I kinda hate how it's an "evaluate" question. In reality, it should just be: "the experiment has no repetitions, so it is not reliable".
Anyway, a couple of points to consider:
Probably better to filter with a Buchner funnel with filter paper (sintered glass is honestly pretty bad since a lot of crap gets stuck in the glass).
Fertiliser is dissolved in an undefined water volume. If they use an absurdly large volume in one test (e.g. 5 L), then solubility in the large volume can become an issue. They should be dissolving in a set volume of water (where it is fully dissolved). Water should also be deionised.
Barium chloride should be in great excess to ensure complete precipitation. They've only included 2.6 g of BaCl2 and there is 1.0 g of fertiliser, which isn't clearly defined either.
Other barium salts could be precipitating (e.g. barium phosphate), considering other anions are used for fertiliser too.