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

by Harvey Washington Wiley

Insoluble.—Twenty grams are treated with water until all soluble matter has disappeared, filtered on a tared gooch, and the filtrate dried to constant weight.

Sulfuric Acid.—The sulfuric acid is precipitated by barium chlorid in the slightly acid filtrate from the insoluble matter. The acidity is produced by a few drops of nitric acid. The rest of the process is conducted in the usual way.

Magnesia.—This is precipitated by ammonium sodium phosphate, filtered, ignited, and weighed as pyrophosphate. The magnesium is then calculated as chlorid. Magnesia is rarely found in excess of one-fourth per cent. When this amount is not exceeded the estimation of it may be neglected without any great error. As has already been said the chlorin is all calculated as sodium chlorid. If a part of it be combined with one-fourth per cent of magnesia it would represent 0.59 per cent of magnesium chlorid instead of 0.73 per cent sodium chlorid. In omitting the estimation of the magnesia therefore the importer is only damaged to the extent of 0.14 per cent of sodium nitrate.

Sodium Iodate.—This body, present only in small quantities, may also be neglected. In case the content of this body should reach one-fourth per cent the estimation of chlorin by titration using potassium chromate as indicator is impracticable. Such an instance, however, is rarely known.

Approximate Results.—When the determinations outlined above have been carefully made it is claimed that the result obtained by subtraction from 100 will not vary more than from two-tenths to three-tenths per cent from the true content of sodium nitrate. The method, however, cannot be considered strictly scientific and is much more tedious and chronophagous than the direct determination. In the direct determination, however, the analyst must assure himself that potassium is present in only appreciable quantities otherwise the per cent of sodium nitrate will be too low.

The presence of potassium nitrate is a detriment in this respect only; viz., that it contains a less percentage of nitrogen than the corresponding sodium salt. As a fertilizer, the value of Chile saltpeter may be increased by its content of potassium.

218. The Application of Chile Saltpeter to the Soil.—The analyst is often asked to determine the desirability of the use of sodium nitrate as a fertilizer and the methods and times of applying it. These are questions which are scarcely germane to the purpose of this work but which, nevertheless, for the sake of convenience may be briefly discussed. In the first place it may be said that the data of a chance chemical analysis will not afford a sufficiently broad basis for an answer. A given soil may be very rich in nitrogen as revealed by chemical analysis, and yet poor in an available supply. This is frequently the case with vegetable soils, containing, as they do, large quantities of nitrogen but holding it in practically an inert state. I have found such soils very rich in nitrogen, yet almost entirely devoid of nitrifying organisms. It is necessary therefore in reaching a judgment on this subject from analytical data to consider the different states in which the nitrogen may exist in a soil and above all the nitrifying power of the soil if the nitrogen be chiefly present in an organic state. Culture solutions should therefore be seeded with samples of the soil under examination and the beginning and rapidity of the nitrification carefully noted. In conjunction with this the nitrogen present in the soil in a nitric or ammoniacal form should be accurately determined. These determinations should be made according to the directions given in the first volume, pp. 448-548.

For the determination of nitrifying power we prefer the following method:

219. Taking Samples of Soil in Sterilized Tubes.—Brass tubes are prepared twenty centimeters in length and one and a half in diameter. One end is ground to a beveled edge and compressed in a mold so as to make the cutting edge slightly smaller in diameter than the internal diameter of the tube. It is then ground or filed until smooth and sharp. The blunt end of the tube is stoppered loosely with cotton and it is then sterilized by heating for an hour to 150°. Rubber caps are provided and each one has placed at the bottom a rubber ball to prevent the rubber from being cut by the edges of the brass tube. The caps should be of two distinct colors. Half of the rubber caps are sterilized by being boiled for an hour in water for three successive days. The caps cannot be heated to 150° dry heat with safety. On removing the brass tube from the sterilizing oven as soon as it is cool enough to handle, a sterilized rubber cap is slipped over its cutting end. An unsterilized cap is then slipped over the other end containing the cotton plug. Inasmuch as the cotton plug is never removed it is not necessary to sterilize the cap covering it. Large numbers of the tubes can thus be prepared for use and they can be safely transmitted to a distance by express or mail. For convenience, each tube is encased in a small cloth bag, which is tied with a cord carrying a tag on which the necessary data can be recorded at the time of taking the sample.

The tubes and their rubber caps thus carefully sterilized should not be removed from their cloth envelopes until the moment of taking each sample. After the sample has been taken and the cap replaced on the tube the latter should be immediately enclosed in the cloth sack and labeled with one of the tags therewith enclosed. The sample should be taken in two kinds of soil, in one instance in a cultivated soil, which is most characteristic of the locality, and in the second place a virgin soil of the same type. The virgin soil may be either soil which has been covered with grass or in forest. The spots at which the samples are to be taken having been previously selected, the tags for each tube should be prepared beforehand so as to avoid delay at the time of sampling. A pit with straight walls should be dug, the sides of which are at least two feet wide and even three feet would be better. The pit should be about forty-two inches deep. One of the sides having been made perfectly smooth and without allowing the loose fragments from the top to fall down and adhere to the walls below, the spots at which the samples are to be taken should be marked with a tape line at the following points; viz., three, fifteen, twenty-seven, and thirty-nine inches, respectively, below the surface. Beginning at the bottom point, carefully scrape off the surface of the wall over an area slightly larger than that of the end of the sample tube by means of a spatula, which, just previous to use, has been held for a moment or two in the flame of an alcohol or other convenient lamp. The sample tube having been removed from its sack, it will be noted that the end covered with black rubber is the one which is to be held in the hand, and this black rubber cap should be first removed being careful not to extract the plug of sterilized cotton which closes the end of the tube. Holding the tube firmly by the end, the fingers extending only about two inches from the end, remove the light-colored cap and push the tube with a turning motion into the side of the pit at the point where the surface has been removed with the sterilized spatula. When this is properly done the tube will be filled with a cylinder of soil equal to the length of the part of the tube penetrating the wall of the pit. The tube is then withdrawn, the light rubber cap first replaced, and then the black one. The light rubber cap should be held in the hand during the process in such a way that no dust or particles of soil are permitted to contaminate its inner walls. For this purpose the open end of the cap should be held downward. For the same reason after the removal of the light rubber cap the brass tube should be carefully preserved from dust or fragments, the open end, that is the cutting end, being held downward until ready for use. After one tube has been filled, capped, replaced in the sack and labeled, the spatula should be again sterilized and samples taken in regular order until the top one is finished.