Principles and practice of agricultural analysis. Volume 2 (of 3), Fertilizers

by Harvey Washington Wiley

If it is in a state of fine powder, such as ground phosphates and certain other fertilizers, it is sufficient to pass it two or three times through a sieve with meshes one millimeter in diameter, taking care to break up the material each time in order to mix it and to pulverize the fragments which the sieve retains. The whole is afterwards spread in a thin layer upon a large sheet of paper, and a portion is taken here and there upon the point of a knife until about twenty grains are removed, and from this the portion subjected to analysis is afterwards taken.

If the sample comes in fragments, more or less voluminous, such as phosphatic rocks or coarsely pulverized guanos containing agglomerated particles, it is necessary first to reduce the whole to powder by rubbing it in a mortar or in a small drug mill. It is next passed through a sieve of the size mentioned above and that which remains upon the sieve pulverized anew until all has passed through. This precaution is very important, since the parts which resist the action of the pestle the most have often a composition different from those which are easily broken.

When the products to be analyzed contain organic materials, such as horn, flesh, dry blood, etc., the pulverization is often a long and difficult process, and results in a certain degree of heating which drives off some of the moisture in such a way that the pulverized product is at the last drier, and, consequently, richer than the primitive sample. It is important to take account of this desiccation, and since the pulverization of a mass so voluminous can not be made without loss, the determination of the total weight of the sample before and after pulverization does not give exact results.

In such a case it is indispensable to determine the moisture, both before and after pulverizing, and to calculate the analytical results obtained upon the pulverized sample back to the original sample.

In order to escape this necessity, as well as the difficulties resulting from the variations in moisture during transportation, some chemists have thought it better to always dry the commercial products before submitting them to analysis, and to report their results in the dry state, accompanied by a determination of the moisture, leaving thus to the one interested the labor of calculating the richness in the normal state, that is to say, in the real state in which the merchandise was delivered.

In addition to the fact that this method allows numerous chances of errors, many substances undergoing important changes in their composition by drying alone, it has been productive of the most serious consequences. The sellers have placed their wares on the market with the analysis of the material in a dry state, and a great number of purchasers have not perceived the fraud concealed under this expression so innocent in appearance. It is thus that there has been met with in the markets guano containing twenty-five per cent of water, which was guaranteed to contain twelve per cent of phosphoric acid, when, in reality, it contained only eight per cent in the moist state.

11. Barn-Yard Manures.—The sampling of stall and barnyard manures is more difficult on account of the fact that the materials are not homogeneous and that they are usually mixed with straw and other débris from the feed trough, and only the greatest care and patience will enable the operator to secure a fair sample.

In the case of liquid manures the liquid should be thoroughly stirred before the sample is taken.

Frear points out the difficulty of securing representative samples of stall manure and describes methods of removing it.[5] The stall manure sampled had been piled in the cattle-yard for a time and the cattle were allowed to run over the heaps for an hour or two each day. Pigs were also allowed free access to the heaps in order to insure a more perfect mixture of the ingredients.

Twenty-nine loads of 3,000 pounds each were taken from the exposed heap and thirty-four loads of 2,000 pounds each were taken from the covered heap. From each load were removed two carefully selected portions of ten pounds each, which were placed in separate covered boxes numbered A and B. When the sampling was completed these boxes were covered. After being removed to the laboratory the boxes were weighed and the contents thoroughly mixed. Two samples of twelve liters volume each, were drawn from each box. One-third of this was chopped in a large meat chopper and the other two-thirds taken into the laboratory without being cut. These samples, on entering the laboratory, were weighed and dried at a temperature of 60°. Smaller samples were then drawn from each of these and ground in a drug mill for analysis. Duplicate samples taken in this way, while they did not give absolutely concordant results, showed a near approximation. A more careful sampling on the line proposed would, in all probability, secure absolutely agreeing results in duplicate samples.