TROPICAL SOILS — PENDLETON 457 



AGRONOMIC PRACTICES IN HUMID TROPICAL LOWLANDS 



As the Belgians and others have found, the most effective methods 

 of managing agricultural soils in the humid tropical lowlands are 

 not the most effective under humid temperate zone conditions. It 

 is always difficult to differentiate between what is traditional and 

 what is empirical, and which will be the most effective soil-manage- 

 ment practices in the long run. The Belgian agricultural scientists 

 in the Congo, for example, insist that it is not possible to build up 

 organic matter to any considerable degree in the soil, and that legumi- 

 nous green manure crops are even less effective than grasses. They 

 have found that the perennial grasses are much better for rejuvenating 

 agricultural soils than legumes. They also emphasize mulches and 

 as nearly continuous a succession of crops on the soil as possible, to 

 keep the soil protected from scorching sun and beating rain. Greene 

 reports ^- that in recent African experiments burning the leguminous 

 cover crops and applying the ashes gives as good stimulation to the 

 following crop as plowing under the green cover. 



Tropical soils thus are, in general, low in plant nutrients, except the 

 very small percentage of recent alluvial and recent volcanic soils which 

 are usually really fertile. There are various reasons for this. In 

 the first place, in the vast basins of the Amazon and the Congo, and 

 in Borneo, the relief of the terrain is low. There is too little erosion 

 to wash away the worn-out surface soil and so expose the less fully 

 weathered-out soil material in deeper portions of the profile. This 

 weathering is hastened by both the heavy rainfall and the constantly 

 high temperatures. The result is that phosphorus, particularly, is 

 strongly fixed in the soil because of the weathering processes which 

 have liberated iron and aluminum; these accumulate in the soil and 

 all too easily combine with phosphorus in forms that the crop plant 

 cannot utilize. Without phosphorus, plants will not grow. Conse- 

 quently, experimental work with fertilizers in central Africa, in 

 humid tropical America, and in southeastern Asia indicates that 

 phosphorus is usually markedly deficient for good crop production. 

 For certain crops, such as sugarcane, nitrogen is an important limiting 

 factor, but for many other crops nitrogen, relatively, is much less 

 deficient than phosphorus, and in only a relatively few cases is potas- 

 sium needed, as for tobacco and certain legumes. In Malaya there 

 seem to be other plant nutrients which limit paddy growth but just 

 what these are, and how they can be made good in plant nutrient, is 

 not yet clear. 



**Dr. Herbert Greene, Adviser on Tropical Soils, Rothamsted, England, in a 

 personal communication. 



