The Soil. 



65 



0.10 per cent of total sulphur trioxide, and 0.2 per cent of potash 

 and 0.5 per cent of lime, soluble in hydrochloric acid. Much 

 larger quantities may, of course, occasionally be present. Plant 

 food is not equally distributed throughout a soil. If a soil is 

 separated by sifting into finer and coarser particles, it will be 

 found that the finer particles are much the richer in plant food. 



The weight of soil on an acre of land is so large that even 

 small proportions of plant food may amount to very considerable 

 quantities. An arable loam to the depth of 1 foot will weigh, 

 when perfectly dry, about 4,000,000 pounds. A pasture soil will 

 be lighter, the first foot weighing when dried with the roots re- 

 moved about 3,000,000 pounds. If such soils therefore contain, 

 when dry, 0.10 per cent of nitrogen, phosphoric acid, potash or 

 sulphur trioxide, the quantity of each in 1 foot of soil will be 

 from 3,000 to 4,000 pounds per acre. 



The following table, partly taken from Vivian, gives the ap- 

 proximate amounts of nitrogen, phosphoric acid, potash and sul- 

 phur trioxide in the first foot of typical sandy loam, clay loam 

 and clay soils : 



Amount of Plant Food per Acre in the Surface Foot. 



The amount of plant food present in the soil is surprising, in 

 view of the fact that it is often difficult to maintain a satisfactory 

 yield of crops. An acre of soil may contain many thousand 

 pounds of phosphoric acid or of nitrogen and yet be in poor con- 

 dition; while an application of commercial fertilizer supplying 

 50 pounds of readily available phosphoric acid in the form of 



