Rothamsted Symposium on Trace Elements 88 



values for the exchangeable manganese than, e.g., calcium and 

 magnesium ions, especially in alkaline soils. The term "exchange- 

 able" evidently means little unless the replacing ion is defined. 



The early investigators stated that lack of exchangeable manga- 

 nese in the soil was a most important factor in causing manganese 

 deficiency in agricultural crops. The values for the computed 

 total amounts of exchangeable manganese led to conclusions 

 which are in accordance with agricultural practice, as well as with 

 field experiments with manganous sulphate and experiences as to 

 the external causes of manganese deficiency in crops. The re- 

 sults of the chemical investigations have further been confirmed by 

 pot-experiments in which different reducing agents (and different 

 amounts of water) were added to manganese deficient soil. 



Observations in the field have shown that a content below 

 2.5 parts of manganous ion, that is exchangeable manganese, per 

 1 kg of air dried soil very often causes attacks of manganese 

 deficiency. 



At present it seems only to be proved that the manganese 

 in the soils exists in two distinct groups or classes: (J) the bivalent 

 ion existing in the soil solution or as exchangeable or non- 

 exchangeable ion, dependent on the displacing ion used, and (2) 

 the insoluble higher oxides of manganese. These forms are in 

 dynamic equilibrium with one another, the equilibrium being 

 dependent on the pH and the redoxpotential of the soil. In spite 

 of the very interesting work done recently by Heintze, Mann 

 and co-workers I shall use the terms "manganic oxides," and 

 "higher oxides" so as to include all formulae from Mn04 to Mn02. 



What are the reactivities of the higher oxides? It seems to 

 me that this is one of the most important questions of the future. 

 It has been known for many years that certain soils contain a 

 highly active form of manganic oxide which is very quickly re- 

 duced by quinol, liberating Mn++ (MnC0H)2). According 

 to Leeper we have a less active form, which reacts with hypo- 

 sulphite at pH 7 but not with quinol. 



But do the plants absorb manganese from those manganic 

 oxides of the soil? In our experience the higher oxides are of 

 importance. During our investigations we have found that the 

 limit for the exchangeable manganese value, below which manga- 

 nese deficiency occurs, is different and usually lower when we 

 move from sandy soils to heavy loams. This may mean that the 

 plants on the heavier soils are able to absorb some manganese 



