HOW TO KEEP THE SOIL IN CONDITION 



301 



a certain amount of organic waste matter into the soil. 

 When a plant has been grown for a term of years in one 

 locality, the soil becomes so full of this poisonous waste that 

 the plant can no longer thrive in it. This substance is not 

 so poisonous to other plants, and thus by a wise rotation of 

 crops this waste need not result in disadvantage to the 

 plants. Rotation also gives an opportunity to kill out 

 weeds that are likely to persist if the same crop is grown 

 year after year. 



Every region must settle for itself the crops that are to 

 be rotated ; but every farmer in any locality should adopt 

 a definite system of plant rotation. In the Middle West, 

 where corn is the staple crop, a three-year rotation is 

 carried on successfully in some places. This consists of 

 oats or wheat, clover, corn. 



One important result that comes with crop rotation is 

 the increase of humus. Humus is added in the plowing 

 under of the oats or wheat stubble, as well as the stems and 

 roots of the clover. The value of humus in any soil cannot 

 be overestimated. From our experiments what do you 

 know of the effect of humus on the capacity of soils to hold 

 moisture? Besides this, humus improves the texture of 

 the soil, adds plant foods, and helps to make available other 

 plant materials that are locked up in the soil. 



What are the sources of humus? We have already 

 mentioned the plowing under of stubble and clover. What 

 plant food does the clover and other legumes add to the 

 soil? Other crops besides legumes are^ sometimes raised 

 and plowed undSr for the purpose of supplying humus. 

 Such crops are called green manures. Rye is frequently 

 used in this way. Stable manure is by far the best source 



