82 Mineral Nutrition of Plants 



soluble materials in soils have been considered in detail by Reitemeier 

 (41), Parker (40), Anderson (2), and others. It must be concluded that 

 the soil solution for a given soil is highly dynamic. 



The role of the soil solution in the absorption of anions by plants 

 has never been wholly clarified. The amount of phosphorus, boron, 

 and molybdenum present in the soil solution at any given time is 

 inadequate for the nutrition of plants. In order that sufficient of these 

 nutrients be available, it is necessary for one of two conditions to pre- 

 vail; namely, that the soil solution be continuously renewed or that 

 plants obtain these ions directly from the solid phase by contact feeding. 

 Studies by Dean and Rubins (//) tend to minimize the importance of 

 contact feeding as a means by which plants can obtain anions from soil 

 surfaces. Studies by Volk (50) and Hunter and Kelley (22) on the 

 absorption of ions from dry soils indicate that phosphates are not 

 absorbed by plant roots from dry soils, whereas cations are absorbed. 

 Studies on the effect of irrigation on the absorption of phosphates by 

 sugar beets have shown absorption to be correlated with soil moisture. 

 Judging from the rather scanty information available, it is not im- 

 probable that plants absorb anions from soils through the medium of 

 the soil solution. 



FACTORS INFLUENCING ANION AVAILABILITY 



Soil organic matter and biological activity 



Aside from being a possible source of available phosphorus, the 

 organic matter itself is credited with having the capacity of increasing 

 the availability of the soil phosphorus. Other reports show that humus 

 reacts with rock phosphate, making it more available. Possible mecha- 

 nisms by which humus makes inorganic soil phosphorus more available 

 are not clear. Experimentally it has been shown by Steele (43) that less 

 phosphorus is adsorbed by clays when humates are present in the 

 system. 



An interesting series of experiments was reported by Gerretsen (17) 

 who showed that more phosphorus is absorbed from insoluble phos- 

 phates when in the presence of microbiological activity than from sterile 

 cultures. 



