THE PURCHASE OF FERTILIZERS.

New Zealand Journal of Agriculture, Volume XL, Issue 6, 20 June 1930, Page 392

THE PURCHASE OF FERTILIZERS.

MEANING AND USE OF THE INVOICE CERTIFICATE.

F. T. Leighton,

Analyst, Chemistry Section, Department of Agriculture,

The Fertlizers Act of 1927 provides that, for the information and guidance of the buyer, the seller of 5 cwt. or more of any fertilizer shall supply an invoice certificate setting out the nature and composition of the fertilizer and the percentage of each of the three recognized fertilizer-constituentsnitrogen, phosphoric acid, and potash. In framing legal measures it is necessary, in order that there shall be no ambiguity, to make use of certain scientific and technical terms the meaning of which is sometimes not understood clearly by the layman. Some questions that are frequently asked by farmers regarding the meaning of statements in- fertilizer invoice certificates are dealt with in the following matter.

The invoice certificate states the 'name and brand of the fertilizer ; the percentages of the fertilizer-ingredients nitrogen, phosphoric acid, and potash, and their solubility in water ; the nature and proportion of each of the components of a mixed fertilizer, including any filler or diluent; and the fineness of grinding in the . case of basic slag or rock phosphate. In certificates for basic slag and ground rock phosphate the seller may also state the percentage of phosphoric acid soluble in citric acid by the standard method of extraction. This, however, is optional, and since the invoice certificate is of the nature of a guarantee of quality by the seller this figure is very frequently omitted.

In the case of phosphates of any kind the essential ingredient is entered in the invoice certificate as phosphoric acid. In trade practice there is, unfortunately, a confusion of terms in respect to phosphatic fertilizers. The grade of superphosphate and of phosphatic guano is usually advertised, and branded on the bags, in terms of tricalcic phosphate (for instance, 44-46 per cent, superphosphate, 60-63 per cent, guano) ; while other fertilizers, such as basic slag, are graded in terms of phosphoric-acid content. One part of phosphoric acid is equivalent to approximately two and one-fifth parts of tricalcic phosphate. It will now be clear why, when buying 44-46 per cent, superphosphate, the farmer receives an invoice certificate showing it to contain about 21 per cent, of phosphoric acid. If basic slag were graded in the same way, a 17-20 per cent, slag would be sold as of 37-43 per cent, quality. In order to minimize this confusion of terms as far as possible the Fertilizers Act requires the quality of phosphatic fertilizers to be stated in the invoice certificate in terms of phosphoric acid only. The statement of components in the invoice certificate is straightforward, and calls for little comment. It tells the buyer in just what form each of the ingredients of the fertilizer has been added ; whether, for instance, the soluble nitrogen is there as nitrate of soda or as sulphate of ammonia, the phosphoric acid as bone phosphate or ground rock phosphate, &c. It also informs him what, if any, filler has been added, and in what amount. A filler or diluent is any substance, other than a fertilizer, that is added to a fertilizer or mixture of fertilizers. A filler, however, is not necessarily an adulterant. Such a substance

as carbonate of lime may be used to improve the mechanical condition of the mixture and enable it to run easily through the drill. Occasionally excessive amounts of filler are used ; mixtures containing about half their weight of ground limestone have come under notice, the percentages of the active fertilizer ingredients being, of course, proportionately reduced. On the other hand, some manufacturers use no fillers in their mixtures, the desired granularity being obtained by using suitable proportions of ground rock phosphate, or by other means. The fineness of grinding of basic slag and ground rock phosphate is shown in the invoice certificate, the figure indicating the percentage of the fertilizer that will pass through a standard sieve having 10,000 holes to the square inch. A minimum fineness of 80 per cent, is required by the Fertilizers Act regulations.

Statement of Solubility

Figures indicating the solubility of fertilizer ingredients appear to perplex many farmers. The solubility of nitrogenous fertilizers is not difficult to understand ; such fertilizers as nitrate of soda and sulphate of ammonia contain water-soluble nitrogen, while the nitrogen of animal fertilizers is in the insoluble form. Generally, no alteration in solubility occurs when nitrogenous fertilizers are used in mixtures (the addition of lime, however, will result in the loss of nitrogen from ammonia compounds and animal fertilizers). In the case of potassic fertilizers, only water-soluble potash compounds are recognized as fertilizers, and these also do not alter in solubility when incorporated in the usual mixtures.. Phosphoric acid may be recorded in the invoice certificate as soluble in water, insoluble in water, or soluble in citric acid. The usual source of water-soluble phosphoric acid is superphosphate, although some of the new concentrated fertilizers contain soluble compounds of phosphoric acid with nitrogen or potash.

It is sometimes asked why an invoice certificate of a mixed fertilizer shows little or no water-soluble phosphoric acid, although the statement of components shows that a fair amount of superphosphate is present. The reason is that the water-soluble phosphate changes (reverts) more or less to a form that is insoluble in water but is still easily soluble in weak acids and is readily accessible to the plant. The well-known basic superphosphate is an instance of the water-soluble phosphate being reverted deliberately, by the addition of lime. Since this reversion of soluble phosphoric acid goes on slowly from the time the fertilizer is mixed, the manufacturer in his invoice certificate allows for the maximum reversion that is likely to take place, and records the soluble phosphoric acid on the low side. Actually, unless lime has been used as filler, there is not usually much reversion if the fertilizer is used reasonably soon after mixing.

The solubility in citric-acid solution (citric solubility) is the cause of considerable confusion of mind, on account of the practice of some sellers of quoting figures showing the citric solubility of their fertilizers by “ modified ” methods. Citric solubility is determined by shaking for a definite time a mixture of definite quantities of fertilizer,. citric acid, and water. By varying the respective quantities and the time of solution, varying degrees of solubility can be obtained. It is necessary, therefore, to have a standard method of procedure in order that solubility results may be compared. . The Fertilizers Act regulations

prescribe the official method, and it is provided that when the citric solubility of a fertilizer is included in the .statement of analysis (invoice certificate) it shall be determined by the official method. . Therefore, when fertilizers are advertised as of high citric-solubility by a modified method, it should be understood that the solubility figure cannot be included in the invoice certificate, and that no action , can be taken under the Fertilizers Act in respect to any guarantee of quality that is not incorporated in the certificate.

The citric-solubility test was devised for the valuation of basic slag, and it is still of considerable use in estimating the probable comparative values of different slags. With the development of ..the sale of finely ground rock phosphates in competition with basic slag it was found that the official method as applied to slag gave results which, it was claimed, did not truly represent the comparative availability of the phosphates. Several modified methods of extraction were proposed, some using a very small amount of fertilizer to a very large amount of weak citric-acid solution, others making use of other weak acid solvents. The effect was to increase the . solubility of the phosphoric acid from two to seven or eight times that obtained by the standard method. As these methods have not so far definitely been proved to give a reliable indication of availability they have not been recognized by the Fertilizers Act. The solubility of basic slag and rock phosphate is in general influenced to a considerable extent by the fineness of grinding of the fertilizer, and, with the modern methods of grinding phosphate rock, fertilizers are obtainable which give a fairly high solubility by the official method.

Unit Values.

The Fertilizers Act no longer requires the “ unit values ” of fertilizer ingredients to be stated, but as this system of valuation is still made use of for advertising purposes (and in the trade as a basis of purchase) a short explanation of the method may be useful. A “ unit ” of a fertilizer ingredient is I per cent, of i ton (that is,. 32-4 lb.). Basic slag containing 17 per cent, of phosphoric acid contains 17 units of that ingredient, and if the price is £5 per ton the price per unit (unit value) will be £5 -7- 17, or ss. nd. This system makes it easy to compare different quotations for the same kind of fertilizer where the quality and price both vary. Suppose, for example, that two lines of basic slag are under offer : y ::

A is quoted at £5 per ton for 17-20 per cent, quality. B is quoted at £5 12s. 6d. per ton for 20-22 per cent, quality. Then the unit price of A is found to be ss. nd., while that of B is ss. yd. Assuming that the solubility and fineness of grinding are approximately equal, it is clear that the higher-priced fertilizer is cheaper by 4d. per unit than that which is offered at the lower price per ton. Similarly o the cost of mixed fertilizers can be . compared by assigning the current unit value to each constituent and multiplying each percentage by its unit value. . . . ‘ The mistake should not be made of attempting to compare the unit values of unlike fertilizers. Comparisons are sometimes made between the unit values of such widely different types of phosphate as superphosphate, basic slag, and raw rock phosphate. No useful information can be obtained in this manner.