There are different types of organic fertilisers in use: manure and biogas digestate, both as liquid and solid type, compost and sewage sludge. Sewage sludge may be in liquid form or dewatered and supplemented with limestone or a synthetic dewatering agent (such as dewatered manure and biogas digestate). Nitrogen in liquid organic fertilisers may be more readily plant available, as a large amount of it is present as NH4-N. If applied in access or before planting, part of it may leach beyond the root zone. It is therefore important to apply liquid organic fertiliser in accordance with plant needs. Nitrogen in solid organic fertilisers is bound to the organic matter and plant available only after mineralisation. In practice, only a small part of the nitrogen in the solid organic fertilisers is taken into account as instantly plant available, the rest has to be remembered for future fertilising measures. Quantifying this future available share of organic bound nitrogen is complicated, as a lot of factors are involved (Klages et al., 2018). Anyhow, the more organic bound N is applied, the higher the risk of unwanted release as mineral N. This is particularly important for solid manure which provides less readily available N in the season after application than slurries but releases more N to crops in subsequent years. Using manure N as a sole nitrogen source may therefore limit overall manure N efficiency (Webb et al., 2013). Figure 5.1 shows for different organic fertilisers minimum availabilities of nitrogen as share of total applied nitrogen in the year of application and the succeeding year according to the German fertilising ordinance (DüV, 2017).
The type of organic fertiliser used may be helpful to estimate the amount of easily mineralisable organic bound nitrogen, which might be subject to leaching.