the natural capacity of the land. With nitrogen we should care- 

 fully look to see just what amount may be used to advantage, 

 for what is not recovered the first year is quite likely to be lost ; 

 with phosphoric acid and potash this is less important since 

 the excess above that utilized by the plant, is retained in the 

 soil and may be had by future crops. 



What has thus far been said may be summarized as follows : 



First. Plants draw their nourishment from the soil and 

 air. 



Second. The term plant food is applied to all substances 

 which help to nourish the growing plant. 



Third. From one to five per cent, of the food of the plant 

 comes from the soil, the remainder coming from the atmosphere. 

 That which comes from the soil is the part that must be looked 

 after by the farmer. 



Fourth. The plant food taken from the soil I have divided 

 into two kinds, one, abundant, and so far as the farmer is con- 

 cerned this requires little or no attention ; the other, deficient, or 

 scarce in an available form when soils are worn out by cropping. 

 Deficient plant food amounts to about fifty per cent., on an av- 

 erage, of the total plant food taken from the soil, (that is the to- 

 tal ash plus nitrogen). 



Fifth. All soils fit for agricultural purposes contain a con- 

 siderable amount of the deficient plant food, even when of low 

 crop producing power. This fact gives rise to a classification of 

 plant food as follows : available plant food, or that which is in 

 such a condition that the roots of the plant can take it up in so- 

 lution ; and unavailable plant food, or that which the roots can- 

 not make use of. 



Sixth. By the action of frost, air, water, carbonic acid, etc., 

 changes are brought about in the soil which annually convert a 

 portion of the unavailable into available plant food, and this 

 portion sustains the natural crop which all soils will produce. 

 This we term the ftatural capacity of the soil to produce crops. 



Seventh. If we desire to produce larger crops we must sup- 

 'ply a sufficient amount of available plant food in manures or 

 fertilizers to feed the increase of vegetation above what the par- 

 ticular soil would produce, but as only a part of the available 

 plant food is recovered by the crop more must be supplied than 

 the analyses of the increased crop would show. 



lO 



