20 AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE 



after harvest is said to cause 

 germination of most tuber pieces 

 in 2 to 3 weeks (6). The concentra- 

 tion necessary to stimulate germi- 

 nation decreases as the season of 

 dormancy passes. An ethylene- 

 releasing substance, ethephon, is 

 also useful in stimulating germi- 

 nation. It is effective in concentra- 

 tions as low as 0.4 percent in water 

 when applied in a 5-minute dip. 



Seed pieces should be handled 

 with great care. They are planted 

 by opening a hole in the soil with a 

 small hand hoe and placing the 

 tuber piece in it. Some persons feel 

 that the orientation of the tuber 

 piece in the hole is very important 

 and that the cut surface should 

 face upward. The seed is covered 

 with about 10 centimeters of soil 

 to prevent its drying. Occasionally, 

 a mulch of dried plant material is 

 placed over the planting site. In 

 West Africa mulching retains soil 

 moisture in the root zone and sig- 

 nificantly increases yields. Plant- 

 ing can be simplified by opening a 

 furrow with machinery, dropping 

 in the seed pieces, and then cover- 

 ing the furrow. 



Optimum planting distances de- 

 pend on many factors, and local 

 trials should always be made. For 

 culture of giant tubers, spacing of 

 2 or 3 meters is justifiable. One 

 meter between plants is usually 

 adequate. When yams are planted 

 in machine-made rows, the rows 

 should be 2 meters apart and the 

 plants, 0.5 meter in the row. 



Fertilization 



Considerable controversy exists 

 with respect to the fertilization of 

 D. alata. Common observations 

 have shown that organic material 

 in large amounts is useful, but that 

 it should be partially rotted before 

 use. Organic materials are added 

 before or at the time of planting. 



Mineral fertilizers have given 

 equivocal results and at times 

 have actually reduced yields. 

 Nevertheless, a pattern of needs 

 has emerged from many experi- 

 ments. During the first 6 weeks of 

 growth, plants rely heavily on the 

 nutrients stored in the seed piece. 

 Then, during the first half of the 

 growth period, they need large 

 quantities of nitrogen to sustain 

 vegetative growth. When tuberi- 

 zation occurs, much potassium is 

 required. 



Among the fertilizer recommen- 

 dations given are the following: 

 Nigeria, 12:12:18, 60 grams per 

 plant (.9) ; Trinidad, nitrogen, 100 

 kilograms per hectare applied 3 

 months after planting (16); Domi- 

 nica, 3:9:18, 30 grams per plant 

 4 to 6 weeks after planting (un- 

 published) ; Barbados, ammonium 

 sulfate and muriate of potash, 

 about 200 kilograms of each per 

 hectare (13). With such a wide 

 variety of recommendations exact 

 advice cannot be given; mineral 

 fertilization trials will always be 

 necessary for particular locations 

 and cultural practices. When inex- 

 pensive organic material is avail- 

 able, it will probably always be 



