Departmental Activities. 213 



Superphosphate should not be mixed with ground rock phosphate 

 or basic slag-, because this mixture renders the water-soluble phosphate 

 of the super less soluble and slower in its action. Farmers do some- 

 times use a mixture of superphosphate and slag to counteract the 

 acidity of the superphosphate, especially when used on light sour 

 soils, but bone dust is certainly preferalale to slag for mixing with 

 superphosphate. 



Superphosphate should not be mixed with nitrate of soda, as 

 apart from the loss of nitrogen, the mixture soon gets pasty and 

 difficult to sow. It can be mixed with sulphate of ammonia, but at 

 present the price of sulphate of ammonia and nitrate of soda is pro- 

 hibitive and their use is limited to special purposes. 



In purchasing fertilizers, the farmer must make up his mind 

 what ingredients the crop requires on his particular soil, and then 

 find where he can get these at the cheapest rate. 



From a knowledge of the percentage amounts of the valuable 

 constituents in a fertilizer and its market value, it is easy to calculate 

 the cost of the actual valuable ingredient "per unit," i.e. the value 

 per ton of each per cent. For example, if phosphates in the water- 

 soluble form are required and the following quotations are obtained — 



Superphosphate. Water SoliiVe F,^.. Pr-ce \)>-v Ton. 



No. 1 15.2 per cent. £10 5 



No. 2 18 per cent. 11 10 



No. 3 17 per cent. 15 10 



the value per ton of each per cent, of the required constituent, i.e. 

 the value per unit can be calculated by dividing the price per ton by 

 the percentage of material in the fertilizer, i.e. : — 



No. 1, ^*^V|^ = 1-^s. 5d. per unit : No. 2, ^l-ii*?: =12s. 9d. 



., ., o £15. 10s. io '^1 

 per unit : No. o, — = INs. del. per unit. 



By comparing the above unit values, No. 2 is seen to be much the 

 cheapest; if the three are f.o.r. the same station it has the further 

 advantage of being high grade, and thus less expensive in transit to 

 the farm. 



At present the unit values of citric-soluble phosphoric oxide in 

 basic slag vary from 13s. 4d. to 16s. 4d., a very wide range in the 

 market value. 



Ill the purchase of bone meal and bone dust, etc., the fineness 

 must be taken into account when comparing the values, as well as the 

 amounts of the different constituents present. Here we are dealing 

 with fertilizers containing nitrogen, phosphoric oxide soluble in 2 

 per cent, citric acid and phosphoric oxide which is insoluble. The 

 phosphoric oxide is sometimes expressed as lime phosphate, which to 

 the uninitiated gives the fertilizer an inflated analysis, but the farmer 

 should take no notice of this figure, as it simply represents the 

 chemical combination of the phosphoric oxide with a portion of the 

 lime, and is arrived at by multiplying the real percentage of phos- 

 phoric oxide by 2. 



The nitrogen in bones costs about 20s. per unit at the present 

 time. Citric-soluble phosphoric oxide costs about '8s. 6d. per unit. 



