Plant Food in Soils. 81 



where 6.38 is the tons of potash per acre-foot, 

 2,000 is the number of Ibs. in one ton, 

 2.4 is the number of 1,000 Ibs. of grain in 40 bush, of wheat, 



5.2 is the number of Ibs. of potash per 1,000 Ibs. of grain, 

 3.6 is the number of 1,000 Ibs. of straw with 40 bush, of wheat 



6.3 is the number of pounds of potash per 1,000 Ibs. of straw, 

 362.9 is the number of crops of wheat. 



When the problem is solved for each of the essential 

 plant foods used by the wheat crop, the results will stand 

 for the clay soil as given below : 



Potash enough for 363 crops of wheat of 40 bush, per acre. 

 Magnesia enough for 2,082 crops of wheat of 40 bush, per acre. 

 Lime enough for 2,260 crops of wheat of 40 bush, per acre. 

 Phosphoric acid enough for 210 crops of wheat of 40 bush, per 



acre. 

 Sulphuric acid enough for 103 crops of wheat of 40 bush, per 



acre. 

 Nitrogen enough for 78.5 crops of wheat of 40 bush, per acre. 



In computing the nitrogen in the soil for this table .132 

 per cent., from the table in (89), was taken and the same 

 weight of soil, 4,000,000 pounds per acre-foot as used for 

 the other plant foods. 



It has been assumed that 404>ushels of grain and 3,600 

 pounds of straw per acre are taken from the ground each 

 crop and that nothing is returned to the soil, and yet chem- 

 ical analyses would indicate that there is enough of every- 

 thing but nitrogen for more than a century of cropping, 

 and this is saying nothing regarding the plant food which 

 is known to exist in the second, third and fourth feet of soil 

 in which the roots of plants regularly feed. Plainly we 

 have very important knowledge yet to discover regarding 

 the feeding of plants from the soil. 



94. Experiments at Rothamstead. The classic experi- 

 ments which have been made by Sir J. B. Lawes and his as- 

 sociates regarding the conditions which determine the fer- 

 tility of the soil, have thrown much needed light upon this 



