The diversion of biodegradable waste away from landfill has been driven by European legislation and national targets for recycling and composting set by governments in the United Kingdom to achieve this. Green wastes and subsequently food wastes have been collected for composting and treated through a range of technologies into products that have different physico-chemical properties depending on their ingredients, processing method and treatment duration. However, little attention has been spent on manipulating the compost end products to suit end user needs, especially in terms of maximising the benefits for soil conditioning and nutrient provision, whilst minimising the impact on the environment. Two composts, one made from green wastes over 13 weeks and one including food wastes processed for over one year, were sieved into particle size grades (0-1mm, 1-2mm, 2-4mm, 4-8mm 8-16mm) and the physico-chemical. properties determined. Laboratory and field scale plant growth trials were conducted using composted materials as soil amendments. It was found that, of the major plant nutrients, potassium had the greatest proportion that was readily available and that this availability increased with particle size in the green waste-derived compost but not in the mature, food waste-derived compost which was soil-like in appearance. Potassium from the composts increased soil available potassium and it was found that it could be utilised as effectively as inorganic potassium fertiliser by plants. It was demonstrated that plant yields were increased through the use of compost due to the soil conditioning effects of the organic matter it contained and that this increased the amount of potassium taken up by the crops. By manipulating the green waste-derived compost product by screening out most of the finer particles less than 8 mm which could be used in landscaping markets, an agricultural product could be made which had a higher organic matter content (66% compared with 54%) and lower total nitrogen (0.95% compared with 1.25%) and phosphate (0.33% compared with 0.43%) contents without reducing the available potassium content significantly. The compost could therefore be applied at 26.3 t ha-1 to maximise organic matter addition (17.4 t ha-1), up to limits on total nitrogen set by the Nitrate Vulnerable Zone regulations, on soils with phosphate indices less than 3. On soils with phosphate indices 3 or above, phosphate supply to match the crop rotation needs becomes limiting and so the manipulated compost could be applied at a lower rate of 20 t ha-1 but still provide levels of organic matter (13.2 t ha-1 ) and plant available potassium equivalent to unamended compost.