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.
To estimate the potash a further 500 cub. cm. of the citric acid solution are evaporated to dryness and incinerated in a basin over a Bunsen burner to eliminate the organic matter. The residue is then treated with boiling distilled water and filtered. An excess of platinum chloride is then added to the filtrate, and the whole is slowly boiled until nearly dry. The precipitate is filtered off and washed with alcohol until no yellow coloration is to be seen in the filtrate. The least possible quantity of water is then added, together with an excess of magnesium powder. The reaction with this is completed by boiling. After cooling, the excess of magnesium is dissolved in hydrochloric acid, the whole being filtered and the precipitated platinum, which remains upon the filter, is washed free from acid, dried, and weighed. From this platinum is calculated the quantity of potash present. One part of platinum =48 parts of potash (K2O).
Available iron is estimated in a further 500 cub. cm. of citric acid solution, this being evaporated to dryness over the water bath, and incinerated over a Bunsen burner to drive off the whole of the organic matter. The residue is dissolved in hydrochloric acid, then evaporated in a conical glass flask. The iron is reduced to the ferrous condition by the addition of metallic zinc (adding only sufficient to get it into complete solution), and titrated with a deci-normal permanganate of potash solution. Each cubic centimetre of permanganate of potash required until permanent coloration is produced indicates the presence of '008 gm. of Fe2O3 or ferric oxide.
This is estimated by 5 gm. of the dried soil in a CO2 apparatus, one portion of which contains a supply of hydrochloric acid and a tube to enable it to be passed into the portion containing the soil, and a further portion contains a tube filled with concentrated sulphuric acid through which the escaping gases pass, in order to retain in the apparatus any moisture that may tend to be carried away. In this way the calcium carbonate is decomposed, the carbon dioxide being evolved. The apparatus is then stood for a quarter of an hour on the water oven to ensure the exclusion of the whole of the gas. The weight of the whole apparatus being taken, with the soil in it, before commencing the operation, it is now weighed again and the difference represents the loss of CO2 gas. 44 parts of the gas represent 100 parts of calcium carbonate that was present. This assumes that there is no magnesium carbonate present.
In estimating the humus, 10 gm. of dried soil are taken and washed on the filter paper with a 1-per-cent solution of hydrochloric acid till free from calcium salts. It is then well washed with hot, distilled water till free from acid, and washed into a long stoppered cylinder with 500 cub. cm. of 4-per-cent ammonia solution. It remains in the solution twenty-four hours, allowing the cylinder to lie in as nearly a horizontal position as is possible without allowing the stopper to leak, and is well shaken at intervals. The cylinder is then stood upright for twelve hours to allow the whole to settle to the bottom. The solution is then filtered, and 100 cub. cm., representing 2 gm. of soil, are taken and evaporated to dryness over the water bath. The dish is then placed in the water oven for a few minutes, to dry off any adhering moisture, and weighed. After weighing, it is incinerated until all the organic matter has burned off. The dish is allowed to cool in the desiccator and the weight again taken. The difference in the weights before and after incineration gives the humus.
This is estimated by boiling 5 gm. of dried soil with 25 cub. cm. of concentrated hydrochloric acid, and evaporating to dryness over a water bath. The residue is moistened with concentrated sulphuric acid, then treated with a mixture of 10 cub. cm. of hydrochloric and 10 cub. cm. of water, warmed, filtered while hot, the filter paper being well washed, and the filtrate treated with an excess of ammonia, boiled, allowed to cool, and filtered. The filtrate is neutralized with acetic acid and treated with ammonium oxalate (about 20 cub. cm.) to remove calcium, and again filtered. The filtrate is evaporated down and treated with successive 5-cub.-cm. lots of concentrated nitric acid to eliminate ammonia salts. As the liquid by this time has evaporated down to a small bulk, it is diluted to about 200 cub. cm., treated with ammonium phosphate, and allowed to stand for twelve hours. It is then filtered and washed with dilute ammonia, dried in the steam oven, and incinerated in a crucible and weighed. From the weight of magnesium pyro-phosphate obtained the magnesia can be calculated as MgO. 1 part of magnesium pyro-phosphate = 36 parts of magnesia oxide. [C. P. C]