622 THE SMALL GRAINS 



greatly exhaust the soil, as the grain removes only about A- 

 of the nitrogen, phosphorus, and potassium of the crop. If, 

 in addition, the flooding is from rivers like the Mississippi, 

 little if any fertility is lost. When the flooding is done 

 with practically pure water, as is often the case, the soil 

 can only be kept fertile by adding to the soil what the 

 crop has removed. It is probable that the constituent 

 most generally lacking is nitrogen. The particular soil 

 amendments required will of course depend upon the 

 conditions of the locality. Kelly (1914) concluded, after 

 experimenting with fertilizers on rice soils of Hawaii : 

 (1) that very little nitrification takes place in submerged 

 soil, but ammonification goes on sufficiently to supply the 

 nitrogen needs of rice, provided there is sufficient organic 

 matter present ; (2) ammonium sulfate should, therefore, 

 be applied instead of a nitrate; (3) nitrates, under sub- 

 mergence, are reduced to nitrites, which are poisonous to 

 the rice; (4) rice soils should not be plowed, unless 

 immediately sown with a rotating crop, until it is time 

 to sow rice again, as the aeration of the soil resulting will 

 permit the formation of the poisonous nitrites. Further 

 information on fertilizers for rice is needed. 



694. Rotations. Instead of the fallow or unplowed 

 land preceding rice, it is far better, usually, to plant a 

 legume to be plowed under as a green-manure, and as a 

 part of a systematic rotation. This, in connection with 

 the application of a locally adapted fertilizer, will com- 

 plete a proper rice crop management. 



In Japan and other oriental countries, rice land is often 

 fertilized in the fall with straw, leaves, rice hulls, fish, 

 and night soil. The field is then planted with vetch or 

 sometimes wheat, for a winter crop, followed in the spring 

 with rice without additional manure. 



