Rothamsted Symposium on Trace Elements 



iron (4). In these instances the latter deficiency has been con- 

 firmed by painting and spraying treatments of the leaves with 

 sulphate of iron. The problem of relationships is further discussed 

 under c. 



Relationships of trace elements to other nutrients.— Various 

 relationships have been shown to exist in plants by means of the 

 visual method between various trace elements and between indi- 

 vidual trace elements and essential major elements, and pointers 

 thus provided for physiological studies. 



The Fe/Mn ratio, first demonstrated by Somers and Shive 

 (5) and later by Pearse (6) and others, is perhaps the best 

 known example. Hewitt (4) at Long Ashton has shown by 

 the visual method that other metals, notably copper, zinc, cobalt 

 and nickel are much more effective than manganese in inducing 

 iron deficiency. 



Interesting relationships between trace elements and major 

 nutrient elements have also been demonstrated visually, notably 

 Fe/K, Mn/Ca and Al/P. At varying levels of iron and potas- 

 sium it has been shown on potato that, when both are at a low 

 level, iron deficiency is evident by chlorotic tip foliage whereas, 

 when potassium is at a high level and iron remains inadequate, 

 the deficiency shows as a slight paling of the whole of the foliage 

 (7). In the case of manganese and calcium it has been demon- 

 strated that, with manganese in excess, its toxic effects are greatly 

 decreased by a high level of calcium (3). 



When aluminium is present at toxic levels the appearance of 

 the leaves and shoots reflect the condition of phosphorus defi- 

 ciency, which can be showoi by chemical tests to exist in these 

 organs (3). 



Soil acidity effects.— An attempt has been made, by applying 

 the visual method in conjunction vdth sand cultures, to determine 

 the causes of various injurious effects produced in crops by soil 

 acidity. Beginning with the viewpoint that the effects might 

 result from deficiencies of the basic elements, calcium, magnesium, 

 potassium and sodium, and of nitrogen and phosphorus, and of 

 excesses of hydrogen ions, manganese or aluminium, various crops 

 have been grown in sand culture and subjected to appropriate 

 nutrient treatments to examine the effects of these various possi- 

 bilities (2, 3, 8, 10). The visual effects produced have been com- 

 pared with effects observed in crops suffering from soil acidity in 

 the field (9). The results obtained with potatoes, cereals, various 

 Brassicae, runner beans and sugar beet have shown how far ex- 



