Manures and Manuring 157 



It is obtained by treating substances containing tricalcium phosphate with 

 sulphuric acid. At first superphosphate was made from bone ash and bone 

 black, but the great bulk is now obtained from natural minerals (phos- 

 phorites, coprolites, apatites). Many millions of tons are produced annu- 

 ally, about 800,000 tons being manufactured in the United Kingdom. 

 Commercial samples contain from 25 to 40 per cent of soluble phos- 

 phates, but there is great variation. When buying, the soluble phosphates 

 only should be taken into account, the insoluble phosphates not being 

 highly valued. 



When superphosphate is applied to the soil, the phosphates, being 

 soluble in water, are well distributed amongst the soil particles by a 

 shower of rain. Then a change takes place. The soluble phosphate 

 reverts into an insoluble state owing to the carbonate of lime (or chalk) 

 and the compounds of iron and alumina present in the soil. This change 

 prevents the phosphates from being washed out of the soil too readily. 



Superphosphate is an acid manure, and therefore tends to use up the 

 available lime in the soil (see p. 161). 



Basic Slag". This is a by-product in the manufacture of Bessemer 

 steel, and is also known as "basic cinder" and "Thomas's phosphate". 

 It is a fine dark-grey powder, 80 per cent of the particles of which should 

 pass through a sieve having 10,000 holes to the square inch. It is only 

 since 1885 that basic slag has been used as a manure, having previously 

 been discarded as a waste product. The estimated production for the 

 whole world in 1885 was 150,000 tons, and in 1906 as much as 2,383,000 

 tons. Of this quantity Germany produces 1,510,000 more than one-half ; 

 the United Kingdom being second with 300,000 tons. 



Basic slag is an alkaline manure, and usually contains from 30 to 

 40 per cent of phosphate of lime, which is equivalent to 9 to 18 per cent 

 of phosphoric acid. The phosphate in basic slag is in combination with 

 lime, and in good samples the greater part of the phosphate can be dis- 

 solved in a dilute solution of citric acid. As the value of basic slag 



o 



depends largely upon its solubility in citric acid, purchasers should obtain 

 a guarantee as to its citric solubility, as there are inferior brands of basic 

 slag in existence. If 90 per cent or more is soluble, the sample is a 

 good one. 



Basic slag is a valuable manure for all soils except those of a chalky 

 or limestone nature. It is particularly valuable where large quantities of 

 stable manure have been applied, and where there is a tendency to acidity. 

 For fruits, flowers, and vegetables of all kinds basic slag may be used 

 at the rate of 2 cwt. to 4 cwt. per acre. There is a general impression 

 that it should be used only during the winter months. In practice it 

 will be found useful if applied to the soil about three months before the 

 crops are likely to require it. It yields up its phosphates slowly, but in 

 the meantime the lime is acting in conjunction with the humus in the 

 soil and excites bacterial activity. When finally potting Chrysanthemums, 

 Zonal Pelargoniums, Begonias, and a host of other plants, a sprinkling of 



