Tropical Agriculture Author:Earley Vernon Wilcox Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: original constituents of the disintegrated lava, but also the fertilizer materials which are applied to such soils. Lime and nitrate of soda are readily leached ... more »out of volcanic soils, while ammonium sulphate and phosphates are fixed in the soils to a rather surprising extent. The humus content of tropical soils is ordinarily high. This is due to the great mass of vegetation produced under tropical conditions. If, however, there are long intervals between the rainy seasons, the humus in soils is rapidly decomposed under the influence of heat and aeration. Practically all tropical soils contain a relatively high percentage of iron. In Hawaii, the iron content of soils is 10 to 45 per cent. (usually about 20 per cent.) ; in Samoa, 15 to 20 per cent.; in Kamerun, 7 to 14 per cent.; in Madagascar, about 10 per cent.; and in India, 2 to 48 per cent. This iron commonly exists in the condition of three oxids, the ferrous, ferric, and magnetic. Fortunately for the farmer the ferrous iron is usually very insoluble except when the soil becomes puddled so that suitable aeration can not take place. Granules of magnetic iron oxid are of much more frequent occurrence in volcanic soils than in the old soils of temperate climates. In Hawaii, for example, magnetic iron is present to an appreciable extent in all soils, as may be seen by passing a magnet over a sample of pulverized dry soil. These magnetic iron granules are black, but soon assume the red color of ferric oxid as the result of further oxidation. In certain localities there are immense quantities of volcanic cinder or black sand which have resulted from volcanic explosions. Several crops make a satisfactory growth upon pure deposits of this volcanic cinder. In some localities in Hawaii the cinder contains a much higher content of potash...« less