Gypsum, bone-dust, superphosphate of lime and nitrate of soda may also be used, and wood ashes are advantageous if the soil contains much vegetable matter; but the best results are usually obtained when farmyard manure is supplemented by artificials, not by using artificials alone.
Nitrate of soda, Peruvian guano and superphosphate of lime in the form of bones dissolved by sulphuric acid were now added to the list of manures, and the practice of analysing soils became more general.
The exhaustion of the soil induced by both barley and wheat is, however, characteristically that of available nitrogen; and when, under the ordinary conditions of manuring and cropping, artificial manure is still required, nitrogenous manures are, as a rule, necessary for both crops, and, for the spring-sown barley, superphosphate also.
Phosphorite, when occurring in large deposits, is a mineral of much economic value for conversion into the superphosphate largely used as a fertilizing agent.
Of superphosphate to the acre at the time of sowing, followed, if the ground is in poor condition, by cwt.
2 The early settlers found the bones of the bison scattered over the prairies, and after the construction of railways the gathering and shipping of these for use in sugar refining and in the manufacture of superphosphate became temporarily a profitable industry.
The artificial manure known as "superphosphate of lime" consists of this salt and calcium sulphate, and is obtained by treating ground bones, coprolites, &c., with sulphuric acid.
In a natural state it is obtained from bones, guano and wood ashes; and in an artificial condition from basic slag or Thomas's phosphate, coprolites and superphosphate of lime.