194 



BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



in 1928, and still more recently by a long list of agriculturally important 

 plants. It is now generally recognized as an essential component of 

 commercial fertilizers for use on low-boron soils. Various plants need 

 from about 0.1 to 0.5 ppm. of available boron in the soil, and many soils, 

 particularly in the more humid regions, have less than this amount. As 

 a result, such crops as sugar beets and alfalfa have frequently suffered 

 from "diseases," which are now known to be due simply to boron de- 

 ficiency and are easily controlled by proper fertilization. Some effects of 

 boron deficiency are shown in Figs. 8-4 and 8-5. 



Courtesy of South Carolina Asricultural Experiment Station. Reproduced from 

 Iluuf/er .S'(V/)i.s in Crops, a publication of tlie American Society of Agronomy and 

 tlie National Fertilizer Association, Washington, D. C. 



Fig. 8-4. Left to right: Ear from healthy boron-treated corn plant, and 



three ears from boron-deficient plants. Note one-sided shriveling of kernels 



on the deficient ears. 



Molybdenum was not recognized as an essential element for plants 

 until 1939 (Arnon and Stout), but it is now known to be required, for 

 example, by tomatoes, oats, and barley. It is also needed by such lower 

 forms as molds (Aspergillm niger) and the nonsymbiotic (free-living) 

 nitrogen-fixing bacteria {e.g., Azotobacter chroococcum) . 



Most soils contain not more than 5 ppm. of molybdenum, but a few 

 areas are known (in Scotland, California, and Wyoming) where the 

 amount is as high as 20-200 ppm. Livestock grazing these areas re- 

 ceive enough of the element from the forage to cause serious poisoning, 



