^8 THE CHEMISTRY 0§ THE FARM 



by sifting, or by currents of water, into finer and 

 coarser particles, the finer particles will be found 

 much the richest in plant food. 



The weight of soil on an acre of land is so enormous 

 that small proportions of plant food may amount to 

 very considerable quantities. Nine inches' depth of 

 arable loam will weigh, when perfectly dry, about 

 3,000,000 lbs. A pasture soil will be lighter, the first 

 nine inches weighing, when dried and the roots re- 

 moved, about 2,250,000 lbs. Supposing, therefore, 

 a dry soil to contain 0*10 per cent, of nitrogen, 

 phosphoric acid or potash, the quantity in nine inches 

 of soil will be from 2,250 lbs. to 3,000 lbs. per acre. 



A large part of the elements of plant food contained 

 in soils is present in such a condition that plants are 

 unable to make use of it. An acre of soil may con- 

 tain many thousand pounds of phosphoric acid or of 

 nitrogen, and yet be in a poor condition; while a 

 dressing supplying 50 lbs. of readily available phos- 

 phoric acid or nitrogen, in the form of super- 

 phosphate or nitrate of sodium, may greatly increase 

 its productiveness. 



The available condition of the plant food depends 

 much on the character of the soil. A much smaller 

 proportion of plant food will render a sand fertib than 

 would be required in the case of a clay. This is partly 

 from the far greater development of the roots in a 

 sandy soil, and partly from the different condition in 

 which the mineral food is held. Hilgard has also 

 pointed out that the presence of lime in a soil, 

 especially when associated with humus, much in- 

 creases the availability both of potash and phosphoric 

 acid, so that smaller quantities of these Fuffice when 

 lime is present. 



