How plant available is the phosphorus in the sludge biochar?
Thursday 21 September, we had a pre-meeting for the project Phosphorus availability in sludge and sludge biochar, in which we will study phosphorus and heavy metal uptake in a four-year frame study. Dorette Müller-Stöver and Jakob Magid visited VA SYD and Testbed Ellinge's project manager David Gustavsson yesterday to discuss the project.
Previous studies of the availability of phosphorus in organic phosphorus fertilisers have been limited to studying short-term fertiliser effects in individual crops in short cultivation trials, often in pot trials (Möller et al., 2018). Linderholm (2011) did a literature study on the subject and concluded that it is difficult to see differences in plant availability of phosphorus in different sludge types, which are not due to liming effects or other substances that can also benefit vegetation.
The forty-year old sludge spreading project in Skåne, Sweden, have, among other things, demonstrated a yield-increasing effect when phosphorus is supplied with sludge in comparison to where phosphorus is supplied only with mineral fertilizer (Börjesson et al., 2014). The phosphorus is thus available for the crops. However, it is unclear when the phosphorus becomes available, but also how much is directly available, which is important information when the farmer plans his phosphorus application.
Phosphorus availability in sludge biochar appears to decrease with increased pyrolysis temperature (Crombie et al., 2015). In addition, phosphorus availability appears to be lower in sludge biochar than in sludge (Frištak et al., 2018). However, phosphorus availability probably also depends on how the phosphorus has been separated at the wastewater treatment plant, chemically (with iron or aluminum) or biologically (assimilation or luxury uptake via biological phosphorus removal).
Linderholm (2011) also stated that there is some confusion around the concept of plant availability. Only the dissolved phosphate ions in the soil water are directly available to the plants.