8o CHAPTER VI 



4. If circumstances arise in which it is desirable to obtain the maximum 

 yield per acre by the application of more than 50 Ibs. nitrogen per acre, sulphate 

 of ammonia should always be used. 



5. Practically on all soils manurings with nitrogen require to be supplemented 

 by phosphoric acid. The most effective form appears to be superphosphate of 

 lime and slag phosphate meal. Mineral phosphates are of distinctly lower value and 

 are not effective unless applied in quantities far exceeding in value those required 

 for either superphosphate or slag phosphate meal ; as a rule phosphates should 

 only be applied to plant cane, their action on ra toons being limited. 



6. On some soils the application of potash salts in quantities from 60 to 

 160 Ibs. sulphate of potash per acre results in greatly increasing the effectiveness. 

 of nitrogenous manuring. Soils containing less than o- 01 per cent, potash soluble 

 in i per cent, citric acid will as a rule respond favourably to this treatment, while 

 those containing between o-oi per cent, and 0-02 per cent, may or may not be 

 favourably affected." 



Harrison 8 has also given a resume of the results obtained from twenty- 

 four years' experimental work in British Guiana. A short abstract of these 

 results is given below : 



Lime. Alternate beds of heavy clay land were treated with five tons of 

 slaked Barbados lime per acre. The canes were grown up to third ratoons 

 and then fallowed for a year. In the plots which were manured in addition 

 to liming, the total increase due to liming was 37-0 tons of cane per acre, 

 and in the unmanured plots at the rate of 33 7 tons per acre. Both the above 

 increases refer to the sum total of ten crops harvested in 13 years. 



Phosphates. Applications of phosphates have not always resulted in 

 financial benefit. It appears that the most satisfactory mode of using phos- 

 phates is to apply 3 cwt. of superphosphate or 5 to 6 cwt. of slag phosphate to 

 plant canes, the dressings of slag phosphate being more remunerative than 

 those of superphosphate of equal cost. Phosphates do not benefit ratoons 

 and Harrison thinks it doubtful if it is necessary to apply phosphates to 

 Demerara soils as often as once in five years. 



Potash. Results obtained with both sulphate and nitrate of potash 

 indicate that potash is not required on the heavy clay soils of British Guiana 

 under the conditions of ordinary agricultural practice. 



Nitrogen. As the mean result of ten crops of cane in 13 years it was 

 found that 10 Ibs. of nitrogen as sulphate of ammonia, when added in pro- 

 portions up to 300 Ibs. per acre, gave an extra return of 1-3 tons of cane per 

 acre, or 2f cwt. of commercial sugar. With nitrate of soda up to 250 Ibs. per 

 acre, 10 Ibs. of nitrogen would probably give I 4 tons of cane, equal to 2| cwt. 

 of commercial sugar, but experiments indicate that it is not wise to apply 

 more than 250 Ibs. nitrate of soda at one dressing. With dried blood the 

 indications over eight crops were that the relative value of nitrogen in this 

 material was 73 per cent, of that in sulphate of ammonia. 



With regard to the effect of manures on the soil, Harrison comes to the 

 following conclusions, basing his results on the analytical figures obtained 

 by the extraction of the soil in i per cent, aqueous citric acid with five hours' 

 continuous shaking : 



" j. That the growth of the sugar cane without nitrogenous manuring is 

 accompanied by a considerable loss of the nitrogen in the soil, amounting in ten 

 years to i86 per cent, on not-limed land, and to 26*7 per cent, on limed land. 

 These are equivalent to losses from the soil to a depth of eight inches of 880 Ibs. 

 and 1250 Ibs., respectively, per acre. 



2. Repeated heavy dressings with farm-yard manure have resulted in an 

 increase in the total nitrogen of the soil. In ten years the increase was 20-3 per 



