574 



VARIOUS FERTILITY FACTORS 



limestone soils are often well supplied with phosphorus, phosphatic 

 limestones being especially rich in phosphate. Phosphorus is also 

 a fixed constituent in most organic matter, and soils rich in humus 

 usually have at least a fair supply of phosphorus. Silicate minerals 

 contain much potassium, and clay soils or silt soils derived from 

 silicates usually contain sufficient undecomposed minerals to in- 

 sure a high content of potassium. These facts serve to indicate to 

 some extent correlation between the physical appearance of the 

 soil and its chemical composition; but these indications have no 

 such value as the chemical invoice of the total plant food contained 

 in the soil. 



peat on the Manito experiment field, but both the soil analysis and the field experi- 

 ments show that the soil is not deficient in phosphorus. (See Tables 15, 16, 17, and 

 9*0 



