91 



Steenbjerg: Microelements from a Practical Point of View 



which gives fairly good results. Secondly, it has been found that 

 the influence of the preceding crop with respect to the amount of 

 available copper in the soil is complex. It has been shown 

 that grass undoubtedly is the crop which causes the highest in- 

 crease of the available copper of the soil, and it seems reasonable 

 to assume that grassland has the same effect with respect to various 

 other microelements. A vegetation of grass and a forest-vegeta- 

 tion, so to say, extract copper and other microelements from the 

 deeper soil horizons and transfer them to the surface soil where 

 they may be used by the crops following. 



Thirdly, one of the most important indirect causes of white tip 

 disease may be the humus in some soils; the ability of humus to 

 combine with cupric ions seems to be so great that copper is made 

 unavailable to the plant. Where it is possible over a considerable 



Table 17 : Copper analyses, percentage Cu. — 



* No analysis of degree of fineness of this mineral was made. The degree of fineness was evaluated and was 

 of the size which we called I. 



period of time, for some years, to decrease the humus of the soil 

 substantially and hence its copper absorbing capacity this may be 

 useful in controlling the deficiency. 



Different copper minerals have been used to supply the ele- 

 ment. During the years 1937 to 1946 pot experiments were made 

 to investigate the question whether plants are able to utilize metal- 

 lic copper (finely ground), copper oxides and copper sulphides, 

 substances which as far as the oxides and sulphides are concerned 

 have been added to the soil partly as minerals and partly as chemi- 

 cals. Malachite was also tried. The following copper minerals 



