80 



Physics of the Soil. 



Table giving the tons of essential plant food per acre-foot 

 of different types of soil. 



From this table it appears that the amount of plant food 

 per acre-foot of field soils, not including nitrogen, ranges 

 from about 2 to 8 tons of potash, 2 to 116 tons of lime, 

 1 to 73 tons of magnesia, 2 to 4 tons of phosphoric acid, 

 and 1 to 2 tons of sulphuric acid. 



93. Number of Crops Required to Remove the Plant Food 

 of an Acre-foot of Soil. The ratio of dry weight of the ker- 

 nels to that of the straw and chaff in a crop of wheat has 

 been found to be as 1 to 1.1 in a dry season, but to be as 

 high as 1 to 1.5 when there has not been an undesirable 

 stimulation to the growth of straw. Taking this ratio of 

 1 to 1.5, a yield of 40 bushels of wheat per acre would 

 mean a crop of 2,400 Ibs. of grain arid 3,600 Ibs. of straw. 

 From these two figures, the data in the table of (91) and 

 that of (92), it is possible to compute the number of crops 

 of wheat yielding 40 bushels per acre which would remove 

 the amount of plant food in an acre-foot of one of the sev- 

 eral types of soil represented in the table of (92). Solv- 

 ing the problem for the potash in the clay soil the case 

 would be 



6.38X2,000 



(2.4X5.2) + (3.6X6.3) 



362.9 



