Soil Fertility Manual, 2004; Appendix 1.)
Conversion of elemental sulphur to the sulphate form
Plants cannot absorb elemental sulphur. For sulphur to be taken up by plants it must be in the sulphate form (SO42-). Elemental sulphur
is converted to the sulphate form in soil through a process of oxidation by bacteria, mainly Thiobacillus species.
The rate of conversion from elemental sulphur to sulphate is influenced by:
1. The particle size of the elemental sulphur source
Because the oxidation process is due to bacterial activity, the more surface area the bacteria are able to access the
quicker the rate of oxidation will be. The smaller the particle size of the elemental sulphur source, the greater the surface
area for a given weight of sulphur, hence the process of oxidation will be quicker for small sized particles of elemental
sulphur than larger sized particles.
The particle size of the elemental-S is the main
condition that can be controlled by fertiliser. Large
particles, bigger than 500 microns in diameter, are
of little value from an agronomic standpoint. The
importance of particle size on the oxidation rate of
elemental-S is shown in Figure 1.
The rate of bacterial metabolism and reproduction
is related to temperature: at very low or very high
temperatures bacterial multiplication is limited.
Oxidation of sulphur to sulphate occurs between 5oC
and 40oC and is most active between 10oC and 30oC;
this is generally true of plant growth as well. At normal
temperatures, in the presence of moisture, bacterial
multiplication is optimal and the rate of conversion
of elemental sulphur to sulphate will be optimal. This
means that elemental sulphur is converted to plantavailable sulphate at the time when the plant requires it
– when soil temperatures favour plant growth they also
favour the oxidation of elemental sulphur to sulphate.
125-175 microns
SULPHUR RECOVERED AS SULPHATE (PERCENT)
Generally, for elemental sulphur to be a useful source
of sulphate, the particle size of the elemental sulphur
needs to be less that around 0.3mm (300 microns).
175-550 microns
550-800 microns
800-1500 microns
1500-4500 microns
Figure 1. S particle size and conversion to sulphate with time.
Study of the effect of particle size in microns on recovery of
sulphate ions over time from 1000ppm of elemental S incubated
in soil at room temperature.
Conversion of elemental sulphur to the sulphate form
2. Soil moisture and aeration
The Thiobacillus bacteria primarily responsible for the oxidation of sulphur are characterised as aerobic, that is they
require oxygen to survive and thrive. They also require a moderate amount of moisture. Soils which are below field
capacity but not too dry for extended periods are ideal for the growth and reproduction of Thiobacillus spp and allow
optimal rates of oxidation. Soils which are waterlogged for extended periods of time become anaerobic and therefore
unsuited to the growth of Thiobacillus spp, thus limiting the conversion of elemental sulphur to the sulphate form.
3. Soil pH (acidity)
Oxidation of elemental sulphur to sulphate can occur over a wide range of soil pH (between pH 2 and pH 9). As soil
acidity decreases and the soil becomes more alkaline the rate of oxidation generally increases. Liming highly acidic soils
will generally increase the rate of oxidation.
4. Amount of organic matter present in the soil
Organic matter is the main source of sulphur in soils: the higher the level of organic matter in the soil, the higher the
rate of oxidation and conversion of elemental sulphur to sulphate. Under conditions favouring oxidation for each 1% of
organic matter in the soil approximately 6kg S are released per annum.
5. Microbial population in the soil
In soils which have had sulphur applied in their recent history, levels of sulphur oxidising bacteria are likely to be
relatively high. In soils with a history of no sulphur addition the bacterial are generally present in the soil, but at low
levels. When sulphur is added to soils with no history of sulphur application the populations of oxidising bacteria build
up quickly because it is a source of energy.
Wengfu SuStain:
The product and the manufacturing process
Wengfu SuStain is a high analysis pastille product containing 90% elemental
sulphur in a matrix of bentonite clay. It is manufactured by adding bentonite to
molten sulphur, shaping the resulting liquid mixture into pastilles, and cooling.
The bentonite pastilles are designed to swell and disintegrate when they come
into contact with moisture, releasing thousands of tiny sulphur particles which are
available to be oxidised to the plant available sulphate form by bacteria in the soil.
Wengfu SuStain contains a range of different sized elemental sulphur particles.
This ensures that some of the sulphur is available for immediate oxidation to the
sulphate form, and some remains in the soil to be oxidised over a longer time
frame, thus ensuring an ongoing supply of sulphate to the plant. Research work has
demonstrated that applications of elemental sulphur can be at least as effective as
applications of sulphate provided that the elemental sulphur particles are less that
around 300 microns in diameter; extremely fine particles (< 75 microns) can oxidise
within 2-3 days under ideal conditions. The majority of sulphur particles in Wengfu
SuStain are less than 300 microns in diameter.
The breakdown of Wengfu SuStain over an 8 hour period: demonstrates the rapid
breakdown of the pastilles and the release of elemental sulphur particles on
contact with moisture, exposing the sulphur particles to microbial oxidation to
the sulphate form.
Combining Wengfu SuStain and Wengfu Pasture King
The combination of Wengfu SuStain and Wengfu Pasture King gives the ideal
balance of immediately available sulphate and sustained release elemental sulphur.
Wengfu Pasture King is a combination of 15.7% phosphorus, combined with 4.6%
sulphate. The high concentration of phosphate gives economies of transport and
storage, and the sulphate is immediately available for plant uptake.
The high loading of sulphur (90%) in Wengfu SuStain delivers elemental sulphur in
a range of particle sizes so that a proportion is immediately available to the plant
and a proportion is slowly oxidised over time to meet the longer term requirements
of the plant as it is actively growing.
Custom blends of Wengfu Pasture King and Wengfu SuStain can be made to meet
the specific nutrient needs of pastures based on soil and plant analyses.