Manuring 307 



cent, phosphoric acid soluble in citrate of am- 

 monia solution. 



If superphosphate be applied to the soil, 

 its phosphoric acid, being soluble in water, 

 becomes disseminated throughout the soil, and 

 is there absorbed by other constituents, as lime, 

 magnesia, iron, and alumina, forming phos- 

 phates which are insoluble in water. In soils 

 containing a fair amount of lime the water- 

 soluble phosphoric acid is precipitated into 

 phosphate of lime, which supplies the plants 

 with available phosphoric acid ; but on soils 

 poor in lime the water-soluble phosphoric 

 acid is either subject to being washed out of the 

 surface soil, or is absorbed by the oxides of 

 iron and alumina, forming compounds the 

 phosphoric acid of which the plants can only 

 assimilate with the greatest difficulty. The use 

 of superphosphates should, therefore, be limited 

 to soils which are not deficient in lime ; on these 

 i Ib. of phosphoric acid in ordinary superphos- 

 phates has the same effect as rib. of phosphoric 

 acid in concentrated superphosphate. 



Thomas's phosphate powder, a by-product in 

 the manufacture of steel free from phosphorus, 

 contains 15 to 20 per cent, phosphoric acid, 

 40 to 50 per cent, lime, and 4 to 5 per cent, 

 magnesia. The phosphoric acid in Thomas's 

 phosphate powder, although insoluble in water, 

 is easily soluble in weak acids, and there- 

 fore readily assimilated by plants. In genuine 

 Thomas's phosphate powder 80 per cent, and 



