12 MASS. EXPERIMENT STATION BULLETIN 290 



bean stubble as residual organic matter. The fourth crop, in 1917, followed a 

 heavy crop of Japanese millet which leaves a thick stubble and root mass, but 

 presumably not as rich in nitrogen as the clover sod. 



The crop of 1900 shows the value of a clover sod as a store of nitrogen. The 

 plots without nitrogen, 4, 7, and 9, equal several of the plots which received those 

 fertilizers, although the average yield is slightly lower than the other averages. 

 The manure plot exceeds any of the averages. 



The crop of 1902 was produced from two distinct varieties of seed; Plots 0, 

 1, 2, and part of 3 received a wholly different variety of seed from that used for 

 the other plots. This may have caused the lower yields on these plots when com- 

 pared with the rest of the field. Manure was noticeably superior to nitrate. 

 Organic nitrogen was superior to ammonia, which at best was little better than 

 no nitrogen. 



In 1904, the manure plot again excelled all other plots. Ammonia again was 

 little if any better than no nitrogen. Soj^ bean stubble is not equal to a clover 

 sod as a supply of nitrogen for a following crop. Ammonium sulfate showed 

 lack of accompanying lime. Plot 10 needs the following note, made in the re- 

 port for 1904: 



.... the potatoes on this plot were covered by hand, while those on the other plots were covered 

 by the use of the plow. The potatoes on this plot came up much more quickly and more vigor- 

 ously than those on the others, and showed decided superiority in growth from the start. 



The crop of 1917 was much superior to the crops of the earlier years, which 

 may have been due to the abundant residual organic matter from the millet 

 stubble. Brooks mentioned the value of organic matter in the soil for potatoes 

 in his report on the crop of 1904, as follows: 



Experiments in continuous potato culture .... have shown in a striking manner the dependence 

 of this crop upon the presence of a suitable proportion of organic matter in the soil. 



Nitrate plots were definitely superior to the others, and organic nitrogen 

 was not far behind. 



These important results stand out with the potato crops here grouped: 

 Nitrogen fertilizers were of comparatively little value following a clover sod; 

 manure was superior to chemicals when the soil lacked organic matter; soy bean 

 stubble was ineffective as a source of nitrogen. 



Table 4 — Yield of Potatoes 



(Pounds per plot.) 



