94 The Potato 



improved for the potato by increasing their organic matter 

 content. Under most American methods of cultivation 

 the proportion is reduced ; consequently yields are grad- 

 ually' reduced, and in the case of some varieties, like the 

 Burbank, even the shape of the tubers may be injured 

 by development in the less mellow soil. Cheap and easy 

 methods for the increase of the supply of organic matter 

 are among the greatest needs of potato-growers. 



Available plant-food 



For several reasons the potato is less able to secure its 

 plant-food from the soil as well as many other plants un- 

 less it is in readily available condition. For example, the 

 use of insoluble phosphoric acid in the form of raw rock 

 phosphate often gives good results on other crops but 

 seldom on the potato. A large supply of organic matter 

 in the soil, good drainage and a slightly alkaline condition 

 are essential to the liberation of the stores of plant-food 

 in the soil for the use of plants. Fertilizers do not pro- 

 duce their full effect on potatoes unless these conditions 

 are present. If the latter are favorable, fertilizers seldom 

 fail to increase the yield. The use of fertilizers on potatoes 

 is increasing very rapidly. In Maine, Long Island, New 

 Jersey and through the coast-trucking section from Nor- 

 folk to Florida, over a ton to the acre is often used. The 

 value of potatoes for human food is so high that the price 

 received for the crop makes such use profitable. There is 

 also a residual effect of the fertilizer on the succeeding 

 crops of a rotation, to be considered. Part of this effect 

 may be due to a favorable effect of fertilizers on the soil 

 bacteria.^ 



1 U. S. D. A. Off. of Exp. Sta. Bui. 194. 



