THE POTATO 363 



that very little or none need be supplied artificially. 

 While an excess of nitrogen may cause the potatoes to 

 " run to tops/' and set few tubers, yet some nitrogen can 

 usually be used to advantage. Soils very often contain 

 less available phosphoric acid than is necessary for 

 the fullest returns in potatoes, and phosphoric acid is 

 usually supplied in some form. Larger returns over a 

 series of years are more likely to be secured by using a 

 complete fertilizer than by using any one or two of the 

 elements separately. A fertilizer which will analyze 

 about 4 per cent of nitrogen, 6 per cent of phosphoric 

 acid, and 10 per cent of potash is one that is commonly 

 used in large potato growing districts. The per cent of 

 potash on many soils could be reduced to 6 to 8 per cent. 



387. Varieties. The selection of varieties of all kinds 

 of fruits and vegetables is always of interest to lovers of 

 plants, and is a question of great importance. However, 

 as late as 1771 there were only two varieties of potatoes 

 recorded, one white and one red variety. But the multi- 

 plication of varieties in recent years has been enormous, 

 and there have been many hundreds of varieties on the 

 market. There are more than one thousand named va- 

 rieties on the market at the present time. Many of these 

 have proved inferior to our standard sorts and many others 

 have proven to be old varieties renamed. 



Each locality must test the standard varieties and decide 

 which will do best under its conditions. Because a variety 

 does well in Maine is no proof that it will do well in Illinois 

 or Colorado, neither does the success of a variety on a clay 

 loam indicate positively what it will do on a sandy or 

 muck soil on an adjoining farm. The confusion which 

 often occurs regarding varieties of potatoes in many cases 

 is caused by the fact that there are several distinct types 



