This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Chemical or artificial manures are valued chiefly by horticulturists and agriculturists for the amounts of nitrogen, potash, or phosphates they contain. The horticultural value, however, does not always correspond with the commercial or market value, as the latter may be affected by such questions as supply and demand, combinations, strikes, etc. The cultivator naturally wishes to obtain the best value for his money. Consequently, if he thinks he is paying too much for his nitrates, phosphates, or potash in a certain manure, he may cease to purchase it, and buy another that will supply his wants at a cheaper rate.
Artificial manures are now valued at "unit" prices for nitrates, phosphates, and potash, but these unit prices are subject to fluctuations.
Nitrogenous manures are valued at a unit price fixed for the percentage of nitrogen they contain; and nitrate of soda and sulphate of ammonia are taken as the standard nitrogenous manures. Thus, if a ton of nitrate of soda contains 155 units of nitrogen, and the price is £10 per ton, the value of the nitrogen will be £10/15.5 or about 12s. 10d. per unit. If 1 ton of sulphate of ammonia has 20 units of nitrogen, and is sold at £11 per ton, the unit price is £11/20 or 11s. It would therefore be cheaper to the grower to buy sulphate of ammonia at £11 per ton than to buy nitrate of soda at £10 per ton, as he would be obtaining better manurial value to the extent of 1s. 10d per unit. Of late years the price of nitrogen has varied from 8s. 3d. to 12s. per unit.
For phosphatic manures superphosphate, basic slag, and ground Algerian phosphates are taken as standards. In a superphosphate containing 32 per cent of soluble phosphate, costing £3 per ton, the cost per unit of phosphate would be £3/32 = 1s. 10 1/2d. Basic slag at £2, 10s. per ton, and containing 40 per cent of phosphate, would represent a unit cost of (£2,10s)/40 = 1s. 3d.
And ground Algerian phosphate at £2, 10s. per ton, but containing 60 per cent of phosphate, would show a unit value of (£2, 10s)/60= 10d.
Insoluble phosphates, although not of such immediate value, are nevertheless reckoned in the price of manures, and vary from 1s. 4d. to 2s. 9d. per unit.
Potash manures are reckoned in the same way as nitrates and phosphates. Kainit, muriate of potash, and sulphate of potash may be taken as standard potash manures. Thus, in kainit containing 12 1/2 per cent of potash, and costing £2, 5s. per ton, the phosphate would cost (£2,5s)/12 1/2 = 3s. 7d. per unit nearly. Muriate of potash containing 50 per cent of potash at £9 per ton costs £9/50 = 3s. 7d. per unit. And sulphate of potash with 50 per cent of potash at £9, 10s. per ton would cost (£9, 10s)/50= 3s. 9 1/4d. per unit.
With such manures as nitrate of soda, sulphate of ammonia, basic slag, superphosphate, kainit, muriate of potash, and sulphate of potash, each valuable for a certain ingredient, it is easy enough to calculate the cost of nitrogen, phosphate, and potash per unit; but it is not so easy with manures containing more than one valuable ingredient. Nor must natural manures like stable or farmyard manure, dried blood, seaweed, etc, be valued on the same basis, because they possess other properties apart from their purely manurial value. If, however, the cultivator has a knowledge of the unit system of valuing artificial manures he will find it advantageous to buy sometimes one kind of nitrate, phosphate, or potash, and sometimes another, and use them as required.
If we take a complete fertilizer, that is, one containing nitrates, phosphates, and potash, at the unit values quoted above, we get an example as follows: -
Nitrogen, 7 units at 11s. =
Phosphate, soluble, 15 units at 1s. 10d. =
„ insoluble, 7 units at 1s. 6d. =
Potash, 5 units at 3s. 1d. =
Add 25 per cent for mixing, storing, bags, carriage, etc.......................
Total cost per ton................
By obtaining a warranty with manures purchased, growers are thus able to arrive at a very fair estimate as to the value of their manures, if they price the percentage of nitrates, phosphates, and potash as given in the above examples.