Lecture 2 — 27 — Micronutrient Elements 



merits were, for most of the elements he considered 

 essential the proof was not conclusive and only one 

 species of plant was investigated, the corn plant. In 

 any event, the work of Maze received but scant atten- 

 tion for a long period of time. The teaching persisted 

 that the plant's indispensable requirements were met 

 by ten chemical elements, although it was often con- 

 sidered probable that plants of some species might 

 have additional specific requirements. 



In the early part of the twenties of this century 

 extensive evidence was presented by McHargue (1922) 

 in Kentucky and by others that manganese must be 

 regarded as an essential element for plant growth. 

 Later experiments were carried out at the Rothamsted 

 Experimental Station (Brenchley and Warington, 

 1927) originally for another purpose, that showed 

 boron to be an essential element for broad bean plants. 

 Interest was especially directed to the relation of 

 boron deficiency to nodule development and nitrogen 

 fixation by leguminous plants. This element was not 

 at that time believed to be necessary for all species 

 of plants, A boron requirement for barley, for ex- 

 ample, was not demonstrated. During this same period 

 SOMMER and LiPMAN (1926) in California inaugurated 

 an investigation of essential elements, using highly 

 refined technique. The culture solutions were made 

 with salts especially purified for the purpose and with 

 redistilled water, and other precautions were taken to 

 avoid contaminations. With this technique it was 

 possible to show that boron was a growth require- 

 ment for every kind of plant tested, including barley. 

 It was not uniquely concerned with the growth of 

 leguminous plants. Other experiments by Lipman 

 and his associates yielded positive evidence of the 

 essentiality of zinc for plant growth for a wide range 

 of species. Lipman and Mackinney (1931) and SOM- 

 MER (1931) showed also that copper was essential. 

 Thus there was a reason to add to the older list of 



