Sw Ives

Plant growth and soil responses to soil applied organic materials in Tasmania, Australia

Biosolids, poppy mulch (PM), and poppy seed waste (PSW) were applied to soils in barley and wheat field trials in two areas of Tasmania to determine crop and soil responses and the potential for these materials to substitute for inorganic fertiliser. Lime-amended biosolids (LAB) was applied at one, two, and five times the nitrogen-limiting biosolids application rate (NLBAR). Anaerobically digested biosolids (ADB) was applied at the NLBAR, and PM and PSW were applied at industry-recommended rates. The results indicated that ADB, LAB, PM, and PSW can substitute for inorganic fertiliser in meeting plant nutrient requirements but may be releasing more plant-available N than guideline assumptions. However, organic amendments are less easily managed and applied than inorganic fertiliser, and release of plant-available N from organic amendments may be too rapid for plant uptake to prevent leaching losses. LAB can also leave elevated residual extractable phosphorus in soil after two cereal crops, and PM and LAB can increase soil pH. Microbial biomass showed an inconsistent, and in some cases unexpected, response to organic amendments after 1 year and no significant relationship to changes in soil carbon.

N Mineralisation from Bioresources Incubated at 12.5°C

Soils treated with lime-amended biosolids (LAB), poppy seed waste (PSW), anaerobically digested biosolids (ADB) and poppy mulch (PM) and incubated at 12.5°C for 56 days released 45%, 36%, 25%, and −8%, respectively, of total applied N as plant available nitrogen (PAN) by the end of the incubation. The mineralisation rates were contrary to expectations based on the C : N ratios of the four products: LAB (5 : 1), PSW (7 : 1), ADB (3 : 1), and PM (16 : 1). PM showed a significant negative priming effect over the incubation period. These results have implications for production agriculture in temperate regions where application and incorporation of bio-resources traditionally occurs in autumn and spring when soil and air temperatures are relatively low. Current application times may not be suitable for nitrogen release to satisfy crop demand.