164 AGRICULTURAL CHEMISTRY 



essary one. By eliminating the ash elements in order, a 

 conclusion may be drawn as to the part which each separate 

 element takes in plant nutrition. After repeated experi- 

 ments with various modifications, aided by chemical and 

 microscopic examinations of the plant, the functions of an 

 element are determined. When a plant develops under 

 normal conditions, there is a definite part which every es- 

 sential element performs during growth. In fact, a plant 

 may be fed, and the effects of the food be observed 

 as accurately as in the case of the feeding of men or 

 animals. 



212. Sand Culture is essentially the same in principle 

 as water culture. Pure sand (SiO 2 ) is treated with strong 

 acids, washed with distilled water, and ignited. When 

 properly prepared, this leaves a perfectly sterile medium 

 to which is added, as desired, known amounts of the 

 various ash elements. 



Occurrence and Function of Ash Elements 



213. Potassium. Potassium is one of the most im- 

 portant and least variable of all the elements found in 

 the ash of plants. It is quite evenly distributed through- 

 out the growing plant and generally occurs in the entire 

 plant in the largest proportion of any of the essential ash 

 elements. It is taken up in the early stages of plant 

 growth and is always present to the greatest extent in the 

 active and growing parts, as in the leaves where the pro- 

 duction of plant tissue occurs. Potassium is one of the 

 elements most essential for the plant's development. 



The function of potassium is apparently to aid in the 



