June, 1940] Experiments With Potatoes 7 



It is pretty generally recognized that the potato crop is a weak feeder, 

 and that generous amounts of available plant food should be provided for 

 the crop during the short period of its growth. This fact has led to the 

 use of a ton or more of fertilizer per acre in New England. Most com- 

 mercial potato growers now use an equivalent of about 2400 pounds per 

 acre of fertilizer approximating 20 units, or almost half that weight of 

 40 unit or double strength materials. Twenty-four hundred pounds of a 

 5-8-7 fertilizer carries 120 pounds of nitrogen, 192 pounds of phosphoric 

 acid, and 168 pounds of potash. 



Brown (5) reports four years' work from Connecticut in which max- 

 imum yields were obtained from fertilizer which supplied approximately 

 100 pounds per acre of nitrogen. 180 pounds of phosphoric acid and 120 

 pounds of potash. In a previous report Brown (5) and Slate stated that 

 for maximum yields on land not recently manured, 80 pounds of nitrogen, 

 100 to 120 pounds of phosphoric acid, and 80 pounds of potash should be 

 applied to each acre. These results are cited to show that in the same 

 state, and even in the same locality, varying amounts of plant food have 

 been found to produce maximum yields. It seems fair to state that the 

 native fertility of the soil, the amount of organic matter, the capacity of 

 the soil to hold and yield water when needed to the growing crop, the acid- 

 ity of the soil, and the supply of rare elements, are factors that, under cer- 

 tain conditions, may affect the amount of plant food that should be used 

 for maximum results. 



During the first World War, scarcity and high costs of potash led to 

 investigations concerning how much potash should be used for optimum 

 yields of potatoes. In general these tests have indicated that the best 

 ratio of nitrogen, phosphoric acid, and potash in the potato fertilizer ap- 

 proximates a 2-4-3. Yet the most common potato fertilizer formula in 

 Aroostook county today is a 2-4-5. It carries considerably more potash 

 than experiments have revealed to be necessary. Continued short rota- 

 tions, or some other factors, probably serve to change conditions sufficient- 

 ly to warrant the greater use of potash under present Maine farming 

 systems. 



Thompson (25) et al. note that in New York state "the most-used 

 analysis on upland soils in the principal potato growing areas is 4-8-7. 

 Except on the very lightest soils, the 4-8-7 carries more potash than is 

 justified. The 4-12-4 and the 5-10-5 mixtures should continue to be en- 

 couraged in most situations in western and central New York." Thus 

 while it appears that in Maine a high potash formula is required. New 

 York research points to the use of a high phosphorus formula. 



Hartwell (15), in arranging twenty crops from low 'to high in their 

 response to the three principal plant food elements, places the potato in the 

 low grouping for nitrogen, but in the medium grouping for both phosphoric 

 acid and potash. Morgan (17) groups all crops into four classes — low, 

 medium, high, and very high, according to their responses to the three 

 nutrients ; and places the potato in the high grouping for nitrogen and 

 potash, and in the very high grouping for phosphoric acid. Morgan's 

 work is given particularly for Connecticut conditions. 



