20 N. H. Agr. Experiment Station [Bulletin 304 



oats as well as the first crop of clover are made into hay and removed, 

 while the second crop of clover is plowed under either in the fall or 

 spring before the potatoes are planted. 



Potatoes have now been grown five years on the difiPerent parts of 

 this field. 



The differences for the 4-8-0, 4-0-7, no fertilizer and for the I/2 ton of 

 8-16-14 are the only significant ones. 



Omitting the phosphorus causes a 42-bushel decrease in yield, but in- 

 creasing this element does not cause so large an increase as has been re- 

 corded on the more acid soils of Coos County. Whether this is due to 

 the better availability of the element on the Lane soil or to larger stocks 

 of native phosphorus is, of course, debatable. We suspect, however, 

 that the Coos soil has a much stronger fixing power for phosphorus than 

 the one in this experiment. 



Omitting the potash causes a yield decline of 58 bushels of potatoes, 

 while doubling the potash has resulted in an increase of 21 bushels per 

 acre. The latter figure is not significant but the gain has usually been 

 positive. 



Two series in this test have received lime at the rate of 500 and 1,000 

 pounds per acre. The original soil pH was 5.3. So far, the lime ap- 

 plications have neither influenced the growth of potatoes nor the de- 

 velopment of scab. 



An Experiment with Potatoes in a Three- Year Rotation 



On this field at the Jackson Farm in Colebrook, potatoes are grown 

 in a three-year rotation of potatoes, oats, and hay. A seeding of mixed 

 clover and timothy is always made for hay, since the soil is very acid 

 and satisfactory pure stands of clover cannot be produced. The fer- 

 tilizer is all applied for the potato crop, the oats and clover that follow 

 being produced on residual fertility. The first crop of hay is cut and 

 removed while the second crop is turned under before the potatoes are 

 planted. 



The experiment as laid out at present has been harvested four years. 

 As compared with the check plots on which one ton of 4-8-7 was applied, 

 significant results were secured by the following treatments : on the 

 4-16-7 plot a 23-bushel increase ; on the 4-16-14 plot, a 46-bushel increase ; 

 on the plot using one-half ton 4-8-7 made up with Ammo Phos A and ni- 

 trate of potash, a decrease of 84 bushels ; on the 2TL, IT 4-8-0 plot, 141 

 bushels decrease. 



The figures serve to emphasize the increase for phosphorus previously 

 reported on this land, and the need for a higher ratio of phosphorus 

 and potash to nitrogen than is found in the 4-8-7 fertilizer. Magnesium 

 in the fertilizer apparently has little effect on yield and the phosphorus 

 in basic slag does not give a significant increase in the potato crop over 

 that from superphosphate. 



Fertilizers made up from Ammo Phos 11-48-0 and nitrate of potash 

 13-0-44, equalling i/^, IT. and II/2 T. 4-8-7 have not given equivalent re- 

 sults with the regular 4-8-7 with potatoes. Moreover, these fertilizers 

 seriously depress clover stands because of the acid residues they leave in 

 the soil. 



