The possibilities for using seaweeds as fertiliser, depend on two components. First, to determine whether seaweed is suitable as organic fertiliser, it is necessary to analyse the nutrients taken up by the seaweed, to gain insight in the amounts of nutrients that could be applied to the soil. Phosphorus content of Ulva lactuca grown under different initial P concentrations in the water (Pi) was measured. Phosphorus content in relation to Pi concentration showed Michaelis-Menten kinetics with Pmax = 0.46% DW and Km 1.19 µM. N content was also measured and P and N content were found to be related.
Second, it was analysed how Ulva lactuca material decomposed over time in an agricultural sandy soil when incorporated in June (2013). The mesh bag method was used to monitor the dry matter decomposition. The relative decomposition rate of Ulva lactuca was found to be 0.0413 d-1.
In addition, the effects of seaweed fertiliser on two crops, lettuce and mustard, was studied. Mustard plant height and lettuce diameter were measured throughout the course of the experiment. After harvest, shoot and root dry weight were determined and P and N content were measured. Ulva lactuca application increased N content (p<0.05) and P content (p<0.10) of mustard plants, either due to Ulva lactuca application or the reduced dry matter production. Ulva lactuca application had a negative effect on crop appearance in both crops and reduced shoot dry matter in lettuce (p<0.05) and mustard (p<0.10). A delay in crop growth and development was visible under Ulva lactuca treatment, but the exact functioning of Ulva lactuca in crop performance remains unknown.