90 CHAPTER VI 



proportion of calcium carbonate, as in such soils nitrification proceeds 

 rapidly. This argument, however, loses much force, as it has been shown 

 in recent years by a long series of experiments, initiated by Pitsch in 1887, 

 and completed by Miller and Hutchinson in 1909, that plants can utihze 

 ammonia salts without their conversion into nitrates. On the other hand, 

 when there is a verj^ large quantity of calcium carbonate in the soil there is 

 danger of loss of ammonia by volatilization. Another objection which is 

 often raised against the use of ammonia salts is that their long-continued 

 use may result in an acid reaction and consequent infertility in the soil. 

 This action has been specially observed at the Woburn Experiment Station 

 on light sandy soils, and has also been studied at several Experiment Stations 

 in the United States. In Java and in Demerara, many years' use has not 

 resulted in this condition being observed, and Harrison^' inchnes to the belief 

 that the Demerara soils have benefited thereby, an action he attributes 

 to the alkaline nature of the subsoil water. In both these districts the results 

 of experiments indicate the superiority of ammonia over nitrate, and a further 

 reason for this may be found in the deflocculating action of nitrate of soda 

 on the clay soils common to both localities. 



The use of nitrate is most extensive in the Hawaiian Islands, and it is 

 also used to a considerable extent in Egypt and in Mauritius. The soil 

 type of the Hawaiian Islands is radically distinct from that of either Java or 

 Demerara, and the deflocculating action on clays would be largely absent. 

 Recentl37, however, some evidence has arisen that the long-continued use 

 of nitrate there has resulted in the formation of " black alkali " in certain 

 soils, and to this cause is attributed the falling off in productivit}^ of the 

 Lahaina cane on certain plantations. An objection to the use of nitrate 

 of soda in the tropics lies in its extremely deliquescent nature, an objection 

 that loses much weight when the locality where it is used is an arid one, 

 as is the case in many parts of the Hawaiian Islands. Where there is reason 

 to suppose that either form is objectionable when long continued, a natural 

 suggestion would be to use the two forms mixed or separately in alternate 

 years. 



The use of oil-seed cake is almost entirely confined to those districts 

 where it is produced, such as Louisiana, where large quantities of cotton 

 seed cake are employed in cane culture. 



Choice of Phosphatic Manures. — All phosphates when applied to soils 

 are fixed, and rendered insoluble ; the rationale of the use of a soluble 

 superphosphate, as opposed to the use of an insoluble phosphate, is that the 

 solution of phosphoric acid is precipitated within the soil in a much finer 

 state of division than can be obtained by grinding an insoluble phosphate, 

 and mechanically ploughing it into the soil. Dependent on the type of the 

 soil, the phosphoric acid will be precipitated within the soil as phosphate 

 of lime, or iron or alumina. The former of these bodies is available to the 

 plant, the latter is not ; hence it is an axiom in manuring that superphos- 

 phates are suitable for calcareous soils or such as contain a considerable 

 proportion of lime carbonate. On heavy clays such as constitute the cane 

 lands of British Guiana superphosphates are contraindicated. On such soils 

 basic slag is the form of phosphatic manure from which benefit is to be ex- 

 pected. It has been shown by many experiments that on clayey and peaty 

 soils, where an alkaline base is required to neutralize the nitric acid formed by 

 soil organisms, this form of phosphoric acid gives the best results. 



