Abstract

Sulfur, an essential element for plant growth, has received lesser attention than it deserves. Current inputs of sulfur to agricultural soils from atmospheric deposition have reduced to less than the amount sulfur required by most crops. In such soils the release of sulfur from organic matter is vital for the supply of sulfur. A pot experiment without plant (incubation study) in a green house was conducted at the experimental station Dürnast TU Weihenstephan to investigate the potential of biogas residues as sources of available sulfur. The pot experiment comprised 20 different fertilization variants; 16 biogas residues and 3 mineral sulfur fertilizer variants, that is, S- 30, S-60, and S- 90 mg/pot, and a control (S-0), replicated trice. Pots were arranged in completely randomized design. Soil SO₄^2- -S and NO₃^--N contents upon were measured trice during the study period. Soil SO₄^2--S and NO₃^--N content were significantly influenced by fertilizer treatments at all times. Generally, liquid biogas residues tend to show higher soil SO₄^2--S and NO₃^--N content. Soil SO₄^2--S content varied from 26.1 to 100.20 mg pot^-1 (625-S, S-90); 30.4 to 100.4 mg pot^-1 (621-S, 626-L) and 31.4 to 98.2 mg pot^-1  (S-0, 626-L), in first, second and third sampling times, respectively. Soil NO₃^--N content also varied from 114.5 to 526.5 mg pot^-1 (629-S, 616-L); 99.0 to 1054 mg pot^-1 (620-S, 616-L); 114.8 to 1045.4 mg pot^-1 (620-S, 616-L), in first, second and third sampling times, respectively. Biogas residues containing more than 0.1% S_t in fresh matter and C_org: S ratio lower than 30 could replace short term sulfate limitation.

Key words: Biogas residues, sulfur mineralization, sulfur availability, C_org:S ratio.