180 Agricultural Chemistry. 



The litter which accumulates during the earlier years of 

 growth will therefore be most effective in increasing the value of 

 the surface soil by stores of plant food obtained from the deeper 

 soil layers. As a result of this process, the manurial require- 

 ments of the forest are low and become much smaller than in 

 ordinary cropping. 



Requirements of truck crops. The various truck crops differ 

 widely in productiveness and feeding habits. Of the more im- 

 portant ones, the cabbage assimilates large amounts of ash con- 

 stituents, with the exception of silica. The heavy yield desired 

 with such crops entails a correspondingly high consumption of 

 nitrogen and necessitates heavy manuring with this element, as 

 well as liberal manuring with potash and phosphoric acid. The 

 high content of sulphur tri-oxide in this crop and in the turnip 

 and other members of the cruciferae, suggests that in some cases 

 this element may become the limiting factor in plant growth, and 

 that the beneficial effects sometimes observed from the applica- 

 tion of gypsum may be due to the sulphur tri-oxide which it 

 supplies. 



The tobacco crop is a comparatively light feeder, but makes 

 positive demands for nitrogen, potash and lime. 



Crop residues, which include the leaves of root crops, the 

 straws of grain crops, the stalks of tobacco and waste parts from 

 trimming, contain sufficient plant food to justify the exercise of 

 care to return them to the soil. Potash and lime are the con- 

 stituents of most concern in the straws and they are of even 

 greater consequence in the leaves of root crops. The common 

 practice of spreading tobacco stalks to decay upon the land, 

 makes possible, as indicated by the table, the returning of con- 

 siderable amounts of potash and also of nitrogen to the soil. 

 These crop residues are frequently reduced to ashes to economize 

 labor in their disposal, but this practice should be discouraged, 

 since it involves a loss of much nitrogen. 



Whenever the soil will profit by the addition of organic mat- 

 ter, these materials should be turned in whole. Another prac- 



