affect them. A few tubers may sprout prematurely, and these sprouts 

 should be removed. Planting can be planned as the rainy season ap- 

 proaches. Tubers are best cut only a few days before planting and are 

 then treated with wood ashes (in some cases) and allowed to dry for 

 several days before planting. 



Spacing will affect yields. With D. transversa, cultivar 'Wael', 

 20,000 plants per hectare yielded 16.5 tonnes, and 60,000 plants yielded 

 23.9 tonnes. Spacing should not exceed 1 meter between plants in 

 any direction. 



The appropriate size of the tuber pieces is 100 to 500 grams. Large 

 tuber pieces sprout more vigorously and yield larger tubers than small 

 tuber pieces. Tuber pieces from the stem end of the tuber sprout more 

 readily than those from the tip or intermediate parts of the tuber. Yield 

 as related to planting-piece size was summarized as follows with D. 

 dmnetonirn {10): 



Piece weight {grams) 125 250 375 500 1,000 



Yield (tonnes/hectare) 13.9 21.5 26.1 32.4 44.6 



Fertilization 



Few experimental data are available on the fertilization of the minor 

 yams. Practical experience has demonstrated that these species can be 

 grown with more or less the same fertilizer treatments as other species. 

 Yams need large quantities of nitrogen during vegetative growth, and 

 potassium during tuber formation. They are efficient in extracting phos- 

 phorus from the soil. 



A trial done in Cameroon with D. dumetorum showed that it re- 

 sponds in the same way as D. cayenensis and D. rotundata when N is 

 raised from 0 to 160 kilograms per hectare and K from 0 to 240 kilograms 

 per hectare. As well as the two other species, it did not respond to a P 

 variation from 0 to 200 kilograms of superphosphate per hectare. In 

 New Caledonia, Bourret (1) has observed that D. nummularia 'Bwet' 

 was less susceptible than D. alata 'Noumea' to an excess of magnesium 

 in the soil and that it accumulates some minerals at a lower level 

 (Fe, Ni, Cr). 



Unfortunately, the needs for different mineral elements do not 

 coincide. A balanced mineral fertilizer applied early during the devel- 

 opment of the plants may not supply sufficient potassium at a later date. 

 Because of their vigorous growth over long periods, yams need to be 

 fertilized s everal times , or continuously. 



A treatment that will supply yams the necessary nutrients over a 

 long period is to add composted organic material or manure to the soil. 

 This fertilizer may be mixed with the soil and used to construct mounds 

 with hand tools or rototilled into the soil and formed with soil into ridges 

 by means of a disk plow. 



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