IMPORTANCE OF ELEMENTAL SULPHUR IN PLANT FERTILISATION

Význam elementární síry pro hnojení rostlin

Cyna Katarzyna, Potarzycki Jarosław, Gaj Renata, Grzebisz Witold

In West European countries, in late 1970s, a systematic decrease in sulphur dioxide occurred. In Norway, in 1991 sulphur falls were almost four-fold lower than twenty years earlier (Singh, 1993). In Central Europe (also in Poland) sulphur emissions to the atmosphere increased until late 1980s, but SO2 falls in Poland in years 1988-97 decreased two-folds. Recently, average sulphur emissions have been slightly over 20 kg S/ha/year showing high diversity in individual regions of the country (GUS). In soils, dominant forms of sulphur are organic compounds (95 % of total sulphur). Only a few percent of sulphur in the arable layer are sulphates, which are easily available for plants (Eriksen et al., 1998). A very important source of this element for plants could be soil organic matter, but only if favourable conditions for the mineralisation process occur.

Reduced forms of sulphur (including elemental sulphur) are oxidised by soil bacteria, which utilise the energy liberated in this process for their own life activities (fig. 1). Autotrophic bacteria, notably neutrophilic thiobacilli, are probably responsible for the bulk of S oxidation in most soils with heterotrophic S-oxidizers having a supplementary role (Germida and Janzen, 1993). The content of sulphates in soil is a resultant of sulphur immobilisation and mineralisation processes, which is associated with the supply, or consumption of S-SO4 by microorganisms (He et al., 1994). Soil enrichment in SO42- can occur only in dry climates. In humid climates sulfites are easily translocated in soil profile and leached into ground waters. The rate of conversion of elemental sulphur into sulfite sulphur depends on many factors (Watkinson and Lee, 1994; Grzebisz, 1996), such as:

· Temperature. The minimum temperature at which sulphur oxidation occurs

is 3-5oC. This process ceases almost completely during the period

of autumn-winter. Hence, autumn application of elemental sulphur poses

no danger of sulfite leaching into ground waters. The process of sulphur

oxidation reaches its peak at the temperature of 30oC.

· Water. Both excess and deficit of water, by limiting the development

of microorganisms, reduce the oxidation rate of elemental sulphur.

· Reaction. The requirements of the majority of crop plants with regard

to soil reaction are identical with pH optimal for the development

of soil bacteria.

· Humus. Soils rich in humus fertilised with farmyard manure, green

fertilizers or straw create conditions favourable for high microb. activity.

· Soil structure. Structural soils, maintained at high culture, are characterised

by good thermal-air conditions and this, in turn, stimulates bacterial

development.

· Mineral fertilisation, espec. by phosphorus, increases sulphur oxidation.

In Poland in mid 1990s, in most rape cultivations of winter crop rape hidden symptoms (not shown visually, but detectable chemically) of sulphur deficiency were observed (Grzebisz and Fotyma, 1996). Pilot studies, which were carried out in the vegetation season 1999/2000 at the Chair of Agricultural Chemistry at Poznań Agricultural University, revealed that sulphur was the limiting factor not only for rape crops but also winter wheat. In Poland is following strategy of sulphur fertilisation:

· Widely known prevention, according to the rule: ”it is easier to prevent

than to cure”. Sulphur could be applied before seeding in different

chemical forms.

· ”The last bell” - this period is associated with sulphur input into the soil

in early spring in the form of potassium sulphate, ammonium sulphate,

kiserite or magnesium sulphate.

· ”Treatment that saves life” - deals with sulphur supply to the plant

(in confined amounts) at the beginning of the critical phase of sulphur

uptake. In this case, the farmer can choose only MgSO4·7H2O (fertilizer

perfectly soluble in water).

In practical conditions, the first of the above-listed objectives can be realised by the application of elemental sulphur (So). In experiments carried out by Riley et al. (2000), a similar yield-forming effect was demonstrated in the case of sulphur derived from ammonium sulphate and micronised So (particles 5-8 ěm). However, the obtained crop of winter wheat and winter rape after treatment with a fertilizer which was a mixture of So (90%) and bentonite (10%) was significantly lower in comparison with the objects mentioned above. The influence of granule size on the effectiveness of elemental sulphur was confirmed by experiments carried out by Malhi et al. (2000). In experiments conducted by the authors, elemental sulphur was applied in the cultivation of spring barley. Sulphur was applied in pre-sowing doses of: 0; 25; 50 and 100 kg S/ha in two fertilizer forms (A and B). The initial concentration of sulphate sulphur in the 0 - 30 cm layer of soil amounted to 0.2 mg S-SO4/kg. The obtained crop of grain ranged from 4.14 - 5.43 t/ha. Irrespective of the type of the applied fertilizer, the introduction into the soil of 25 kg S/ha resulted in 22-28% increase in the yield of barley grain. The application of higher doses of the fertilizer failed to have a significant impact on the obtained yield.

Analysing the dynamics of So on objects fertilised with sulphur, a similar course of curves should be emphasised (fig. 2). In the initial stage of vegetation (up to day 49, stage EC 37), a systematic increase in the concentration of S-SO4 in the soil was observed. This was followed by a 2-week period during which the content of sulphates in the arable layer of soil did not change. After another 10 days, the amount of sulphate anions in the soil decreased rapidly and, at the stage of EC 65, it was found at the level similar to the control object. From then on, the quantity of sulphates in the soil remained relatively stable and differences between objects fertilised with sulphur decreased. S-SO4 liberation from the control soil was found to be most intensive between day 66 of vegetation (stage EC 49) and day 93 (stage 75). After that, the curve dropped dramatically and the amount of sulphur sulphate decreased three-fold (fig. 3).

The soil sulphur content in the period of shooting up to the beginning of earing affected the grain yield of spring barley (fig. 4). The obtained results indicate that the concentration of sulphate sulphur in soil during the period of flag leaf formation of spring barley was closely correlated with grain yield. The optimal concentration of sulphur in soil at the EC 37 phase calculated from the regression equation (fig. 4) amounts to 9.6 kg S-SO4 /ha.

Summing up

· Reduction of atmospheric sulphur deposition makes it increasingly

necessary to apply sulphur fertilisation more and more often.

· Fertilizer value of elemental sulphur is determined by environmental

conditions and physical and chemical properties of the fertilizer.

· Spring barley fertilisation with elemental sulphur at the dose of 25 kg S/ha

can increase significantly its grain yield.

· The concentration of sulphate sulphur (S-SO4) in soil at the phase

of the appearance of the flag leaf (EC 37) can be used as an indicator

allowing the estimation of grain yield of spring barley.

References mentioned in the paper are available from authors.

1. Conversion of So in the soil (according to Spiess Urania)

2. Net oxidization of the elemental sulphur (fertilizer A and B)

3. Content of the S-SO4 in the control soil

4. Relationship between grain yield of barley and S-SO4 content

in the soil ( stage EC 37 ).