Tillage^ Fertilizing and Rotation 109 



dondition generally and it has an important influence on 

 soil temperatures. 



Although a liberal supply of humus is very desirable 

 on sweet potato land, it should be well decayed, the soil 

 well-drained, and it should not be acid or sour. Humus 

 becomes worthless with many croppings and its supply 

 in the soil must be renewed from time to time. This 

 is best done by the application of barnyard manure and 

 the use of green cover-crops turned under. However, 

 such applications of humus should be made to crops pre- 

 ceding the sweet potatoes in the rotation if it is to decay 

 properly and be of most value. 



Oreen-manures. 



The question of providing in the rotation a legu- 

 minous crop to supply needed nitrogen and humus has 

 been briefly discussed under rotation. Even when these 

 crops are removed from the land, an actual gain in 

 nitrogenous fertility results. The Ehode Island Ex- 

 periment Station, in a five-year pot experiment, found 

 that cowpeas and soybeans grown on land resulted in 

 a gain in nitrogen. " The approximate five-year net 

 gain of nitrogen in the presence of these two crops 

 and the vetch which was grown alternately with each, 

 was a ton of nitrogen per acre, about seven-tenths of 

 which was contained in the twenty-five tons of moisture- 

 free summer crops removed, and, the remainder in the 

 soil itself. These quantities should be considered with 

 reference to the following facts, namely : the soil in the 

 pots was 10 inches deep, somewhat deeper than the 

 surface soil in many localities; the winter vetch was 

 grown in a heated greenhouse; and furthermore, suf- 



