Asterisks are used in place of the word 'shit' because shit is a 'rude' word. If you refer to something as 'bullshit' (i.e., bull's excrement), you think that it is nonsense or completely untrue. Manure is animal faeces that is spread on the ground in order to make plants grow better. Organic manure contains no synthetic fertilizer. The humour resides in the play on the word bullshit, which could refer to the composition of the manure or to the fact that organic manure is no better than the non-organic variety.