22 PLANT PRODUCTS 



and 2 to 3 per cent, silica. The amount of nitrogen varies 

 from 17 to 20 per cent. Calcium cyanamide, when kept in 

 store, slowly absorbs water from the air, so that it increases 

 in weight. In consequence of this fact the percentage of 

 nitrogen decreases at the rate of i per cent, in two or three 

 months, but the owner does not thereby lose anything. 

 At the same time a small amount of decomposition does take 

 place, and traces of ammonia are given out into the air. 

 Calcium cyanamide in itself is no use to the plant, and when 

 acted upon by the water in the soil it will produce the poison 

 di-cyanamide, which will slowly decompose into ammonia, 

 and then nitrify. It is only suitable for application some 

 time before sowing. It is a slow-acting manure, and is quite 

 unsuited to top dressing. It can be mixed with basic slag, 

 but not with super-phosphate or with sulphate of ammonia. 

 The amount of lime present is generally beneficial, and the 

 graphite is absolutely harmless. At first calcium cyanamide 

 will act as a poison ; it will therefore have the value which 

 will be alluded to again under the head of the " Partial 

 Sterilization of Soils " (see p. 90). 



The Organic Nitrogen Fertilizers. Fish refuse, fish 

 meal, or fish guano, is one of the most important in this 

 group. 



Refuse fish is often used locally by farmers, but the 

 manufacture of fish meal and fish guano are definite industries 

 in connection with fisheries. The best qualities are used only 

 for feeding purposes, but the other qualities are applied to 

 the soil. A very large proportion of the fish guano in Great 

 Britain comes from herrings. The heads, tails, and guts 

 that are discarded in salting the herrings are dried, and 

 then the fat extracted by petroleum spirit. The resulting 

 material when used for fish guano contains about 9 to 12 per 

 cent, nitrogen, 3 to 5 per cent, of phosphoric acid, and about 

 i per cent, of potash. The amount of oil should not exceed 

 i to 2 per cent. In other parts of the world other systems 

 are often in use. In some parts of America the fish is boiled, 

 the fat skimmed off, and the resulting mass dried and used 

 as a manure. In India much refuse fish is dried on the beach, 



