96 the farmers' handbook. 



The term is used locally to include a much more extensive class of sub- 

 stance. The clung of bats, flying-foxes, and marsupials goes under this name. 

 A large number of artificial fertilisers are also sold as guanos. 



The nitrogen exists often in the three forms of organic nitrogen, ammonium 

 salts, and nitrates, the last form being the most soluble form in which 

 nitrogen can be applied to the soil. They are excellent manures for cereals, 

 especially as a top -dressing in the spring. 



They likewise contain an appreciable percentage, 2 to 3 per cent., of potash. 



The bulk, however, of the mineral phosphates and of phosphatic guanos 

 is converted by treatment with sulphuric acid into superphosphate. 



Superphosphates. — When bones, bone-ash, or any of the substances 

 mentioned in the previous paragraphs are acted upon by oil of vitriol 

 (sulphuric acid), the phosphate of lime which they contain, and which exists 

 in the form of tri-calcium phosphate, is converted into a different lime 

 compound, known as mono-calcium phosphate, or superphosphate. This 

 substance differs from the original phosphate in being readily soluble in 

 water. We have already seen that the availability of a manure as plant-food 

 depends largely upon its ready solubility ; consequently a phosphate of lime 

 soluble in water is the most available form of this substance. Plants, how- 

 ever, are not dependent for their food-supply only upon such compounds as 

 are soluble in water, but are able, by means of the acid juices secreted in the 

 roots and root-hairs, to avail themselves of a class of compounds soluble 

 in vegetable acids. It is, indeed, very improbable that superphosphate 

 remains in the soil for any length of time in that condition, for it is readily 

 decomposed, both by carbonate of lime and by the iron and alumina salts 

 present in the soil. Carbonate of lime converts it into a third compound of 

 lime and phosphoric acid, known as bi-calcium phosphate. This substance 

 is insoluble in water, but being in a very finely-divided state is readily 

 dissolved by the plant-acids. Iron and alumina attack the superphosphate 

 even more rapidly, converting it into phosphates of iron and alumina, to 

 which the same remarks apply. 



This point is of further importance in reference to the composition of the 

 superphosphates sold as manures, for they also usually contain a considerable 

 quantity of the bi-phosphate in question. 



In the valuation of fertilisers the term " citrate-soluble phosphate," some- 

 times employed, is just this bi-calcium phosphate, which is formed to a 

 greater or less extent in superphosphates by the inter-action of the super- 

 phosphate and unchanged tri-calcium phosphate. In a superphosphate, 

 therefore, as usually purchased, there will be not only the water-soluble 

 superphosphate, but also a certain quantity of the reverted phosphate, which 

 is insoluble in water, but which is, nevertheless, available for plant-food, and 

 of only slightly lower fertilising value than the water-soluble phosphate. 



Superphosphate is essentially the manure for wheat, cereals, and root-crops, 

 and in all cases where a rapid return is wanted. For autumn manuring and 

 for the manuring of permanent pasture, bone-dust is equally effective ; 

 and the conditions under which it will be advisable to use the one or the 

 other form of phosphate will vary with circumstances. 



Superphosphates are more specially beneficial on soils of medium nature 

 and such as contain lime. Indeed, in sandy soils poor in lime it has been 

 found to be of very little benefit. It must not be forgotten that it is an 

 acid manure, and in the absence of lime to neutralise it, may even prove 

 injurious. 



