[449] SCIENTIFIC MANUAL. 163 



Assuming ten bushels as the average yield per acre a fair as- 

 sumption for errain growing regions a calculation on that basis 

 from analysis of Wolff and Knop shows the following quantities 

 of the principal elements of plant-food are removed in every ten 

 bushels of wheat sold from the farm, compared with that removed 

 in lint from an average acre in cotton : 



Wheat. Lint cotton. 



Nitrogen 12.40 pounds 1.00 pounds. 



Potash 3.30 pounds , 50 pounds. 



Lime 36 pounds 75 pouuds. 



Magnesia 1.40 pounds 25 pounds. 



Phosphoric acid .. 4,90 ponnis 25 pounds. 



Total . .32 36 pounds 2.75 pounds. 



This represents a most remarkable contrast between the ex- 

 hausting effects of wheat and cotton in the amounts of the ele- 

 ments of plant food removed by sale from the soil, and yet the 

 cotton soils are being more rapidly exhausted than those on which 

 wheat is the principal staple produced for market. 



These seem to be contradictory facts which demand explana- 

 tion. 



The apparent contradiction arises from the existence of other 

 factors which are operative to a greater extent in aid of exhaus- 

 tion on the cotton than on the wheat farm. 



In wheat growing regions the soil is not denuded of vegetable 

 matter during the leaching rains of winter and spring, but is pro- 

 tected, either by small grain or grass, from surface washing. 



The summer fallow, in the preparation for seeding wheat, neces- 

 sarily returns more or less vegetable matter to the soil, and with 

 it not only mineral elements of plant- food, derived from the soil 

 in an available form, thus returning, in an improved condition, 

 all that the plants turned under have taken from the soil, but this 

 return is augmented by whatever organic matter the plants have 

 extracted from the atmosphere. Not only are more stock kept, 

 and consequently more animal manure produced, but more at- 

 tention given to its collection, and more care taken to protect 

 it from the injurious effects of leaching and evaporation. It will 

 thus be observed that while much plant-food is removed in the 

 produce sent to market, but little is wasted of the natural ma- 

 nurial resources of the farm. 



On the cotton farm, the fields are left bare after the crops are 

 gathered, and exposed throughout the winter to leaching and 

 washing action of the rainy season. A single heavy rain in win- 

 ter or early spring, when the surface is finely pulverized by recent 

 freezes, oiten causes greater injury to the naked fields of the South 



