192 



■drawn from the air by leguminous crops. It is true that the latter, by 

 their deep tap roots, pump it up from the lower strata of the subsoils 

 and by turning them under as green crops, the phosphoric acid is trans- 

 ferred from the subsoil to the surface soil, and is thus placed within the 

 reach of the roots of the subsequent cane crop. If, however, both soil 

 and subsoil be deficient in this ingredient, recourse must be had to some 

 commercial fertiliser containing it, in order to grow maximum crops. 

 A crop of cow peas can and does supply a soil with nitrogen drawn 

 directly from the air, ard in doing so makes a positive addition to the 

 store of fertility in the soil. By its tap roots it may transfer phosphoric 

 acid from subsoil to surface soil, but in doing so it has not increased 

 the stock of plant food on hand, but simply transferred it to a more 

 eligible position for assimilation by cane. Therefore phosphoric acid is 

 equally needed by both plant and stubble cane. (^) 



EXPLAXATIOXS OF FORMS OF PHOSPHORIC ACID. 



Dissolved Bone Black. — Bones are subjected to destructive distilla- 

 tion in the process of " Charring," by which the larger part of their 

 organic matter is driven off, and with it their nitrogen. A part of the 

 carbon existing in bones as organic matter, is now left as charcoal, en- 

 crusting each grain of phosphate of lime. When thus prepared it is 

 called Bone Black," or " Bone Charcoal," which is extensively used in 

 sugar refineries for decolourising and defecating syrups destined for 

 white sugar. After constant use, the bone black becomes " spent," i.e., 

 no longer exercises a decolourising influence upon syrups. It is then 

 sold for fertilising purposes. Sometimes it is without further treat- 

 ment, applied directly to the soil, but without immediate benefits, since 

 the phosphates, being encased in charcoal, are protected from rapid de- 



(1) The part played by phosphatic fertilisers in relation to sugar cane presents 

 features which at present are involved in some obscurity. While for the most 

 part it may be assumed that on the majority of soils canes are benefited by the 

 application of small doses of phosphates, other experiments have shown that 

 moderately large applications are not beneficial, and may even result in dimin- 

 ished yields. The experiments conducted at the Antigua Experiment Station 

 tended to show that, under the conditions of soil and climate prevailing there, 

 the application of phosphates, instead of increasing, actually diminished the yield, 

 and this, whether the phosphate was applied in the form of super-phosphate, 

 mineral phosphate or basic phosphate. Again, at the Barbados Experiment Sta- 

 tion in the first series of experiments conducted, it was observed that while a 

 small application of super-phosphate increased the yield of canes somewhat, 

 larger applications resulted in smaller yields, and in later experiments during 

 1896 and 1897 similar results are arrived at. Experiments conducted in Demerara, 

 give results tending in the same direction. It is significant that unexpected re- 

 sults of this kind should be reported from places so distant and widely separated 

 as Demerara, Barbados and Antigua. 



Although on the whole phosphatic manures may be desirable and beneficial, it 

 is evident that considerable care should be exercised in their use. It is fre4uently 

 suggested that the prejudicial action is due to a deficiency of carbonate of lime in 

 the soil. It is true that the soil on which the Antigua experiments were con- 

 ducted was very deficient in carbonate of lime, but it appears improbable that the 

 injurious action of the phosphates can be wholly traced to this as a cause, seeing 

 that not only super-phosphate, but also neutral and basic phosphates «xercised 

 similar influence. The Barbados soil contains more carbonate of lime than that 

 of the Antigua Station, though in this case the quantity is but small. This ques- 

 tion calls for careful investigation. It seems possible that the application of phos- 

 phates, even when in a neutral or basic form, may so affect th» activity of various 

 soil ferments as to lead to the results observed. — F. "W. 



