SOUTHERN INDIA.
| Madras Presidency. | |||
|---|---|---|---|
| Laterite Soil. | Upland. Red Soils. | ||
| Sidapet. | Madura. Dindigal. | Trichinopoli. Parambalur. | |
| Insoluble matter | 76.86 | 90.47 | 86.74 |
| Soluble silica | |||
| Potash (K₂O) | .09 | .24 | .05 |
| Soda (Na₂O) | .17 | .12 | .15 |
| Lime (CaO) | Trace. | .56 | .48 |
| Magnesia (MgO) | .77 | .70 | .70 |
| Br. ox. of Manganese (Mn₃O₄) | .19 | .08 | .10 |
| Peroxid of iron (Fe₂O₃) | 10.09 | 3.51 | 5.70 |
| Alumina (Al₂O₃) | 8.84 | 2.92 | 5.68 |
| Phosphoric acid (P₂O₅) | Trace. | .09 | .05 |
| Sulfuric acid (SO₃) | |||
| Carbonic acid (CO₂) | .12 | .30 | .11 |
| Water and org. matter | 2.87 | 1.01 | .24 |
| Total | 100.00 | 100.00 | 100.00 |
| Nitrogen | .015 | .006 | .021 |
| Madras Presidency. | |||||
|---|---|---|---|---|---|
| Alluvium. Averages. | Regur. | ||||
| Loamy Soils. | Sandy Soils. | Regur. Avg. 18 Soils. | Trichinopoli. Parambalur Taluk. | Kistina Narsaopet. | |
| Insoluble matter | 71.79 | 93.09 | 68.41 | 65.16 | 68.29 |
| Soluble silica | |||||
| Potash (K₂O) | .22 | .04 | .41 | .14 | 1.14 |
| Soda (Na₂O) | .17 | .07 | .31 | .01 | 1.30 |
| Lime (CaO) | .54 | .13 | 2.90 | 2.18 | 3.43 |
| Magnesia (MgO) | 1.29 | .33 | 2.27 | 2.47 | 1.94 |
| Br. ox. of Manganese (Mn₃O₄) | .13 | .04 | .17 | .25 | .09 |
| Peroxid of iron (Fe₂O₃) | 9.59 | 2.46 | 7.13 | 9.27 | 6.96 |
| Alumina (Al₂O₃) | 9.98 | 1.74 | 10.14 | 13.76 | 10.28 |
| Phosphoric acid (P₂O₅) | .11 | .05 | .06 | Trace. | .00 |
| Sulfuric acid (SO₃) | Trace. | Trace. | |||
| Carbonic acid (CO₂) | .09 | .11 | 1.62 | .91 | 1.88 |
| Water and org. matter | 6.09 | 1.94 | 6.58 | 5.85 | 3.96 |
| Total | 100.00 | 100.00 | 100.00 | 100.00 | 99.27 |
| Nitrogen | .037 | .015 | .03 | .024 | .012 |
The Brahmaputra Alluvium in Assam.—Aside from the immediate alluvium of the Indus, of which no definite data are available, the Indo-Gangetic plain represents the drainage of the southern slope of the Himalaya chain. That of most of the northern slope is represented by the Brahmaputra, which not only originates in a region of heavy precipitation—Thibet—but continues in the same throughout its course, and rounding the easternmost spur of the Himalaya range enters, in southern Assam, upon the region of the maximum rainfall known. Its alluvial deposits should therefore show the reverse characteristics of those of the Ganges; they should, as thoroughly leached soils, be poor in lime, magnesia and potash. We have fortunately on this subject the excellent work done by Mr. H. H. Mann for the Indian Tea Association, the report of which was published in 1901, and contains, besides a large number of analyses, good descriptions of the general soil and cultural conditions of the Assam tea districts, with suggestions for their improvement.
The tea plantations of Assam are located almost wholly on the new and old alluvium of the Brahmaputra river, bordered on the north by the eastern spur of the Himalayas, on the south by the low ranges of the Khasia hills. The soil is mostly quite sandy, the late alluvium gray in color, the older reddish and more loamy. Of the four analyses given in the table and fairly representing the average character of these soils, the two first are from the north side, the latter two from the south side of the river.
It will be noted that the prominent feature of all these soils is an extremely low percentage of lime, the general average being about .08% as against nearly 1.0% in the average Indo-Gangetic soils. In the latter, potash ranges between .65 and .70%; in the Assam soils between .25 and .35. Magnesia averages nearly 1.3 in the Indo-Gangetic, against about .50 in the Assam tea soils. It is thus apparent that the same general facts as regards the leaching-out of soil ingredients already shown for eastern and western North America are strikingly verified in northern India; but reversed as regards the points of the compass. The preferential leaching-out of lime as compared with magnesia and potash, is here again well exemplified. It would be interesting to have an analysis of the Brahmaputra water to compare with that of the Ganges. That tea should flourish for twenty to thirty years in such soils, is a good indication of one cause at least of the total failure of tea culture in California, where tea plants are difficult to maintain alive, and after 25 years form rounded, scrubby bushes not over four feet high. Similar failures of tea on calcareous soils are on record from India. The low lime-content of the Assam soils, then, does not necessarily imply that these soils should be limed to maintain tea production. According to Mann, the main deficiency is in nitrogen, as the figures imply; but whether his recommendation of green-manuring with leguminous crops to increase the nitrogen-supply is practicable without first supplying more lime to the Assam soils, is questionable. Since phosphoric acid is also low, his recommendation to use freely the basic or Thomas slag is doubtless a good one, since lime would thus also be moderately increased.
Bamber gives a number of analyses of tea soils from low ground in Assam, which are very rich in vegetable matter and quite acid. Like those reported by Leather, these “bhil” soils are very poor in lime and nitrogen, but fairly supplied with potash and phosphoric acid.
The Regur or Black Cotton Soils of Southern India.—The second-greatest reasonably uniform soil-area of India is that covered by the regur, or black cotton soils, in south central India, notably the Deccan, where these soils are said to have been cultivated without fertilization for 2000 years and are still fairly productive.[159] Both in their physical character, chemical composition, and cultural characteristics, these regur soils are very similar to the “prairie soils” of the Cotton states and especially to the “black adobe” of California. Like the latter they are of unusual depth without change of tint; they crack wide open during the dry season on account of their high clay content; and the soil is thus partly inverted by the surface soil falling into the cracks. To the latter fact Leather ascribes, in part, the long duration of fertility in the regur lands. The regur also contains fragments of calcareous hardpan (here called guvarayi), just as in the Great Valley of California. The eighteen analyses of regur given by Leather agree so nearly in their essential points that it is admissible to average them; two other examples are however also given in the table.
It will be noted that while the contents of lime, magnesia and alumina are uniformly high, the content of potash has a wide range; it rises very high (1.14%) in the maximum, while the average is fair.
One conspicuous defect of these soils is their extremely low content of nitrogen, in view of which their lasting productiveness is difficult to understand; unless it be that, as in California, their high lime-content causes a copious crop of leguminous weeds, constantly replacing the nitrogen supply.[160] Unfortunately we have no determinations of humus or of its nitrogen-content. Leather attributes the black color of the regur to some mineral substance rather than to humus; but his arguments are not quite convincing, so long as the Grandeau test has not been made. In view of the low rainfall and the closeness of the texture of regur, it is probable that little if any nitrates are currently washed out of the black cotton lands.