Soil and plant growth benefits resulting from applying biosolids, poppy mulch and poppy seed waste as soil amendments to texture contrast soils in Tasmania

Organic materials are used as soil amendments in productive agriculture to increase or replace soil organic matter and provide essential plant nutrients. Two field trials were undertaken in Tasmania (a temperate region located between latitudes 40° and 44° south and between longitudes 143° and 149° east) over two years to quantify changes to biological, chemical and physical properties of soil and to determine crop responses from applying locally available organic materials to a texture contrast soil. Lime amended biosolids (LAB) and anaerobically digested biosolids (ADB) were applied at both sites with application rates calculated from local EPA guidelines. Lime and fertiliser (L+F) was applied at both sites, with application rates based on nitrogen requirement of the crop. Poppy mulch (PM) and poppy seed waste (PSW) were applied at one site only, with application rates based on industry recommendations. Results showed that the application of bio-resources can produce equivalent cereal crop yields to inorganic fertiliser, for two successive seasons following application. LAB applied at 1NLBAR (for cereals) and PM applied at 17.5 wet t/ha increased soil pH by 0.9 and 0.6 units respectively within 9 months of application. Without further application of P, a season of growing cereals did not reduce soil Colwell P from pre-trial levels for the LAB treatment. However, an increase in Colwell P after the second year is of major concern for potential leaching and surface run-off of mobile P. A partial nitrogen balance after the first year showed that actual mineralised N from LAB was > 30% higher than calculated mineral N from EPA guidelines, whilst mineralised N from ADB was 19% lower than calculated mineral N from EPA guidelines. Furthermore, contrary to previous research, an inverse relationship was found between increasing rates of LAB and mineralised N according to partial N balances after the first season. A further field trial and an incubation experiment were conducted to study nitrogen mineralisation kinetics of the different bio- resources. Results confirmed that current EPA guideline assumptions for application of ADB and LAB do not adequately reflect actual release of mineral nitrogen from either product. They also showed that eight weeks after application, PAN as a percentage of total N applied in PSW was 6 times higher than PAN from ADB, even though the application rate for ADB was 6 times higher than PSW and total N of the initial products were 4.1% and 4.2% respectively. After 56 days incubation at 12.5° C (temperature of autumn/winter period when bioresources are applied to soil) and constant soil moisture, PAN from total N applied in ADB, PSW and LAB was 35%, 49% and 62% respectively. The PM treatment showed a drawdown of PAN over the same period, suggesting that applying this product requires additional nitrogen to satisfy plant demand. A modelling component was included in the research program using APSIM (Agricultural Production Systems Simulator) with data from the field trials to interpret and improve understanding of the results obtained. The model simulation of mineral nitrogen accumulation in the soil following application of LAB was in good agreement with the measured data. However, measured mineral nitrogen for ADB and the higher application rates of LAB were not in agreement with the simulated model. This result together with partial nitrogen balances performed as part of this research suggests that the nitrogen equations used in the model may require additional information such as a constant that allows for the (non) uniformity of the soil to product contact when incorporated. This constant may then be used in general application guideline calculations to better reflect nitrogen release from bio-resources after application to soil.