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TBM AOniCtJ LTUBAL JOURNAL. 



vised of imitatiug the solvent powers of 

 root-juices, and so distinguishing this 

 phosphate from the other two. 



Basic Slag. 

 An irregular form of calcium phos- 

 phate occurs in basic slag or Thomas' 

 phosphate, which is not constructed in 

 either of the forms already mentioned, 

 and is perhaps not found in any other 

 substance. Up to the present it has 

 hardly been thoroughly investigated, but 

 appears to be partially soluble in citric 

 acid, and upon the amount so soluble the 

 value of a sample largely depends. 

 It is usual to give only the total amount 

 of phosphoric oxide present in an analy- 

 sis of this manure, but it would be better 

 if the citrate-soluble portion were also 

 estimated. Basic slag is a by-product in 

 the Thomas-Gilchrist process of steel- 

 making, and is not a single chemical 

 compound, but a mixture of several ; 

 accordingly, the percentage of phosphoric 

 oxide is not strictly defined, but varies in 

 different samples from 15 to 20 percent. 

 It needs to be very finely ground to be of 

 any use. 



Phosphatic Manuring. 



Several phosphatic manures have been 

 mentioned in this and the preceding 

 chapter ; it only remains to say that as a 

 general rule phosphoric acid in one form 

 or another is the one ingredient of fer- 

 tilisers that cannot be omitted, and in 

 some cases is the only one required. 

 Turnips especially demand that they shall 

 have a full supply from the commence- 

 ment of their life. On soils, with a fair 

 amount of lime in their composition, there 

 is probably nothing better or cheaper 

 than superphosphate, or the mixtures 

 made from it. If the soil is poor in lime 

 it is generally better to use non-acid 

 manures, such as bones, basic slag, or 

 guano ; it must not be forgotten, how- 

 ever, that a somewhat heavier dressing of 

 these is needed, because being less 

 soluble the roots must have a larger 

 (quantity to act on. But the excess re- 

 mains in the soil for the benefit of the 

 next crop. If the soil has a tendency to 

 sourness, /.c;, acidity, l)asic slag should be 

 tried, as it contains some free lime, 

 which helps to neutralise the vegetable 

 acids which are detrimental to plant 

 growth. 



Phosphates in Analysis. 

 A little explanation is necessary as to 

 the manner in v/hich the percentage of 

 phosphates is given in analyses of soils 

 and manures, as there is unfortunately a 

 good deal of confusion, or at any rate a 

 want of agreement, which is likely to lead 

 to confusion. We shall probably find, if 

 we consult an analysis, that so many per 

 cent, of phosphoric acid and lime are 

 given. For analytical purposes it is much 

 the most convenient plan to consider a 

 salt as made up of an acid-forming oxide 

 and a basic oxide, and we have seen that 

 salts can be formed in this way, though it 

 is not the most scientific way to look at 

 it. Moreover, it has been the custom, 

 handed down from the darker ages of 

 science, to speak of this acid oxide as the 

 acid itself, which is certainly incorrect. 

 Therefore, when we see phosphoric acid 

 in an analysis, we must understand that 

 phosphoric oxide is meant ; and in some 

 places, th^ Cape, for example, the correct 

 term has been brought into use. Another 

 plan formerly common, and still adhered 

 to by manure manufacturers, is to reduce 

 all phosphates to the equivalent quantity 

 of tric-calcic phosphate. If, for instance, 

 a superphosphate is said to contain 30 per 

 cent, of soluble phosphate, this does not 

 mean that there is 30 per cent, of mono- 

 calcic phosphate, but that if the mono- 

 calcic phosphate were converted into 

 tri-calcic phosphate, there would be 30 

 per cent, of the latter. No doubt the 

 larger figures look better ; but the method 

 appears to be one of those " trade 

 customs" which are not always under- 

 stood by the ordinary purchaser. If the 

 average farmer has the choice of two 

 manures, one stated to contain 36 per 

 cent, of soluble phosphate and the other 

 20 per cent, of soluble phosphoric acid 

 (cr oxide), the odds are he would choose 

 the former ; yet a little calculation with 

 the figures given below will show that 

 36 per cent, of tri-calcic phosphate is only 

 equal to about K;^ per cent, of phosphoric 

 oxide. 



The factors nece sary for the compari- 

 son of phosphoric oxide with the phos- 

 phates are as follows : — 



One part of phosphoric oxide equals 

 1.648 parts of mono-calcic phosphate. 



One part of phosphoric oxide equals 

 1.915 parts of di-calcic phosphate. 



