TROPICAL SOILS 23 



plow back and forth across the field. Outfits of this sort cost 

 about $25,000 and are sufficiently strong to endure the great 

 strain required to turn up the clod-packed, laterite soils. 

 Traction engines have been found rather unsatisfactory for 

 these heavy soils, although in some instances it is possible to 

 plow 12 inches deep with a traction engine. 



Another difficulty encountered in plowing laterite soils of 

 the Tropics lies in the fact that these soils do not scour the 

 plow. Experiments have been carried on in modifying the 

 surface of plows with the idea that they might be made to 

 scour, even in the gritless laterite soils. No success, however, 

 has ever been achieved along this line, either by modifications 

 of the surface of the plow or of the kind of metal in the mold 

 board or by the use of a slat mold board in place of a solid 

 one. The only condition under which laterite soils will scour 

 is the muddy stage in which some of the rice soils are plowed. 

 The draft upon the plow in soils which do not scour is of 

 course much heavier than in scouring soils. 



The investigations which have thus far been made on heavy 

 tropical soils, particularly laterite soils, have shown clearly 

 that the mechanical condition of soils is of prime importance 

 in their use for agricultural purposes. Most of these soils 

 contain plant food in abundance, but when the soils become 

 thoroughly packed and impervious to air and water the 

 chemical compounds in soil become less soluble or fixed in com- 

 binations from which plants cannot take sustenance. The thor- 

 ough aeration of these heavy soils not only brings about the 

 rapid nitrification of the organic nitrogen and ammonia, but 

 renders all chemical compounds more soluble and increases 

 the rate of movement of soil moisture. Under proper tillage 

 it has been found that fertilizers applied to the soils are much 

 more beneficial than when applied to improperly tilled soils. 

 If the laterite soils are allowed to become thoroughly packed, 

 it has been found that fertilizers are fixed by these soils 

 beyond the power of plants to take them up. 



