28 PLANT PRODUCTS 



a dressing of lime may be provided by any of the forms of 

 lime mentioned above. To endeavour to represent in some 

 way the value of basic slag for replacing lime a conventional 

 calculation is adopted. As a means of obtaining information 

 of degrees of solubility, citric acid is commonly taken as 

 a convenient standard, but there is no real theoretical reason 

 why citric acid should be preferred to any other acid, though 

 it is certainly convenient, and has amply justified itself in 

 practice. In the case of basic slag it has become a recognized 

 standard to extract the slag by shaking for half an hour 

 with 2 per cent, citric acid solution, and to consider that the 

 portion dissolved has a special value to the plant. If we take 

 the lime that has been dissolved by citric acid, and deduct 

 from that the lime equivalent of the phosphoric acid also 

 dissolved, we shall obtain the lime soluble in citric acid, 

 over and above what may be regarded as neutralized by the 

 phosphoric acid. This figure is generally known as the 

 available lime in the slag, and may fairly represent the relative 

 ability of the slag to replace the ordinary operation of liming 

 the soil. It is, df course, purely conventional. There is 

 a good deal of evidence to show that the citric-acid soluble 

 phosphate in a slag has a distinct value in pot experiments, 

 and in all cases where the crop has only a short period of 

 growth. There is also plenty of evidence to show that in 

 the case of pasture such solubility is of little advantage. 

 Citric solubility must, therefore, be regarded as a test 

 of distinct value, in its proper place, but its importance can 

 easily be exaggerated. The degree of fineness to which 

 basic slag has been ground is also a very important point. 

 The basic slag must be distributed much more completely 

 than is necessary for a manure soluble in water, and this 

 can only be achieved if the material is very finely divided 

 (see p. 6). Basic slag must be put on early to get a full 

 value. Probably the maximum result is obtainable about 

 two years after application, but with slags of high citric 

 solubility the maximum may be reached earlier. Soils 

 containing much humus or peaty material are especially 

 benefited by slag. To what extent this benefit is attributable 



