Determining the agronomic value of composts produced from garden organics from metropolitan areas of New South Wales, Australia

About 0.3 million t/year of composted garden organics (CGO) including mulches and soil conditioners are produced annually in New South Wales, Australia, although only a small proportion of this material (< 4%) is used in agriculture. A lack of information on product characteristics and agronomic performance has limited the development of agricultural markets for CGO products. These CGO products are the coarse and fine fractions separated by screening after composting. This paper presents the results of a survey of CGO mulches and soil conditioners ( unblended or blended with a mixture of other organic materials including biosolids, animal manures and paper), which are commercially produced in the metropolitan areas of New South Wales and assesses their agronomic and soil amendment values in terms of chemical and biological properties. It also evaluates the short-term effects of applying increasing rates ( 0, 25, 50 and 100 t/ha) of selected composted soil conditioners on radish growth in a pot experiment. The mulch products had low nutrient concentrations but had high carbon ( C) contents ( mean C = 45%) and C/ Nitrogen ( N) ratios ( mean C/ N = 72) and are most suitable for use as surface mulch. The unblended soil conditioners were low in nutrients, particularly N ( average total N = 1.0%, range 0.9 - 12%), and had lower and variable C contents. The pot trial results indicated lack of growth response of radish at application rates up to 100 t/ ha of unblended soil conditioners from garden organics. The blended soil conditioners were more variable in quality and as confirmed by pot trial results produced highly variable plant responses. The high variability in product quality and performance of the soil conditioners, particularly the blended products might be related to the source and type of blending material as well as the composting conditions used in the manufacturing process. These results highlight the need to improve compost quality and consistency and the need for further research to advance understanding of the benefits using CGO in terms of improving soil quality, crop productivity and net economic returns to growers.