D, rotundata and D. cayenensis. Dipping tubers of other species 

 5 minutes in an 8-percent solution of ethylene chlorohydrin in 

 water stimulates sprouting. Similarly, ethephon, an ethylene- 

 releasing substance, is used at concentrations as low as 0.2 percent 

 in water as a 5-minute dip. Both substances reduce sprouting 

 time from 1 to 2 weeks and increase sprouting uniformity. 



The formation of the sprout has been studied in some detail 

 by Onwueme (23). Larger tuber pieces sprout more rapidly and 

 give rise to more shoots than smaller pieces. In tuber pieces that 

 do not include the crown, an inactive meristematic layer close to 

 the tuber surface is stimulated to rapid cell division and gives 

 rise to new buds. The buds are protected by a white calluslike 

 tissue from the same meristem. This is ruptured by stem growth. 

 Externally, sprouting loci appear as swellings on the upper 

 surface of the tuber. Sprouting may proceed very rapidly in tuber 

 pieces cut near the normal sprouting time. 



Fertilization 



For optimum growth of D. rotundata and D. cayenensis well- 

 composited organic material mixed with the soil is desirable. 

 Under such conditions, mineral fertilizers have sometimes caused 

 reduced yields. The effects of organic materials are probably 

 multiple: improved soil texture, improved nutrient availability 

 and retention, and improved aeration and water availability. 



However, in most instances organic material is not available. 

 Furthermore, population increases in areas where slash-and-burn 

 practices are traditional are gradually eliminating the practice of 

 permitting used lands to lie fallow 5 years or more. Therefore, 

 it is becoming necessary to add mineral fertilizer to the soil to 

 compensate for loss of natural fertility. The problem is that the 

 needs of the African yams for fertilizer are not well known. 



The newly developing shoot is sustained by food reserves of 

 the tuber, and therefore few or no additional nutrients are needed 

 during its first weeks of growth. As these reserves are ex- 

 hausted, yams need large quantities of nitrogen to stimulate 

 vegetative growth. Later, as the plants turn from foliage to 

 tuber production, potassium becomes the limiting element. Yams 

 appear to be very efficient in extracting phosphorus from the 

 soil and seldom need added quantities. Very little is known about 

 yam requirements for minor elements, but significant yield im- 

 provements were obtained after addition of about 1 metric ton 

 of magnesium oxide per hectare to the soil in Puerto Rico. 



In one of the best experiments in this field, Koli has 

 shown that in Ghana application of 67.2 kilograms per hectare of 

 nitrogen increased yields 22 percent, phosphorus at 33.6 kilograms 

 per hectare increased yields 3.6 percent, and potassium applica- 



22 



