The Sweet Potato and its Cultivation. 343 



while yielding a good crop diiring tlie first year after planting on 

 new gronnd, the second year the harvest is less and the third year 

 \ery little is produced in the way of tubers, although there is 

 abindance of foliage in response to treatment with farmyard manure. 

 Such manure is admittedly very suitable for sweet potatoes, especially 

 if previously thoroughly mixed with leaf mould and allowed to rot, 

 turning over from time to time for some months, in order to secure 

 uniformity of texture. Even then, however, continuous cropping of 

 one piece of ground with a potash-loving culture like the sweet potato 

 is bound to exhaust it, and rotation should therefore be practised. 

 In the pre-war days, when the German potash syndicate used to 

 furnish supplies of j)otasli salts for fertilizing purposes, it is possTl>le 

 that exhaustion would have been prevented, even under such circum- 

 stances as those just quoted, by dressing the lands, say once every 

 third year, with sulphate of potash, to the extent of from 80 to 100 

 lb. per morgen; but, in any case, such continuous cropping is not 

 sound agriculture, and Mr, Robertson affords a good example to sweet 

 potato growers by setting his face against it. The furthest one could 

 go in the direction of continuous cropping is to vary the crop the third 

 year by putting the land under peas or beans after having taken 

 off two sweet potato crops, but Mr. Robertson — wisely — practises an 

 even more rigid rotation. 



For the sweet potato grower rotation is of advantage not only as 

 a preventive of soil-exhaustion, and a consequent means of increasing 

 the yield, but it is at the same time of service in controlling diseases 

 of the crop and in generally improving the soil. In the United 

 States it was found that a rotation which brought sweet potatoes on 

 the land once every three or four years was most effective in prevent- 

 ing loss from disease. Moreover, if in the course of rotation green 

 standing crops are ploughed under, particularly clover or vetch, the 

 fertility of the soil is improved and larger yields of all crops included 

 in the rotation result. Failing clover or vetch, rye and oats have 

 been successfully employ jd in the States. It is, however, advisable 

 in any case to include in the rotation, if at all practicable, a 

 leguminous crop like cowpeas, soya beans, clover, or vetch of some 

 kind, to furnish the soil not alone with humus, but above all with 

 nitrogen, especially in a poor sandy soil. 



Mr. Robertson, as already stated, invariably practises rotation 

 and strongly advises against putting any field under sweet potatoes 

 two seasons in succession, because, apart from any question of soil- 

 exhaustion, voluntary growths from the previous year's crop are 

 always apt to appear during the second season and to choke the newly- 

 planted cuttings. He therefore considers it better to plant cereals 

 after sweet potatoes, and when the cereals are harvested the ground 

 is prepared afresh without any risk of voluntaries or opslag. For 

 that reason too the suggestion to grow two crops of a three-months 

 variety in succession is not favoured by Mr. Robertson. Hence he 

 does not consider that larger annual harvests will thus be obtained 

 than by the cultivation of a six-months variety ; moreover, the keeping- 

 qualities of the latter have to be taken into account, as well as the 

 time lost in double preparation of the ground. 



Manuring. 



The tables of analyses above set out give some indication of the 

 lines whereon the manuring of the sweet potato should proceed, 



