Contribution to the turbidimetric method for sulphur determination in arable soils

Sulphur (S) plays an important role in agriculture, being the fourth major contributor to improved quality of crops and increased yields. The applied methods for the estimation of different forms of S in soil aimed at assessing the sulphur availability to plants in various conditions. Nowadays, the wider spreading of regions with sulphur deficiency imposes optimisation of the soil testing procedures in order to increase their availability for laboratories. This study contributes to improving the analytical performance of the turbidimetric method in determining water-soluble sulphate in soil after leaching with the CaCl2 reagent. The modified testing protocol showed: method limit of quantification of 5.0 mg/kg; precision as relative standard deviation less than 3%; recovery of fortified soil samples 103 ± 18%. The expanded uncertainty was 2.3 mg/kg SO42--S (K = 2, norm.). The proposed testing protocol was inexpensive, fast, used simple equipment and procedures, easily adoptable in regular laboratories, and showed characteristics suitable for the estimation of water-soluble sulfate in arable soils. A set of 546 soil samples was tested, and 74% were found to be sulphur deficient with SO42--S < 10 mg/kg and sulfur availability index < 6.0. Thus, the availability of laboratory analysis to a broader group of farmers could contribute to effective fertilisation programs, as the newly proposed fertiliser blending technologies are based on adequate estimation of sulfur availability in arable soils.