New Yoke Agricultural Experiment Station. 45 



soil composition. 



The most extensive soil types in New York State contain in the 

 plowed soil of an acre (to a depth of 7 ins.) approximately 



4,000 pounds of nitrogen 



1,200 pounds of phosphorus 

 30,000 pounds of potassium 



8,000 pounds of calcium 

 10,000 pounds of magnesium 

 No pounds of lime carbonate. 



By means of these figures, and those in the preceding table we 

 can obtain some idea of the relation between the amount of plant food 

 in the soil and the needs of crops. If the entire stock of plant food 

 could be drawn upon as fast as needed by crops of the size mentioned 

 above there would seem to be enough to last for a comparatively 

 long time. However, viewed even in this way, the supply of seme 

 of these constituents is by no means inexhaustible. If the stock 

 of phosphorus had been drawn upon at this rate during the entire 

 lifetime of men now living, an amount equal to that contained in 

 the surface seven inches of soil would have been removed. 



Available plant food. — But the entire stock of plant food in the 

 soil exists in a comparatively insoluble (unavailable) state. If this 

 were not so it would not have remained after many centuries of 

 weathering to which the soil has been exposed. Only a trifling 

 amount of plant food becomes soluble (available) each year. 

 Factors which make plant food available are: favorable physical 

 condition of the soil as modified by texture, cultivation, drainage, 

 etc.; moisture supply; decaying organic matter; amount of lime 

 carbonate; action of plant roots; and many chemical and biological 

 influences, all of which are modified by the above conditions. In 

 any soil the amount that does become available during a given 

 season depends upon the intensity of the factors which make it 

 available and upon the total amount in the soil, somewhat as a banker's 

 income is the product of his rate of interest and the amount of 

 capital he has invested. With radical variation in the soil type, 

 especially as to method of formation, differences may occur as to 

 the form in which a part of the total plant food exists; and this 

 may properly be regarded as another factor in determining the 

 amount of available plant food. 



In considering the amount of plant food in a soil the question 

 is not so much how many years it would last if it could all be drawn 

 upon as fast as needed, but, rather, is the total stock large enough 

 so that with other conditions as favorable as it is practicable to 

 make them there will be enough plant food liberated for good- 

 sized crops. 



