Soil water and residue amendment effects on nutrient availability and wheat growth in silt loam soils of South Australia

A study was conducted for 45 days to examine the influence of soil residue amendments with different C:N ratios and soil wetting regimes on soil nutrient availability and plant growth. Four soil water treatments i.e. 20 days moist (20 M), 5 days moist followed by 15 days dry (15 D), 10 days moist followed 10 days dry (10 D) and 20 days dry (20 D) and three residue treatments i.e. control (CO), dry faba bean residue with C:N ratio of 60.2 (FB), dried kikuyu grass residue with C:N ratio of 20.1 (KK) were tested. The experimental soils were wetted and maintained at 50% of water holding capacity (WHC) for 10 days. 10,000 mg kg-1 of ground residues of KK and FB were added to pots containing 0.35 kg soil each, and the soils were subjected to water wetting treatments for 20 days. On day 20, all soil treatments were brought up to 50% of WHC and pregerminated seeds of wheat were transplanted. No inorganic fertilizers were applied to wheat. Soil samples were collected at 5, 10, 15, 20 and 45 days after water treatment (DAWT) and were analysed for inorganic nitrogen (IN), percent ammonium nitrogen (% NH₄-N) of the inorganic N and the available phosphorus (AP). Water treatments' effects were significant with higher values found under 10D and 15 D water treatments. The KK treatment resulted in better plant growth, higher nutrient uptake and an increase in the soil IN and AP while the lowest in the FB soil. This study highlights the short-term detrimental effects of residues with relatively high C:N ratio and nutrient immobilization could present a real challenge to crop growth in the short term in such soils.