146 MINERAL SALTS ABSORPTION IN PLANTS 



small amounts of various trace elements required by plants, their 

 availability in the soil often limits growth. This may happen even 

 when the soil contains adequate amounts of the element in question. 

 Iron and manganese, for example, are precipitated as insoluble 

 oxides at alkaline pH values and this may lead to micronutrient 

 deficiencies in calcareous soils. Some iron is still available in alkaUne 

 soils in the form of soluble complex salts of humic acids, and this 

 may suffice for the growth of some, but not all, species. Some 

 calcifuge plants, e.g. rhododendrons, grow badly in calcareous soils 

 because they suffer from iron deficiency. Improved growth of these 

 plants in such soils can sometimes be obtained by supplying addi- 

 tional iron in the form of soluble chelate compounds, such as iron 

 E.D.T.A. (Fig. 10, p. 39). 



Manganese deficiency is likewise more pronounced at alkaline 

 pH values, but it is observed also in acid soils when a large amount 

 of organic matter is present. This is apparently due to the formation 

 of insoluble complexes with organic substances, which are not 

 absorbed, and from which manganese is not easily released. 

 Manganese toxicity generally occurs on very acid soils and can often 

 be remedied by hming. Limitation of plant growth as a result of 

 copper deficiency is best known for cereals growing in soils which 

 are rich in organic materials, where stable complexes between 

 copper and organic substances are formed. 



Zinc deficiency symptoms seem to be due more often to unavail- 

 ability of the element in the soil, than to its absence. Absorption of 

 zinc is reduced with increasing pH, and this is attributed to a 

 reduction in the concentration of zinc in the soil solution. Healthy 

 crops can be grown in neutral soils, low in zinc, if fresh organic 

 material is added as an available source of zinc. Decomposition of 

 the organic materials into humus seems to render the zinc unavail- 

 able, again by formation of insoluble complexes. 



It is well known that application of lime to soils enhances the 

 development of boron deficiency symptoms in boron-deficient soils, 

 but the reason for this is still in doubt. Drake et al. (1941) concluded 

 that boron is not adsorbed by clay or organic matter at alkaline 

 pH values, or rendered insoluble by calcium. In contrast the growth 

 of crops in molybdenum-deficient soils is improved by liming, 

 and this is due to increased availability of molybdates with in- 

 creasing pH. 



