Diagnosis and prediction of sulphur deficiency in winter wheat and barley

Abstract

The sulphur (S) status of winter cereal crops grown at potentially S-deficient sites and their requirement for fertiliser S were assessed in a crop survey and in field experiments by soil and/or crop analysis.

In the crop survey, representative leaf and grain samples were collected from commercial winter wheat and winter barley crops at over 90 locations in England and Wales in 1992 and 1993. Concentrations of total S and sulphate-S either in just fully emerged flag leaves or in second and third leaves at anthesis were significantly related to soil type in the order sands < shallow calcareous < light-textured loams < heavy-textured loams, and were lowest in regions away from industrial activity, but there was no regional or soil type variation in grain S content. Leaf S concentrations were greater in wheat crops than in barley crops although not always significantly so. Similarly, S contents of wheat grain were significantly greater than of barley grain in 1992 only, when they were elevated because of small grain weights. Leaf total S and sulphate-S were significantly lower in malting varieties than in feed varieties of barley, but were not significantly different between breadmaking and feed varieties of wheat. Grain S and N:S ratio contents were generally lower in malting barley varieties and greater in breadmaking wheat varieties compared to feed varieties. There was no relationship between S or N: S ratio in leaves at anthesis and in grain at harvest, and crops which could be classified as S deficient by leaf analysis were not deficient according to grain analysis. Grain S analysis in isolation suggested an unrealistically large number of S-deficient sites and is considered unreliable for diagnosing a deficiency of S. Leaf analysis and grain N:S analysis indicated that only about 5% of sampled crops were short of S during vegetative growth.

Soil and crop samples were taken in autumn (emergence), spring (GS21-25), summer (GS39-65) and/or at harvest from 41 sites evaluating the yield response to 40 kg S/ha applied as potassium sulphate in spring during 1993-94. Grain yield responses estimated by quadrat and/or plot combine of between 4 and 18% were obtained at seven sites and increases in straw (and chaff) or in total dry matter yield of between 5 and 34% were obtained at ten sites. Straw yield responses were obtained more frequently in 1994 and, also, in the absence of grain yield responses. Similarly, grain yield responses were obtained in the absence of straw yield responses. Distinct S deficiency symptoms were observed at one site in 1993 and seven sites in 1994. At sites showing deficiency symptoms, S application significantly reduced thousand grain weight but did not always increase grain yield. Soil extractable sulphate-S in spring and uptake of S by full flag leaf emergence stage were significantly lower in 1994 than in 1993. Differences in the ratio of N and S uptake during the growing season between 1993 and 1994 also suggested a shortage of S relative to N during early stem extension in 1994, relating to a 30% higher average winter rainfall over the 1993/94 winter compared to the 1992/93 winter. The average recovery of applied S was <10% in both years. Neither soil analysis, whole crop analysis or grain analysis could satisfactorily distinguish yield responsive sites or sites showing S deficiency symptoms. Analysis of individual leaves undertaken in 1994 indicated that yield responsive sites could best be predicted by total S and N:S ratio analysis of second and third leaves at flag leaf emergence or at anthesis, but critical values could not be reliably estimated and were different for the two growth stages. Further work is required to develop a more reliable predictive or diagnostic indicator for S deficiency in cereals.

AICC review by R. Bryan (D&N Management Systems Ltd)

Introduction

Falling atmospheric deposition and the susceptibility of Brassica species have led to the widespread use of sulphur products on oilseed rape. Research work and farm experience has shown sulphur application to oilseed rape to be very cost-effective on deficient sites. The aim of this work was to establish the current position with winter cereals and the method for identifying deficient sites.

Techniques used

Crop Survey. A total of 375 winter cereal crops sampled over the 1992 and 1993 growing seasons in England & Wales.

Field Experiments. Responses to 40 kg/ha sulphur were tested at 21 sites in 1993 and 1994 harvest years. Sites were chosen in low sulphur deposition areas and on sands or shallow chalks.

Observations and results

a. Crop Survey Results

• Less than 5% of cereal crops were judged to be sulphur deficient in the survey.
• The analysis of leaves 2 and 3 at the flag leaf to flowering stage for total S and N:S ratio still appears to be the most reliable method of identifying deficient sites.
• There was a poor relationship between the analysis of sulphur in the leaves and the analysis of sulphur in grain.
• Using a critical level of 1.2mg/g, grain analysis suggested much higher levels of deficiency than was observed by visual symptoms or leaf analysis.
• Averaged over the 2 years, barley had a lower N and S concentration in the leaves than wheat.
• Malting barley had significantly less total S in leaves 2 & 3 at flowering than feed barley. Lower N applications on malting barley and traditional cropping of lighter soils contribute to the lower total S effect.
• Grain analysis showed a higher N:S level for breadmaking wheat compared to feed.

Field Experiment Results

Grain Yield. From 41 sites over two years, only 3 sites gave significant yield responses to added sulphur in 1993 and 4 sites in 1994. Typical yield increases at responsive sites ranged from 0.3-0.8t/ha (4-18%). Four sites with the most obvious visual sulphur deficiency had lost those symptoms by the flowering stage of the crop and did not produce grain yield increases from added sulphur.

Straw and chaff yield. There were significant increases at 3 sites in 1993 and 7 sites in 1994. The range of responses was 0.4-2.0t/ha (5-34%). On sites with crop colour indicating deficiency in 1994, there was a reduction in the thousand grain weight and specific weight from sulphur treatment. The higher straw & chaff yields supports the conclusion that this effect was caused by an increased tiller number on those sites.

Crop & Soil Analysis

Soil analysis, whole crop analysis or grain analysis could not help to identify sites which responded to added sulphur or had visual symptoms of deficiency. The analysis of leaves, as discussed above, was the most reliable method of diagnosis.

Agronomist analysis

• The application of sulphur to winter cereals is cost-effective on deficient sites.
• The survey and experiments indicate that sulphur deficiency in winter cereals was uncommon over the period 1992-94.
• However, the number of deficient sites may have increased since the study was completed in 1994.
• We now have more confidence in the leaf analysis technique to identify deficient sites.
• Leaf analysis of less than 2mg/g and N:S of more than 17:1 from flag leaf to flowering being a reliable indicator of a deficient site.
• There is less confidence in soil, whole crop and grain analysis to identify sites.
• Lower deficiency thresholds for winter barley should be considered, particularly malting barley.
• More routine sampling should be carried out where oilseed rape is responding to sulphur.
• In some years, visual sulphur deficiencies in the spring can disappear following further mineralisation in early summer.