Optimising sulphur management to maximise oilseed rape yields and farm profitability (OPTI-S): Project Report No. 595

Sulphur (S) is an essential plant nutrient that has important effects on both the yield and quality of crops. The aim of this project was to develop improved guidance for farmers on S management.
(i) Optimum S rates for oilseed rape
Sulphur response field experiments were carried out at ten sites cropped with winter oilseed rape over four harvest years (2014 to 2017). Data from a further eight S response experiments carried out between 2011 and 2013 are included in the data set giving a total of 18 site years of data collected between 2011 and 2017. Ten of the eighteen S response experiments showed a yield response to S fertiliser of between 0.1 and 4.4 t/ha and economic optimum S rates at these sites varied between 30 and 79 kg SO3/ha. There was no relationship between the optimum S rate and yield at the optimum rate and therefore no evidence to suggest that higher yielding oilseed rape required higher fertiliser S rates. The optimum S rate was insensitive to typical changes in fertiliser and crop price.
Leaf tissue testing for malate: sulphate, S content and N:S ratio and seed analysis for S and N:S ratio were able to identify differences between some, but not all, of the S sufficient/deficient sites. All of the sites which showed a yield response to S were light or medium textured i.e. loamy sand, sandy loam and sandy clay loam soils. Soil texture and over-winter rainfall was a better predictor of S deficiency than soil analysis or tissue testing.
We recommend updating the guidance for S application to oilseed rape to include soil texture and winter rainfall in assessing the risk of S deficiency and increasing the current S recommendations to 50-80 kg SO3/ha. The wording in the AHDB Sulphur information sheet should be revised to highlight potential uncertainly in diagnosing S deficiency based on visual symptoms and tissue analysis.
(ii) Sulphur supply from organic materials
At five of the S response field experiments additional organic material treatments were included to quantify the S supply from organic materials. Four of these sites showed a yield response to S fertiliser and a yield increase from the organic material treatments which indicates that organic materials supply crop available S that can contribute towards crop S fertiliser requirement. Based on the results of this project, we recommend updating the guidance on S availability from organic materials to increase the S use efficiency from autumn applications of organic materials (from 5-10% to 15% for livestock manures and from 10-20% to 25% for biosolids) to oilseed rape and grassland. This change reflects the S uptake by these crops in the period between application and the start of over-winter drainage (and subsequent reduction in S leaching losses). We also recommend increasing the S use efficiency figures for spring applied slurry (from 35% to 45%) and biosolids (from 20% to 35%) for all crops.
This work has produced a robust evidence base to support S recommendations to oilseed rape and led to a better understanding of the crop available S supply from organic materials.