TROPICAL SOILS 21 



ices extending downward and in all directions so as practically 

 to meet the crevices formed by neighboring explosions. 



On account of the fact that most tropical crops are allowed 

 to remain in the soil for five years or more the soil is evi- 

 dently subjected to a slow packing process which interferes 

 with aeration and consequently with nitrification. In almost 

 any sample of laterite soil which has not been disturbed for 

 a year, nitric nitrogen occurs only as a mere trace, while the 

 total nitrogen may indicate a fertile soil. By thorough tillage 

 and furnishing proper drainage conditions, aeration may be 

 set into active operation, thus providing the proper conditions 

 for the rapid growth of all plants which require nitrogen in 

 the nitrate form. 



In the growth of sugar cane, the usual system of taking one 

 plant crop and two rattoon crops requires from 4^2 to 5 years. 

 In order to prepare the soil as thoroughly as possible for this 

 long period of plant growth, deep plowing has been adopted. 

 This is accomplished either by traction engine, by ox and 

 mule teams, or still better by stationary engines and cable. By 

 the latter means, it is possible to plow from 16 inches to 3 feet 

 in depth. Obviously, the greater the depth of soil thus stirred 

 up and pulverized, the longer the time required for it to be- 

 come packed and impervious again. When plowed by steam 

 plows to a depth of two feet or more, laterite soils are thrown 

 up largely in huge lumps which require exposure to the sun 

 and water for slacking into a granular form. 



In some of the sugar-growing countries, notably in Hawaii, 

 it was long maintained by the sugar planters that no attention 

 need be paid to cover crops or humus. It was assumed that 

 the soils were inexhaustible and that efforts put forth to restor- 

 ing humus were merely lost and useless. During a consider- 

 able series of years, the sugar cane was harvested by setting 

 fires in the fields at the time of maturity. These fires went 

 raging through the field like a forest fire, destroying everything 

 except the green stalks of cane. These were immediately har- 



