There is no clean way to say this but…sewage sludge might just be the next thing to help grow the produce you consume. A recent study published in the journal Frontiers in Nutrition states that thermally conditioned sewage sludge could replace commercial chemical fertilizer as a sustainable option for improving soil properties. The greatest advantage of this alternative fertilizer is that it re-uses essential and finite phosphorus resources, which are commonly sourced from non-renewable phosphate rocks.
Sewage sludge is now a readily available substitute of commercial fertilizers in agriculture due to technological improvements that have increased the phosphorus content of it. Therefore, Andry Andriamananjara from the University of Antananarivo (Madagascar), along with his colleagues, decided to assess its effectiveness using a phosphorus radiotracer technique to measure the availability of phosphorus for plants in thermally conditioned sewage sludge.
Here’s a description of their experiment and the results:
They grew ryegrass in pots filled with soil that underwent isotopic P-labelling, and added either no fertilizer, thermally conditioned sewage sludge as fertilizer, or commercial triple superphosphate fertilizer. Shoot and roots were harvested at fixed intervals, and their radioactivity was analyzed to measure phosphorus uptake.
As expected, fertilizer application increased shoot biomass significantly over the harvest time, while only a trend towards an increase in root biomass was found. The addition of fertilizer increased phosphorus uptake by the plants. This wasn’t only because the fertilizer offered an additional phosphorus source, but also because the plants increased their soil exploitation.
However, total phosphorus uptake from thermally conditioned sewage sludge was lower than that from the commercial fertilizer. This could be explained by the fact that the phosphorus in the water-soluble commercial fertilizer is immediately available for plant uptake after application, whereas the phosphorus in thermally conditioned sewage sludge is present in a lower available form. Moreover, the other nutrients present in the sludge stimulated the microbial activity, thereby inducing competition between microorganisms and the plant roots for phosphorus uptake.
Dr. Andriamananjara would still recommend the usage of sewage sludge as fertilizer. He said: “It was shown to have a higher agronomic effectiveness in comparison with commercial fertilizer. Although on the short term it enhanced the microbial biomass and therefore phosphorus immobilization, on the longer term the phosphorus captured by this microbial biomass can again become available for the plants. Moreover, sewage sludge is a non-limited continuously available and sustainable fertilizer source.”