Are we underestimating biosolids crop available nitrogen supply?

Taylor, M.J.1, Rollett, A.J.1, Martindale, T.2, and Chambers, B.J.1, 1ADAS Gleadthorpe, 2Severn Trent Water, UK



Biosolids are a valuable source of plant available nutrients, particularly nitrogen (N) and phosphorus. Many research studies that have quantified the N supply properties of biosolids have only measured release to the next crop grown, and have not quantified N supply to following crops. At ADAS Gleadthorpe, the N supply properties of two contrasting biosolids products were measured over four winter wheat cropping years (2006 to 2009). Mean N supply in the first cropping year was c.13% of total N applied and c.10%, c.4% and c.4% in cropping years two, three and four, respectively. Over the four cropping years, N supply was equivalent to 31% of the total N applied. Based on a single application of 250 kg/ha total N, biosolids crop available N supply over the four years was equivalent to c.77 kg/ha N or £77/ha (based on a manufactured fertiliser N price of 100 p/kg N)

Key words:

Biosolids, crop available nitrogen, fertiliser, nitrogen, plant nutrient, sewage sludge.


The application of biosolids (i.e. treated sewage sludge) to agricultural land is regarded as the Best Practicable Environmental Option in most circumstances (Defra, 2007). Biosolids provide a valuable source of plant available nutrients and organic matter that can be of benefit to soil quality and fertility. In 2008, c.77% of UK sludge production was recycled to farmland as biosolids (Water UK, 2010). Digested cake is the most common biosolids product applied to farmland. However, the development of new technologies, such as thermal drying, has produced new enhanced treated biosolids products that are likely to have different nutrient supply properties compared with conventional digested sludge cake products. Thermally dried biosolids have a number of advantages compared with conventionally treated products, viz.: the volume of material to be transported is reduced, the material can be pelletised and is therefore relatively easy to spread, and the product is low in odour, however, they are energy intensive to produce. Typical nitrogen (N) content data for contrasting biosolids products are summarised in Table 1 below (Defra, 2010). However, the nutrient characteristics of these products will vary depending on the individual source and treatment processes undergone.

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