it, and the fact is that a very large proportion of the 2,000 pounds 

 or more of phosphoric acid, or potash, or nitrogen, that is in the 

 soil, is unavailable. But so far as the soil produces crops, even if 

 small, plant food is furnished. The average supply of plant food 

 taken up by the 13^ bushelsof wheat and 1,125 pounds of straw, 

 in Lawes and Gilbert's experiments, would amount to 20 pounds 

 of nitrogen, 17 pounds of potash, and 10 pounds of phosphoric 

 acid, it is evident that this soil is capable of supplying these 

 quantities annually, else the crops would not grow, and this 

 small but necessary allowance comes from the change of una- 

 vailable plant food into available by the action of air and water 

 containing carbonic acid, as well as other more complex influ- 

 ences, which render soluble a limited amount of plant food each 

 year, and this amount is a measure of the natural capacity of any 

 given soil. In raising crops, therefore, we need not be at the 

 expense of supplying all of the deficient plant food contained in 

 the crop we raise, but only the excess which the crop contains 

 over and above that which the soil is capable of supplying, year 

 in and year out, indefinitely. It must be remembered, however, 

 that not all of the plant food supplied will be received by the 

 crop. Thus only one-half (50%) of the nitrogen applied in the 

 fertilizer is recovered, the remainder is either lost in the drain- 

 age water, or is carried down into the subsoil or held in the soil. 

 It has been demonstrated that very little if any infiuence is ex- 

 erted by the ?iitrogen in chemical fertilizer after the second year, 

 and even the second year this influence is very slight. With 

 potash and phosphoric acid the case is different, the effect of a 

 large application being felt for many years. This is explained 

 by the fact that nitrogen in the form suitable to be used by 

 plants is readily washed out of the soil, while potash and phos- 

 phoric acid combine with the soil, and though not immediately 

 available are gradually made so, and hence in time will be large- 

 ly recovered. 



On this point the conclusions of Lawes and Gilbert are ex=' 

 pressed as follows : " While the soil fixes potash and phosphoric 

 acid independent of vegetation, nitric acid is only fixed by the 

 agency of vegetation." 



This being true, it follows that we must apply a sufficient 

 excess of plant food so that the roots may avail themselves of a 

 quantity equal to the excess which the full crop contains above 



9 



