Superphosphates 



contain a great deal of humus. It may, however, be 

 appUed to soils which do contain lime. 



Superphosphate and basic slag and the infre- 

 quent precipitate phosphates are the best forms 

 of phosphate, and are nearly always preferable. 

 In alkaline or neutral soils good results ought 

 not to be expected from mineral phosphates. 

 They are not suitable for soils rich in lime, but 

 under certain conditions — for example in wet soils, 

 decidedly acid and containing a great deal of organic 

 matter, they give encouraging results usually, and 

 sometimes very good ones. It is understood of 

 course that when applying mineral phosphates lime 

 is not also applied, because by neutralising the acids, 

 and so arresting their action for the formation of 

 assimilable phosphate from the tricalcic phosphate 

 the benefit of the operation would be altogether lost. 

 Large applications of mineral phosphates may be 

 recommended on certain waste lands, before the 

 first ploughing, seeing that the price is low ; but 

 even under the most favourable conditions, which 

 do not occur frequently, they do not give such good 

 results as slag. 



Superphosphates. 



The phosphates met with in nature, though 

 sometimes very rich in phosphoric acid are not, how- 

 ever, very assimilable by plants, when regarded as 

 manure, a fact which ought, without doubt, to be 

 attributed to their chemical form, tricalcic phosphate 



[(CaO)3P205]. 



Liebig, in 1840, was the first to suggest treating 

 them with sulphuric acid to make them more 



49 E 



