Quantifying the sulphur (S) supply from farm manures to winter wheat crops

Abstract

The overall aim of this project was to quantify sulphur (S) supply from organic materials to winter wheat crops, in order to improve current recommendations on the use of farm manures and biosolids as sources of crop available S for arable crops.

An enhanced database of the S content of organic materials found good agreement with the ‘typical’ values given in Defra’s Fertiliser Manual (RB209) for livestock manures (cattle farmyard manure - FYM, pig FYM, cattle slurry and pig slurry), digested liquid, lime stabilised and thermally dried biosolids. However, for digested cake and composted biosolids, the mean total SO₃ content was higher than the ‘typical’ values given in Defra’s Fertiliser Manual (RB209) (Defra, 2010). The S content of organic materials (both total SO₃ and ‘extractable’ SO₃) was shown to be variable; hence, analysis of a representative organic material sample is advisable to ensure that crops grown on potentially deficient sites receive adequate S.

Field experiments were carried out at 3 sites cropped with winter wheat over 3 harvest years (2 harvest years at each site; 6 harvest years in total). At each site, there were 7 organic material treatments, including autumn applied cattle FYM, pig FYM, broiler litter and two biosolids products, and spring applied broiler litter and pig/cattle slurry. Three of the 6 sites responded to S and at these sites the organic material treatments were compared with inorganic (water soluble) fertiliser S response treatments (0, 12.5, 25, 50 and 75 kg/ha SO₃) to determine the fertiliser S replacement value and hence S availability from the applied organic materials.

For the spring applied organic materials, ‘extractable’ SO₃ (i.e. readily available SO₃) appeared to be a good indicator of S that was available to the crop. Analysis of the organic materials used in this project showed that ‘extractable’ SO₃ varied between around 15% of total SO₃ for cattle FYM, up to around 60% of total SO₃for broiler litter. Results from this project showed that for spring applied organic materials, ‘extractable’ SO₃ was equivalent to inorganic fertiliser S i.e. the S use efficiency for spring applications is 15% of total SO₃ for cattle FYM, 25% for pig FYM, 60% for broiler litter, 35% for slurry and 20% for biosolids.

Readily available S from organic materials applied in the autumn may be lost via overwinter leaching with losses dependant on soil type and overwinter rainfall. Results from this project showed a lower S use efficiency from autumn compared with spring applied organic materials; typical autumn S use efficiencies in the range 5–10% of total SO₃ for livestock manures and 10–20% of total SO₃ for biosolids.

This work has led to a better understanding of the crop available S supply from organic materials and produced guidance to farmers on the availability of S from applications of organic materials. This is likely to improve farm profitability by ensuring that crops receive adequate amounts of S from applied organic materials or, where necessary, that supplementary inorganic S fertiliser additions are made to meet crop needs.