Metal oxide nanoparticles (MONPs) are increasingly being incorporated in domestic and industrial products. They are expected to pass into waste water treatment facilities, and may inadvertently be applied in biosolids to agricultural soils. Very few nanotoxicity studies have been conducted in soils and the risk that MONPs pose to soil organisms are poorly understood. The aim of this study was to investigate the behaviour and effects of two MONPs with different solubilities on terrestrial organisms in soils. Exposure of earthworms to soils amended with up to 10,000 mg/kg nano-sized TiO2, a virtually insoluble nanomaterial, resulted in no adverse effects on earthworm population parameters such as survival or reproduction. Earthworms avoided nano-TiO2 amended soils, but only at nanoparticle concentrations higher than those expected for agricultural soils. The mechanisms resulting in the avoidance response, and nano-TiO2 transformations in soils, could not be further investigated because techniques to track nanoparticles in complex media are lacking. Subsequent studies focused on a sparingly soluble nanomaterial, nano-CuO, which releases Cu2+ ions as it dissolves. To identify nanoparticle-specific effects on organisms, the effects of the Cu2+ ions must be determined but little is known about the effect of nano-CuO on Cu2+ activity in soils. Tests revealed that Cu2+ activity in nano-CuO amended soils increased over a 56 d period, but were not comparable to those in soils amended with similar concentrations of micrometer-sized CuO or Cu(NO3)2, which are commonly used as treatments to control for the effects non-nanosized Cu in nanotoxicity tests. These results have implications for the design of experiments that test effects of dissolving particulates. A subsequent barley growth test demonstrated that there was no significant difference in plant growth or shoot Cu concentrations between soils amended with nano-CuO, micrometer-sized CuO or Cu(NO3)2 once growth was normalised to Cu2+ activity. These results demonstrate that release of metal ions can play an important role in toxicity of soluble metal-based nanoparticles and highlight the importance of direct measurement of potentially toxic products of nanoparticle dissolution. There was no evidence of nanoparticle-specific toxicity under the conditions studied either for earthworms exposed to nano-TiO2 or barley exposed to nano-CuO. The study highlighted some of the challenges of understanding the fate and effects of nanomaterials in soils, including the lack of techniques to track nanomaterials in complex media and the difficulty in designing toxicity tests that control for temporal changes to both nanoparticles and soils.