64 THE CHEMISTKT OF THE FABM 



present. Heavy land is greatly lightened, and made 

 more pervious to air and water, by ploughing in fresh 

 manure. Sandy soils are consolidated, and their power 

 of retaining water much increased by the admixture of 

 rotten manure. 



Seaweed.— This manure when fresh is, on the whole, 

 similar in value to farmyard manure. It becomes 

 more valuable as it loses water. 



Guano.— This consists chiefly of the dried excre- 

 ments of sea fowl. When guano has been deposited 

 in the absence of rain it contains a large amount of 

 nitrogenous matter, phosphates, and alkaH salts. If 

 exposed to rain the original nitrogenous matter is de- 

 composed and disappears, the alkali salts are washed 

 out, and the guano remaining is then almost purely 

 phosphatic. The largest deposits of guano are on 

 the Peruvian coast and the adjacent islands. Guano 

 is also imported from the south-west coast of Africa. 

 The present imports contain 2 — 10 per cent, of nitro- 

 gen, 10 — 30 per cent, of phosphoric acid, and 0*2— 3'4 

 per cent, of potash. From its great variation in 

 composition, guano should always be purchased on 

 analysis. 



To obtain a constant composition different kinds are 

 sometimes mixed, and any deficiency in nitrogen made 

 up by the addition of sulphate of ammonium; such 

 mixtures are known as equalised guano. 



In a nitrogenous guano the nitrogen is chiefly pre- 

 sent as uric acid and as ammonium salts. The stronof 

 smell of a damp guano is due to carbonate of ammonium. 

 The phosphoric acid exists principally in tbe f :na of 



