30 THE CHEMISTRY OF THE FARM. 



is their effect. Bones are usually employed for pasture, 

 and for turnips. 



Ground Phosphates. — Some phosphates when finely 

 gi'ound may on certain soils be successfully emjDloyed as 

 manure without previous conversion into superphosphate. 

 The phosphates most suitable for this purpose are phos- 

 phatic guanos, bone-ash, and South Carolina phosphate. 

 The soils most suitable for such manures are those rich in 

 humus, and poor in carbonate of calcium ; these being the 

 conditions (presence of humic and free carbonic acid) most 

 favourable to the solution of phosphate of calcium. Pas- 

 ture soils are especially suitable for such treatment. The 

 solution of the ground phosphate may be facilitated by 

 forming it into a compost with farmyard manure before 

 its application, or by employing with it sulphate of 

 ammonium. The phosphate should be employed in very 

 fine powder. 



Superphosphate. — An abundance of mineral phosphates 

 (phosphates of calcium) occur in nature ; many of these 

 are so little soluble that their effect as manure is but 

 small ; by treating them with sulphuric acid the sparingly 

 soluble tricalcic phosphate is converted into the readily 

 soluble monocalcic phosphate, sulphate of calcium being 

 at the same time produced. Superphosphate is thus a 

 mixture of monocalcic phosphate, and generally some free 

 phosphoric acid, with gypsum, and various impurities (as 

 sand and compounds of iron and aluminium), derived from 

 the original mineral. A superphosphate will always 

 contain more or less of undissolved phosphate; this 

 amount will be more considerable if the manure is badly 



