Climate, Soils and Rotation 103 



the potato crop that methods for maintaining its supply 

 need careful study. In the natural wild state the soils 

 of humid regions receive constant additions of organic 

 matter from the death of plants and the leaves of trees. 

 The richness of newly cleared forest land and of the west- 

 ern prairies when first broken are examples familiar to all. 

 Under cultivation most crops annually produce more 

 organic matter than do wild plants, but in most cases only 

 a fraction is returned to the soil. Part is shipped away 

 from the farm for the use of man or of animals, part is 

 destroyed by the animals fed on the farm in the process of 

 digestion and part is lost in handling the manure. Opera- 

 tions of tillage and exposure of bare soils to the air rapidly 

 use up organic matter. A few American soils are being 

 handled in waj's which increase their organic matter, but 

 most of them probably are not. 



The principal method of maintaining soil organic 

 matter is that of crop residues. All crops leave part 

 of their organic matter in the soil after being harvested, 

 the proportion varying greatly. When potato tops are 

 burned to prevent disease, very little organic matter is 

 left in the soil from the roots. Other plants have more 

 extensive root systems and also leave other residues in 

 the form of stubble. A larger proportion is left when 

 only the grain or seed of crops is removed and all stalks 

 or straw is left on the land. Feeding off crops in the 

 field, as in "hogging down" corn, leaves all the organic 

 matter except that destroyed in the process of digestion. 

 In practice, hay crops leave more residues than others 

 like grain and potatoes. It is unfortunate that so little 

 attention has been paid to hay crops in the United States 

 compared with that given to the inter-tilled crops and 

 to the grains. Tillage, manure, fertilizer and care are 



