6 MINERAL SALTS ABSORPTION IN PLANTS 



symptoms of malnutrition become visible in its absence which can 

 be corrected by supplying that element, and in no other way. The 

 procedure for testing indispensability usually involves growing 

 plants from seed in solutions lacking the element and comparing 

 them with others grown in a complete medium. Using the technique 

 of "solution" or "water" culture, it was established by Sachs, Knop 

 and other late nineteenth-century investigators, that, in addition to 

 carbon, hydrogen and oxygen, seven elements are universally 

 essential for plant growth. These are nitrogen, phosphorus, sulphur, 

 potassium, calcium, magnesium and iron. This list differs in only one 

 respect from that compiled by De Saussure in 1804, namely in the 

 substitution of iron for silicon. 



Since the time of these early investigations, further research 

 along similar lines, employing highly purified chemicals and more 

 refined techniques, has established that at least six other elements 

 are required in small amounts by some, and probably by all, plants. 

 These, together with iron, are the so-called "micronutrients" or 

 "trace elements" and they include boron, chlorine, copper, manganese 

 molybdenum and zinc (Table 5). In the absence of a sufficient 



Table 5. A List of the Essential Elements of Plants, and the Forms in 



WHICH They are Mainly Absorbed 



Non-ionic sources in brackets 



amount of any one of these elements distinctive symptoms of 

 malnutrition become visible, which are sometimes referred to as 

 "deficiency diseases", and which can be corrected by supplying the 

 missing nutrient. 



There are indications that various elements, in addition to those 

 which are known to be essential, are beneficial to the growth of 



