22 TROPICAL AGRICULTURE 



vested and ground. This system of burning the cane resulted, of 

 course, in the total destruction of the leaves and other trash 

 which would otherwise have been returned to the soil. The 

 folly of this tremendous waste of vegetable substance has at 

 last been realized, and the cane tops and leaves are returned 

 to the soil, supplemented with green manuring crops which 

 are planted immediately after the second rattoon crop is re- 

 moved. The same change of habit has occurred among the 

 pineapple growers, who found that a good tilth of laterite soils 

 cannot be maintained without giving strict attention to the 

 humus content of the soils. 



In Hawaii, an excellent opportunity was offered for the 

 study of the leaching and weathering processes which natu- 

 rally occur in the lava rock. A chemical study was, therefore, 

 made of the life history of Hawaiian soils. For this purpose, 

 samples of lava were taken from the historic flows known to 

 have occurred in 1910, 1907, 1883, 1868, and 1823. These 

 flows were all from the same volcano. It appeared from a 

 chemical study of these samples of lava that lime is the element 

 which is leached out to the greatest extent. This also gives 

 an indication of the necessity of supplying lime to these soils 

 in order to balance the loss from excessive rainfall and leach- 

 ing. The supply of suitable lime for agricultural purposes is 

 at hand in the immense deposits of coral sand on the seashore. 



From the standpoint of cultivation, many tropical soils are 

 exceedingly heavy. The power required, for example, in plow- 

 ing soils in Hawaii and in Poona, India, is much greater than 

 would be needed in plowing to the same depth in the familiar 

 soils of the United States. Three or four mules are required 

 for an ordinary mold-board plow turning the soil to a depth of 

 six inches. The difficulty of plowing these heavy soils has 

 induced most planters to resort to power plows. In very heavy 

 soils, as already indicated, the most satisfactory device is the 

 system with two stationary engines, one at either end of a 

 long cable which hauls a heavy, single mold-board or gang 



