MANURING AND FERTILIZING 43 



Phosphorus occurs in fertilizers, as : 



1 i ) Insoluble phosphate of lime e.g. , floats, bone 



meal, tankage. 



(2) Soluble phosphate of lime e.g., acid phos- 



phate, dissolved bone. 



Insoluble phosphate of lime is considered to 

 be but slowly available. It is converted 

 into " soluble " by treating it with an acid, 

 usually sulphuric acid. 



Soluble phosphate of lime, as a rule, is more 

 active than insoluble in promoting plant 

 growth, but on acid soils insoluble phos- 

 phate often gives better returns. The 

 soluble phosphate of lime and a phosphate 

 soluble in weak acids constitute the ' ' avail- 

 able phosphoric acid ' ' of the chemist. 

 Potassium is the valuable ingredient found in : 



Wood ashes, kainit, sulphate of potash, double 



salts, and muriate of potash. 

 It usually gives good returns when applied to 

 light, sandy, and peaty soils. As kainit 

 contains chlorides and muriate of potash is 

 a chloride, it is often advisable to apply 

 them some time previous to planting the 

 crop, in order that the injurious substances 

 may be removed by the soil water, chlorides, 

 in excess, being injurious to potatoes. 

 Value. All fertilizers may be valued according to 

 the percentage of nitrogen, soluble phosphate of lime, 

 insoluble phosphate of lime, and potash present. They 

 are often valued on the unit system. A unit is one per 

 cent, of a ton, or 20 pounds; the ton, 2,000 pounds. 



