CHAPTER XV 

 MEASURING PLANT FOOD REQUIREMENTS 



108. Forms of Plant Food. There are two forms of 

 each element of plant food present in every soil : the insolu- 

 ble, or unavailable; and the soluble, or available. The 

 former may be termed the potential plant food, and the 

 latter the kinetic. The amount of available plant food is 

 the limiting factor of plant growth, however much potential 

 plant food may be present. The unavailable plant food is 

 by natural processes slowly changed chemically so as to 

 become soluble and these changes may be hastened by 

 appropriate treatment. The depletion of the total food 

 content of the soil must be avoided by application of fertilizer. 



109. Soil Analyses. The plant food in the soil is present 

 in salts, minerals, and organic matter, which vary to a marked 

 extent in their solubility in different solvents. The fact 

 that solvents in the soil vary in their composition and 

 therefore in their solvent power makes it extremely difficult 

 to select a chemical solvent that truly represents the solvent 

 power of the soil solvents; hence it is practically impossible 

 to determine accurately by chemical means the absolute 

 amount of available plant food in any soil. Soil analyses 

 can only determine what elements are present, in what 

 form and in what amount. They are suggestive of the 

 treatment that should be given the soil and, therefore, in 

 many ways are of value; but it should be remembered that 

 soil analyses do not afford definite data of the amounts of 

 various kinds of food plants may obtain. 



Hopkins' " Soil Fertility and Permanent Agriculture " 

 estimates that, by the most approved agricultural methods, 

 2 per cent of the total nitrogen content, 1 per cent of the 



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