Download 4.3 Sulphur and pyrite Sulphur is essential to all plants, especially

Rocks for Crops - 27
4.3
Sulphur and pyrite
Sulphur is essential to all plants, especially for the synthesis of proteins. Sulphur deficiencies in soils are
becoming more and more apparent in recent years, especially in areas far away from the sea and from
industry. These sulphur deficiencies are partially due to depletion of S through heavy crop removals,
intensive cropping and lack of organic matter recycling but also due to the expanding use of S-free
fertilizers such as TSP, MAP, DAP, and urea.
Sulphur and pyrite (FeS2 ) are naturally occurring minerals that can provide sulphur to plants, but in
oxidizing environments they can also produce acids to lower the pH. Crops with a high demand for S
include sugar cane, protein-rich leguminous crops like lucerne and clover, oil crops such as rapeseed,
mustard, and cruciferous plants like cabbage and cauliflower.
Sulphur as elemental sulphur is found in relatively small amounts in many volcanic areas but also as part
of sedimentary gypsum- (CaSO 4@ 2H2 O) and anhydrite-(CaSO 4 ) bearing sequences. Sulphur is abundant or
even over-abundant in many countries as a byproduct of the refining of sulphide ores and of treating sour
gas wells. Pyrite and marcasite (both FeS2 ) occur in many sedimentary successions in varying purity, and
as a minor constituent in hard coal deposits. ‘Coal pyrites’ are by-products of the upgrading and
purification of hard coals. Pyrite is also the main sulphide mineral, occurring together with many base
metals. It is often discarded together with low grade metal-containing rocks and wastes on mine tailings.
Elemental sulphur applied to alkaline soils for pH reduction is not a very common practice because high
amounts of S are required to be effective. Sulphur applied in combination with phosphate rock has been
tested to achieve in-situ phosphate solubilization and increase supplies of P and S to soils and plants
(Swaby 1975; Rajan 1983, 1987a; Loganathan et al. 1994). When S and PRs are mixed together and
applied to the soil, the sulphur oxidizes, forming sulphuric acid which in turn assists in the dissolution of
phosphate rock. The actual rate of P release can be controlled to a great extent by the amount of S used.
Sulphur mixed with phosphate rocks and inoculated with the sulphur oxidizing bacteria Thiobacillus ssp,
has been tested in many soils and proved as effective as superphosphates (Logonathan et al. 1994),
particularly as slow-release P and S fertilizer. ‘Biosuper,’ as this blend is called, is very effective in many
tropical soils and is superior to single superphosphate in areas that receive more than 635 mm of rain
(Swaby 1975). Under these conditions it can be used as controlled P and S fertilizer for pastures and longterm agricultural and horticultural crops.
The iron sulphide mineral pyrite has not been widely regarded as a promising agromineral because of its
common association with potentially toxic metal impurities. But in some parts of the world, for example,
India, pyrite is widely used as an S (and Fe) fertilizer (Tiwari et al. 1985). Agricultural pyrite from
Amjore (Bihar) of sedimentary origin contains 22-30% S and is used successfully on calcareous soils of
northern Bihar State as a S and Fe fertilizer for production of chickpeas, peas and lentils (Tiwari et al.
1985).
Another use of pyrite is to reclaim alkaline sodic soils. Pyrite has been tested as a soil amendment to
reclaim sodic and calcareous soils by reducing the pH and improving soil structure (Banath and Holland
1976; Dubey and Mondal 1993). In many experiments, pyrite applied to alkaline sodic soils was however
outperformed by another sulphur-bearing mineral – gypsum. Gupta et al. (1988) incorporated pyrites and
Mussoorie phosphate rock (MPR) from India in decomposing cattle manure and tested the effectiveness of
the P- and S-enriched manure in pot trials. The level of available P for mustard can be increased by
decomposition of MPR in organic manure in the presence of acidifying pyrite.
Pyrites from mill tailings have also been tested as an inexpensive material to correct Fe deficiencies in
calcareous soils (Barrau and Berg 1977). Depending on the particle size of the pyrite and susceptibility to
oxidation, pyrites can serve as a continuous slow release Fe source on sodic and Fe-deficient soils (Vlek
and Lindsay 1978).
28 - Part 1
Few experiments with pyrites have been carried out in Africa. Among them is the experiment by Lowell
and Weil (1995), who tested pyrite as a means of enhancing phosphorus availability from phosphate rock
in a laboratory study. Pyrite and various African phosphate rocks in several ratios were incubated and the
soluble P and pH was measured in the leachates. Soluble P measured in the leachate was greatest in
Pyrite-PR mixtures with Togo PR and the Sukulu PR from Uganda. Soluble P released from pyrites mixed
with phosphate rocks from Tundulu in Malawi and Minjingu in Tanzania was virtually zero (Lowell and
Weil 1995).