THE ROTHAMSTED EXPERIMENTS 389 



even in acid phosphate, is one of the smallest items in the produc- 

 tion of this expensive and valuable crop. 



RESIDUAL EFFECT OF FERTILIZERS ON Hoos FIELD 



Any one who has made himself acquainted with the 26-year 

 potato experiments on Hoos field will naturally be interested in the 

 further history of those plots. The data reported since 1901 are 

 given in Table 69, following a summary of the soil treatment and 

 potato yields. 



The barley yields for 1902 are in harmony with the common ex- 

 perience that potatoes leave an excellent seed bed for a succeeding 

 crop of barley or wheat; and the residual effect for one year is also 

 very marked where nitrogen has been applied, as was the case with 

 continuous wheat on plot 16 of Broadbalk field. Even the first 

 barley crop on plots 9 and 10 are no better than on plots i and 2, 

 clearly showing that nitrogen was the limiting element for the quick- 

 growing barley crop. Aside from the farm-manure plots, much 

 less residual effect is apparent after 1902; and, in all cases where 

 the treatment is comparable, the barley yields of these plots in 

 1903 were less than on corresponding plots in the same field (Hoos) 

 where barley had been grown every year for more than half a 

 century. 



If we keep in mind that nine of the eighteen plots of continuous 

 barley produced more than 36 bushels per acre in 1902, also that 

 four of the ten plots where potatoes had been grown for 26 years 

 produced less than 36 bushels of barley in 1902, and that the 

 largest average yield of potatoes from the farm-manure plots (3 

 and 4), either for one year or for five years, was secured after pota- 

 toes had been grown on the same land every year for more than 

 fifteen years, then the following statement by Whitney seems 

 clearly inapplicable: 



"One of the most interesting instances going to show that toxic substances 

 are formed and that what is poisonous to one crop is not necessarily poisonous 

 or injurious to another is a series of experiments of Lawes and Gilbert the 

 growing of potates for about fifteen years on the same field. At the end of 

 this period they got the soil into a condition in which it would not grow potatoes 

 at all. The soil was exhausted, and under the older ideas it was necessarily 

 deficient in some plant food. It seems strange that, under our old ideas 



