Ko. 4.] COMMERCIAL FERTILIZERS. 121 



soil l)uikler, than as a direct fertilizer in special kinds of 

 farming. On soils rich in vegetable matter (humus), where 

 decay proceeds rapidly and organic acids are formed, the 

 ground phosphate rock will give in many cases satisfactory 

 returns if applied in sufficient quantities. Ground phosphate 

 rocks are also suitable for sour soils, which sourness is not 

 due to excess of water. Ground phosphate rocks are not suit- 

 able for soils that have been farmed for a long time, and in 

 which the vegetable matter is exhausted. They answer just 

 about as well for this purpose as ground cobblestones. They 

 are not to be depended upon for quick-growing crops, as those 

 planted in spring and harvested within two or three months. 



Another source of supply of phosphoric acid, which is very 

 largely used in foreign countries and with great satisfaction, 

 is basic slag, or Thomas phosphate powder, derived in the 

 manufacture of steel from pig iron. This phosphate pos- 

 sesses several characteristics which are not possessed by any 

 of the phosphates already discussed, and should not be con- 

 fused wdth them. 



In the first place, it is an alkaline product, carrying 40 per 

 cent to 50 per cent of lime; and in the second place, the 

 phosphoric acid is combined with lime to form what is called 

 " tetra-basic " phosphate, or four parts of lime combined 

 with one part of phosphoric acid ; thus the phosphates are 

 more loosely combined than in the case of the untreated 

 phosphates from other sources, and when ground to the same 

 degree of fineness are likely to be much more available. 

 Theoretically, one molecule of the tetra-basic phosphate 

 breaks up into two molecules of di-basic phosphate, making 

 the phosphate correspond to what is known as " reverted " 

 phosphoric acid in superphosphates. 



For many purposes, therefore, this material is superior 

 even to superphosphates, because carrying no free acids, and 

 because the lime associated with it has a tendency to neu- 

 tralize acidity in soils. Thus far this product has not been 

 largely used in Ihis couuti-v, and this is probably due to two 

 reasons: first, its cost makes the phosphoric acid more ex- 

 pensive, as a rule, than in superphosphates; and second, the 



