the dow v big mak, page-14

Down on king island they process around 2600 tonnes of Bull Kelp a year and that is when it washes up in abundant supply on the beaches. Bull kelp has about 0.15% phosphorus - so out of the 2600 tonnes you get 3.9 tonnes of pure phosphorus I assume.

The world produces just over 2 million metric tonnes of seaweed a year, phosphorus at 0.2% (max) = 4,000 tonnes of pure phosphorus and this of course assumes no consumption as food. (potash is in low % in seaweed so no luck there either)

Fish meal production is around 6.3m tonnes per annum (approx 6% phosphorus) but is used as a high protein supplement and expensive.

So to produce 100m tonnes of rock phosphate equivalent you would need to suck the living daylights from the ocean and create an ecological disaster.

Think about that next time you think you are being 'green' by using seaweed-based fertilizer.