(21)Certain substances and mixtures, commonly referred to as inhibitors, improve the nutrient release pattern of a nutrient in a fertiliser by delaying or stopping the activity of specific groups of micro-organisms or enzymes. For inhibitors made available on the market with the intention of them being added to fertilising products, the manufacturer should be responsible for ensuring that those inhibitors fulfil certain efficacy criteria. Therefore, those inhibitors should be considered as EU fertilising products under this Regulation. Furthermore, EU fertilising products containing such inhibitors should be subject to certain efficacy, safety and environmental criteria. Such inhibitors should therefore also be regulated as component materials for EU fertilising products.