388 INVESTIGATION BY CULTURE EXPERIMENTS 



It should be noted that the average yields on plots 3, 4, and 8 

 increased during the fifteen years previous to the last five, when the 

 " Beauty of Hebron " variety was introduced; and, as an average, 

 the farm-manure plots yielded higher during the five years ending 

 1896, than during the six years beginning 1876, notwithstanding 

 the addition of acid phosphate during the earlier period. 



During the first six years the use of $150 worth of plant food on 

 plot 8 produced $615 worth of potatoes, above the 85-bushel yield 

 on the untreated land, which is also the lo-year average yield of 

 potatoes for New York State. Even when used in addition to 

 manure, during the first six years, acid phosphate, as well as phos- 

 phate and nitrate, paid 100 per cent net profit on the investment; 

 but no test was made with manure and nitrate without phosphate. 



These Rothamsted data furnish no information concerning the 

 effect of potassium, except that it failed to pay its cost on plot 10. 

 It might be said that all but 9 bushels of the i96-bushel increase on 

 .plot 7 should be credited to the minerals (compare plot 5), but how 

 much of this increase would have been produced by acid phosphate 

 and ammonium salts is not revealed; on the other hand, nitrogen 

 must be credited with the increase from plot 7 above plot 10; all 

 of which means that phosphorus is the first limiting element and 

 nitrogen the second, for the growth of potatoes on this normal soil. 



To maintain satisfactory soil texture and to provide for the 

 liberation of potassium, magnesium, etc., from the immense supply 

 in the soil, liberal applications of manure should be made, and for 

 the improvement of the subsoil the growing of clover in rotation will 

 produce benefits that manure cannot produce. On the other hand, 

 in such intensive agriculture, there is large profit in a moderate 

 use of commercial nitrogen, especially in such form as sodium 

 nitrate, which also furnishes sodium as a soil stimulant. 



Whether one should use raw phosphate or acid phosphate, in 

 connection with the manure, clover, and sodium nitrate, is not 

 established, but the Rhode Island and Wisconsin data indicate that 

 potatoes are able to utilize the raw phosphate to some extent, and 

 (in Rhode Island) even without adequate provision for decaying 

 organic matter. It would seem advisable, however, to use the acid 

 phosphate until the raw rock has been more thoroughly tested for 

 potatoes, especially considering that the expense for phosphorus, 



