174 Mineral Nutrition of Plants 



specific inorganic transformations of nutritional importance carried out 

 by autotrophic organisms. Elements such as nitrogen, sulfur, and iron 

 in various forms are involved. These are ordinarily relatively easy to 

 determine; the significance of the transformation and its quantitative 

 aspects can be evaluated. Those indirect effects on solubility of other 

 elements said to be due to products of metabolism of microorganisms 

 are much more difficult to assess. The clay components of the soil and 

 the less weathered rock fragments contain many elements required or 

 utilized by plants in varying amounts. Some of these may occupy posi- 

 tions in the lattice of the clay or silicate; others may be cations retained 

 by the clay or organic matter. At various times investigations have 

 shown that the maintenance of carbon dioxide concentrations in soil 

 may result in an increase in the availability of certain elements. Some 

 of these experiments have been quite artificial, in that lengthy extrac- 

 tions with carbon dioxide-saturated water have been carried out. In 

 others, however, gaseous carbon dioxide has been passed through the 

 soil and changes in availability have been measured by plant uptake. 

 More attention has perhaps been given to phosphate in such studies 

 than to other elements, and it appears to be established that increased 

 solubility of phosphate from rock phosphate or basic calcium phosphate 

 may result, especially in neutral or calcareous soils. The amount of car- 

 bon dioxide evolved in soil by microbiological activity is substantial. 

 Under optimum conditions in soils well supplied with organic matter, 

 carbon dioxide evolution may attain rates as high as ioo pounds per 

 acre per day, though figures of 20-30 pounds per acre per day are more 

 general. Any increase in the hydrogen ion concentration might be 

 expected to increase the solubility of bases such as potassium and mag- 

 nesium, but, as Dr. Jenny indicates, it is not- -now assumed that these 

 must be in the soil solution to be available to the plant. Moreover, ex- 

 periments with plants growing in electrodialyzed colloidal clay have 

 shown a greater availability of certain elements, such as iron, alumin- 

 ium, and silicon as measured by plant uptake, than pass into solution 

 as a result of long carbon dioxide saturation of the clay suspension. 



Much is sometimes made of the presence of organic acids in soils. 

 Organic acids, however, are not a usual product of bacterial activity 

 under aerobic conditions. They may be produced by certain fungi, but 



