11 Wallace: The Method of Visual Diagnosis 



action of oxidising enzymes, that is to say on the level of the oxida- 

 tion-reduction potentials. This influence is not restricted to man- 

 ganese (Cu, Co, Fe etc.) 



Miss Brenchley:— 



1) In growing plants in sand culture, is it possible to maintain 

 a constant pH? In water cultures the pH of an acid solution may 

 be changed by the plant within a few hours, almost to neutral in 

 some cases. 



2) It is not easy to understand why when a sprouted potato is 

 put into a very acid soil the shoot should so soon die from lack 

 of calcium. One would have thought that the reserves of calcium 

 in the tuber would have carried the shoot on for much longer. 



Prof. Wallace:— 



1) The pH of culture media in sand is changed by growing 

 plants in a similar way to water culture. 



2) We do not understand why potato sprouts die so quickly 

 in a calcium deficient medium. Since sprouts grow healthily in 

 air we can only assume the calcium content is sufficient for growth 

 under those conditions but insufficient when other nutrients are 

 added externally without further increase of Ca. The effect can 

 be produced in sand in a neutral medium and is thus not due to 

 specific toxins of acid soils. 



Dr. Lohnis:— 



1) Has injury from excess of aluminium been noted in field 

 crops? And if so does it go along with excess of manganese? 



2) Is it more advisable to apply the direct method of estima- 

 tion of micronutrients to petioles or to the leaf blades? In 

 Phaseolus my results were more clearcut when blades were ana- 

 lysed. 



Prof. Wallace:— 



1) Plants of the excess aluminium type have been observed 

 in crop failures in acid soils, e.g., cereals and sugar beet. We 

 should always expect excess aluminium condition in such instances 

 to be accompanied by a high level of manganese. 



2) In using quick tissue tests we prefer to use petioles rather 

 than leaf laminae as the former contain less interfering pigments. 



