560 NATURE AND PROPERTIES OP SOILS 



321. The maintenance of soil fertility.^ — The practical 

 management of a soil, whereby profitable crops may be grown 

 without materially reducing the fertility of the land rests 

 on fiive fundamental principles. The basic factors are: (1) 

 drainage, (2) tillage, (3) organic matter, (4) lime, and (5) 

 fertilizers. Obviously, the removal of excess water depends 

 on adequate drainage, while aeration and all of the activities 

 that attend it rests both on drainage and tillage. The upkeep 

 of the soil organic matter by the use of crop roots and resi- 

 dues, by farm manure, and by the turning under of green- 

 crops has already been emphasized as fundamental to con- 

 tinuous productivity. 



These factors are by no means the whole program of ra- 

 tional soil management. Artificial additions must be made. 

 Of these lime is of vital importance. Calcium and magnesium 

 are lost from the soil in such large amounts that outside 

 sources must be drawn on. Every arable soil will ultimately 

 come to the point where liming will be profitable. Finally, the 

 judicious use of commercial fertilizers must receive attention. 

 The addition of* phosphoric acid will probably be the first 

 fertilizer element to be considered seriously, especially in gen- 

 eral farming. Under special conditions of soil and crop, nitro- 

 gen and potash will also be a part of the program. The adap- 

 tation of crops in suitable rotation to climate and soil, with 

 adequate attention to the factors emphasized above, are the 

 prime essentials of a paying system of permanent soil pro- 

 ductivity. 



^Hartwell, B. L., and Damon, S. C, Sisc Years* Experience in Im- 

 promng a Light Unproductive Soil; Jour. Amer. Soc. Agron., Vol. 13, 

 No. 1, pp. 3741, 1921. 



Lipman, J. G-., and Blair, A. W., The Lime Factor in Permanent 

 Soil Improvement, J. JRotations without Legumes, II. dotations with 

 Legumes; Soil Sei., Vol. IX, No. 2, pp. 83-114, 1921. 



