106 The Potato 



advisable to save as much moisture as possible for the 

 potatoes. Another danger from the use of any form of 

 organic matter plowed under shortly before planting is 

 that movement of the soil water may be hindered and 

 the plants injured for lack of it. So much of the money 

 value of grain crops is in the seed that it usually pays to 

 harvest the crop, sell or feed the grain and return the 

 stalks or straw only to the soil. Many experiments have 

 shown that the amount of organic matter in any crop 

 increases most rapidly in the latter part of its life ; so 

 that in the straw of a grain crop there is likely to be found 

 more than that in the whole crop if plowed under a few 

 weeks before maturity. The value of the grain is several 

 times the cost of harvesting and threshing. The use of 

 the hay crops seeded Avith the small grains to plow under 

 gives larger net returns of organic matter to the soil, 

 because, unlike rye, buckwheat, or soy beans, there is no 

 destruction of organic matter by tillage in planting. 



The need of winter cover-crops for soils is least where 

 the ground is constantly frozen or covered by snow 

 throughout the winter. Mild and open winters, as in 

 the South, make it desirable to have the soil covered by 

 plants which wdll prevent erosion and save plant-food 

 which would otherwise leach away. Unless left to grow 

 too long in spring, there is little danger in humid sections 

 of loss of enough water to injure later money-crops like 

 potatoes. 



The choice of catch-, cover- and green-manure-crops 

 for any particular rotation, soil and climate depends on 

 many factors which differ from those governing the choice 

 of the money-crops of the rotation only because cover- 

 crops seldom have a cash value. Those crops of any 

 rotation which have the greatest value, as is generally the 



