148 



MISSOURI AGRICULTURAL REPORT. 



United States, and 73.176 pounds as the mean weight of a surface foot 

 of soil, the following table gives the amount of the different essential 

 plant food elements per acre, expressed in tons : 



Tons, per acre, of essential plant food elements in the surface foot 

 of soil : 



These amounts constitute 2.68 per cent, of the dry weight of the soil 

 and their aggregate volume is represented relatively by the fifth cube on 

 the chart, 2.73 inches on an edge and weighing nearly an even 2 pounds 

 for each cubic foot of ■ surface soil. Thus it is seen that of the 

 nearly 43 tons of essential plant food elements carried, about one- 

 half is potash (K), one-fifth is lime (Ca), one-sixth is magnesia (Mg), 

 one-twelfth is nitrogen (N), one-eighteenth is phosphoric acid (P.j, and 

 one-eightieth is sulphuric acid (S). 



But the effective root zone in any soil adequately underdrained and 

 of the proper degree of openness is fully 3 to 4 feet in depth, and hence 

 the roots of a crop may be spread out through a soil mass carrying 3 or 

 4 times the amounts of all of the plant food elements designated in the 

 table above excepting only nitrogen, and in this case Warington found 

 the surface four feet at Rothamsted to carry 5.2 tons per acre. Four 

 thousand crops of wheat yielding 40 bushels of grain and 3,600 pounds of 

 straw per acre will not gather from the soil as much potash, lime or mag- 

 nesia as are carried in the surface four feet of soil. There is phosphoric 

 acid enough for 1,100 such crops; sulphuric acid enough for 700, but ni- 

 trogen enough for only 155 such crops of wheat. Nevertheless, it must be 

 remembered, that willi more plant food even of nitrogen than will suffice 

 for 100 such crops of wheat within the reach oi roots beneath tiic sur- 

 face of every field; with the rainfall, with the sunshine and with the 

 temperature to a measurcable extent unchanged ; with better seed, better 

 varieties, fuller knowledge, more efficient tools and higher skill in 

 management the richest of our virgin soils have fallen from a i)roduclive 

 capacity of 40 bushels of wheat per acre to one as low as 20 and even 

 much less during periods so short as 25 years. What has been the 

 cause of these shrinkages? Can the virgin productive capacity of such 

 soils be restored? Can it be economically maintained if restored? If 

 it can be economically restored and afterward maintained, then by what 

 means and by what methods? These are the vital and fundamental 

 problems of soil management. 



