PHOSPHORUS FERTILIZERS 123 



according to Wagner's standard method of shaking up 

 5 grammes of slag with \ litre of 2 % citric acid for half an 

 hour, end over end, in a rotary shaker at a speed of about two 

 revolutions per minute. When the citric soluble methods 

 were almost universally adopted, it was found convenient to 

 estimate the lime in the citric soluble extract, as one could 

 safely assume that the citric soluble lime was of an active 

 character. The available lime was calculated from the 

 amount of lime soluble in citric acid, from which had been 

 deducted the amount of lime combined with phosphoric acid 

 in the citric soluble extract. Now that the determination 

 of citric solubility is less frequently performed, the nearest 

 comparable figure is obtained by taking the total lime and 

 deducting the lime needed to combine with the total phos- 

 phoric acid. The figures would be somewhat higher than 

 those given before, but would exhibit the same general 

 relationship. In addition to the method of attempting to 

 arrive at the available lime as expressed above, Hendrick 

 devised a method of distillation with ammonium sulphate. 

 Worked by his method, the amount of available lime is very 

 near to that obtained by calculations from the citric soluble 

 lime as given above. Usually his method gives 2-3 % 

 higher. If the method suggested for the future of taking 

 the total lime less the amount combined with the phosphoric 

 acid is used, the figures obtained by such a calculation will 

 be very close to those of Hendrick's method. One of the 

 great advantages of basic slag over acid phosphatic manures 

 lies in the fact that the former contains a considerable 

 quantity of lime, which is capable of neutralizing strong 

 acids like sulphuric acid in sulphate of ammonia, or feeble 

 acids produced by organic matter in the soil. The products 

 of bacterial growth in the soil also need lime or other bases. 

 The lime in basic slag is probably present partly as phosphate 

 and partly as silicate, yet this latter breaks up in the soil, 

 so that the lime is undoubtedly useful. The true free lime, 

 that is to say lime which is in the form of calcium oxide, is 

 only present in slags to a very small extent, and has probably 

 no significance. 



