Introduction. 21 



of the ash. For the reason that plants have been brought to 

 maturity in their absence, it has been generally accepted that they 

 are non-essential. However, it is necessary to remember that 

 such experiments have generally extended over a single genera- 

 tion and that it is possible that an attempt to grow the crop 

 through successive generations from its own seed in a soil devoid 

 of sodium, chlorine, silica, or manganese might meet with failure. 



How ash elements occur. The ash elements named above oc- 

 cur in part in the plant as salts, being combined with phosphoric, 

 nitric, sulphuric and various vegetable acids of which acetic, 

 oxalic, malic, tartar ic and citric acids are the most common. It 

 is also very certain that part is in combination with the organic 

 or combustible part of the plant. Sulphur occurs partly as sul- 

 phates and also as a constituent of proteins. Phosphorus as a 

 phosphate in the stem and root of the plant, but in organic form 

 in its seeds. In addition, such ash elements as potassium, mag- 

 nesium, calcium, iron and silicon are very probably in part con- 

 stituents of the organic compounds of plants. 



It is usual to speak of the combustible ingredients of a plant 

 as organic, and of the non-combustible ingredients as inorganic. 

 This is not accurate, as these ash constituents, which are essential 

 for the growth of the plant, have during its life as much right 

 to be called organic as the carbon of starch or protein. 



Can one element displace another? The fact that some of the 

 elements found in plant ash, as sodium and potassium, are chem- 

 ically very much alike, has led to the attempt to displace the ex- 

 pensive and less commonly occurring potassium by the inexpen- 

 sive and relatively abundant element, sodium. If it were pos- 

 sible to do this, the farmer's fertilizer bills for potassium salts 

 would be materially reduced. However, experiments have dem- 

 onstrated that sodium cannot take the place of potassium in the 

 growth of the plant. 



A definite amount of all the essential elements is needed for <\ 

 certain yield and none of the elements can be replaced by another. 

 A crop will be limited by the quantity of the essential element 



