A STUDY OF FACTORS INFLUENCING THE YIELD OF POTATOES 1181 



Whitney (1910) summarized the many fertilizer experiments on potatoes 

 in this country as to the influence of increasing the amount of fertilizer. 

 So far as nitrate of soda, acid phosphate, and muriate of potash, used singly, 

 are concerned ^no consistent increase in yield has resulted from increasing 

 the amount up to 500 pounds per acre. Increasing the amount of complete 

 commercial fertilizer up to a ton and more per acre has, on the contrary, 

 consistently increased the yields. The figures shown, however, would 

 indicate that the increase in yield caused by amounts exceeding a ton has 

 not been profitable. The same conclusions can be drawn regarding the 

 use of manure in these experiments up to an amount not exceeding 20 

 tons per acre. 



One of the oldest and best series of fertilizer experiments on potatoes, 

 covering many years, was begun on Long Island in 1895, by the New York 

 Agricultural Experiment Station at Geneva (Van Slyke, 1895). Com- 

 parison of the yields from the use of 500, 1000, 1500, and 2000 pounds of 

 fertilizer, up to 1898, showed that it was not profitable to use more than 

 1000 pounds per acre. The tests in 1898 showed 1500 pounds to be the 

 most profitable amount. The curve of relation between the cost of fer- 

 tilizer and the yield of potatoes, altho irregular, shows a positive correla- 

 tion. Jordan (1900), reporting on a continuation of these Long Island 

 tests in 1900, showed that whereas the highest yields were obtained with 

 2000 pounds of fertilizer per acre, the greatest net gain from the crop 

 resulted when only 1000 pounds was used. Rane and Hall (1904), at the 

 New Hampshire station, found that 1500 pounds of commercial fertilizer 

 was the most profitable amount to use, whether or not normal applica- 

 tions of manure were used. Greater amounts of fertilizer, either with or 

 without manure, were not profitable. Kohler (1910), in a triplicate series 

 of plots conducted at the Minnesota station in 1910, showed that under 

 Minnesota conditions it would not pay to use more than 800 pounds 

 of fertilizer per acre, and in most of his tests 650 pounds gave the highest 

 gain. The gain in yield from the elements used singly was almost negli- 

 gible, their efficiency showing only when in combination. Kohler recom- 

 mended the use of commercial fertilizer only 'when the supply of stable 

 manure became insufficient in quantity. The experiments of Zavitz 

 (1916) at the Ontario station, covering cooperative and station tests for 

 five and three years, respectively, show a gain in yield, in most cases, 

 resulting from an increase in either the amount or the value of the ferti- 

 lizer used. Manure and fertilizer in combination, and manure alone, 

 gave the greatest yields per acre and formed the cheapest fertilizer in 

 both sets of experiments, not counting the cost of freight and application. 

 So far as profit is concerned, therefore, the results of the Ontario experi- 

 ments must be discounted. General experience has shown $h&t the 

 high cost of handling stable manure for potatoes on a large scale is often 

 prohibitive. 



