68« ANNoAL R3PORT OF THE Oft. Doc. 



Phosphoric Acid. — It is an admitted fact that most soils are rela- 

 tively poor in phosphorus. Of the ten or more elements known as dis- 

 tinctly soil elements used by plants for making growth, only three 

 are ever found deficient in most agricultural soils. Tw o only may be 

 lacking, and oftentimes only one element. In considering the mat- 

 ter of fertilizing land for potatoes, I have tried to show that pur- 

 chased nitrogen may pay, and it may not. Concerning this we judge 

 by the character of growth the soil make:;, and by field tests. Like 

 wise, potash may or may not be lacking in available form. Concern- 

 ing the third one of these three elements, phosphoric acid, the same 

 may be said, but we assume that in nine soils out of every ten, if any 

 one of these three elements is lacking, that one is phosphoric acid. 

 If two elements are wanted, phosphoric acid will prove to be one of 

 these two. The content of this element in the potato is small, and 

 there remains a general impression that phosphoric acid is of minor 

 importance in fertilization for potato<?s, but a careful study of all 

 Experiment Station tests is convincing that phosphoric acid cannot 

 be left out of any potato fertilizer, and that the requirements for 

 this element are wholly out of proportion to the amount actually 

 stored in the tops and tubers of the plants. So pronounced is this 

 chat some cai^ul experimenters have been led to say that phos- 

 phoric acid is the controlling element in a potato fertilizer. A 

 formula for a fertilizer carrying the elements in the proportion 

 found in the potato tuber would require four and one-half per cent, 

 of nitrogen, two of phosphoric acid and ten of potash. 



The Long Island growers have raised the percentage of phosphoric 

 acid to 8, cutting the nitrogen to 4, and Station experimentation indi- 

 cates that the formula should show a still higher percentage of phos 

 phorus, while the potash may be cut down. The whole matter of 

 fertilization must be left within wide lines, and the individual grower 

 must find by farm experiment just what plant food ma^- be given 

 with profit to his land, but the assumption should be that any potato 

 fertilizer should be strong in phosphoric acid. Until the opposite is 

 proved true in the field, I should assume that this element should be 

 the dominant one in the potato fertilizer used. The assumption is 

 based not only upon Station experiments in sands as well as clays, 

 but also upon the trend of opinion among those growers who test 

 these things for themselves. 



Let no reader get the impression that cropping should be done witL 

 phosphoric acid alone. Much laud is being ruined to-day by such 

 practice. But if he is a grower of potatoes he may begin experimen- 

 tation with the fact that if any one element is lacking in his soil, 

 this one probably is so lacking. Then he must learn whether he is 

 supplying suflQcient nitrogen through manures and clovers, and he 

 must take stock of his supply of available potash in the soil, and sup- 



