according to the following article 5% of phosphate might be all that Amazon may be shooting for and which they achieved already in today's news:


Phosphate Concentrates
Besides the natural difficult characteristics for mining, processing and solubilization that apatites present, when compared to phosphorites,Brazilian igneous rocks have some additional unfavorable properties :very low P2O5 content in the run of mine(5% to 10%); high iron and aluminum oxides content; some accessory minerals similar to the phosphate ore which prejudice the separation in the beneficiation processes.

Due to the peculiar characteristics of Brazilian rocks, it was necessary a huge effort to develop technology for processing and to produce concentrates comparable to those available in the international market. Leaded by Professor Paulo Abib Andery, the effort succeeded and today Brazil is a leader in technology for processing low grade phosphate rocks. By using this technology, an extraordinary increase in phosphate production was reached.

The Brazilian production of phosphates started in 1943 by SerranaS.A. The mining and concentration of a residual ore with highP2O

Ps the article. you have to translate it
http://avepagro.org.ve/fagro/v17_14/v171a010.html