370 Irrigation and Drainage 



plants and be sure to submerge the highest points 

 in the field and start the rice there. This done, 

 the water is drawn to a gauge and changed every 

 seven days until the stage for dry growth has 

 arrived, after 21 days, or the fifth irrigation. 



The water is now held off during 30 days and 

 the fields are given two dry hoeings. This stirring of 

 the surface of the rice fields appears to have two 

 important objects to secure: (1) to destroy weeds, 

 and (2) to so aerate the soil as to admit air to 

 the roots and to the niter germs for the develop- 

 ment of nitrates. If the soil is not stirred, the 

 plants take on a yellow color, which quickly changes 

 to a dark green after the cultivation, proving this 

 tillage very important. During this time the dry- 

 growth roots are formed, which penetrate the soil 

 sufficiently to enable the plants to stand securely, 

 while at the same time they absorb the nitrates, 

 potash, phosphoric acid and other ash ingredients 

 required to mature the grain. 



The cultivation is made more urgent on these 

 fields because of the fine silt borne in the river 

 water, which settles and overspreads the surface, 

 forming so impervious a film that air can only pass 

 it slowly, and if not broken would set up the pro- 

 cesses of denitrification, which in turn must check 

 the growth of the crop and cause it to turn yellow. 



After the dry -growth stage has been passed and 

 the head is ready to form, the 7 -day irrigations are 

 resumed and maintained until the crop has been 

 matured. The frequent irrigations are necessitated 





