Sulphur nutrition of cereals in the UK: effects on yield and grain quality

Summary

The incidence of sulphur (S) deficiency in agricultural crops in the UK has increased in recent years due to a reduction in the amount of S falling onto agricultural land from industrial emissions of sulphur dioxide and the almost complete absence of S in modern fertilizers. Whilst multi-cut systems of grassland production and oil seed rape are the most S demanding, there has been increasing concern over the incidence of S deficiency in cereal crops in the UK and the implications for crop yield and quality. Whilst HGCA-funded research projects are currently evaluating the development of S deficiency in cereals and identifying appropriate plant and soil analysis procedures for its diagnosis and prediction, there is an interim need to provide guidelines to farmers and the agricultural industry on the most appropriate methods of correction of S deficiency. Drawing on previously unpublished experimental data and advisory experience of ADAS and SAC, this review summarises the role of S in the nutrition of cereals, draws together information on the most appropriate methods of S fertilization and discusses the future significance of S deficiency to the UK cereals industry.

Sulphur is an essential nutrient required for protein formation and a number of enzyme reactions within plant cells which are required for satisfactory crop growth. The S- containing amino acids cysteine and methionine are particularly important in forming disulphide bonds during breadmaking. Although the S requirement of cereals is comparatively small (10-30 kg S/ha), grain yield responses of up to 30% have been obtained in UK field experiments on sandy and/or shallow soils in areas receiving < 20 kg/ha/year from the atmosphere. Deficiency symptoms of leaf paling and crop stunting occur during stem extension and have been recorded in cereal crops in southwest, northern and eastern England, Wales, Scotland and Ireland. They are often transient in nature and their occurrence varies considerably from season to season depending on the supply of both nitrogen (N) and S. Deficiency is more pronounced at high rates of N application and is characterised by wide N:S ratios (> 17:1) in both young leaf tissue and grain. A knowledge of soil type and location together with retrospective crop analysis is currently the most reliable guide for S fertilizer need. It is recommended that potentially susceptible crops are sampled between full flag leaf emergence and anthesis to assess the future need for S fertiliser on individual fields.

Although there has been limited opportunity to evaluate alternative methods of S fertilization for cereals in the UK, experimental evidence to date indicates that applications of 10-20 kg S/ha applied as soluble sulphate fertilizer just prior to stem extension is the most appropriate means of preventing S deficiency. Elemental S fertilizers have been shown to be less effective than soluble sulphate fertilizers when applied at the same rate and time. Elemental S fertilizers have potential residual value but further work is required to optimise their performance. It is not known to what extent any yield loss associated with S deficiency can be controlled by application of S once symptoms appear.

Numerous experiments have shown that a shortage of S relative to N supply alters the balance of amino acids and the protein composition of the grain. These changes are considered to reduce the nutritional value of feed varieties and flour quality in breadmaking wheat, although quantitative evidence for UK varieties is lacking. It is not known to what extent grain S content affects the malting process.

Crop surveys in recent years indicate that up to 10% of the UK cereal crop may be sufficiently low in S to impair either crop yield or grain quality. With the increasingly rigorous controls over the industrial emissions of sulphur dioxide to the atmosphere, the incidence and severity of S deficiency in cereals in the UK is forecast to increase. There is little doubt that S has become as important a nutrient as phosphorus or potassium and a continued research effort is required to ensure that cereal grain production and quality are not impaired. Future research priorities are identified.