Lecture 2 — 33 — Micronutrient Elements 



Failure to reco^ize the essentiality of micronu- 

 trient elements in earlier periods led not only to mis- 

 understanding of many diseases of plants growing in 

 the field, but also to doubtful interpretations of sup- 

 posedly well controlled experiments in greenhouses. 

 When nutrient solutions were prepared without de- 

 liberate addition of micronutrient elements, there was 

 no assurance that fully adequate amounts of these 

 would be accessible to the plant, even though its de- 

 mands were very small. Some lots of nutrient salts 

 were probably freer from impurities than other lots, 

 and contaminations from culture vessels and from dis- 

 tilled water varied in unknown degrees. Consequently, 

 the possibility was not excluded that some of the effects 

 of the culture solutions on plant growth attributed 

 mainly to elements in the classical list, did in fact 

 have a relation to the uncontrolled micronutrient ele- 

 ments. Some specific instances could be mentioned to 

 show that the earlier investigations of plant nutrition 

 by artificial culture methods were subject to these com- 

 plications. 



There is another aspect of the physiology of one of 

 the micronutrient elements of marked agricultural in- 

 terest. Boron, essential as it is, can become toxic 

 when present in the nutrient medium of plants in 

 concentrations not much higher than those which are 

 favorable. In some parts of California irrigation 

 waters contain enough boron so that it accumulates 

 in the soil to a point of injury to sensitive crops. 

 (Kelley and Brown, 1928 ; Eaton, 1935) . The boron 

 toxicity problem has been extensively investigated in 

 a Federal laboratory at Riverside established for the 

 purpose. Many controlled sand culture experiments 

 were made which had not only practical but also 

 physiological interest. Different species of plants differ 

 most significantly in their ability to absorb boron 

 from a given medium and in susceptibility to injury. 

 Some citrus species are very sensitive ; alfalfa tolerant. 

 In citrus large amounts of boron can accumulate in 



