AVAILABLE PLANT FOOD 109 



plant food into available forms, such as the products of decaying 

 organic matter, including carbonic, nitric, and various organic 

 acids, the different forms of lime, and most soluble salts, each of 

 which is more fully discussed in its proper place, it is also known that 

 the plant roots themselves influence the availability of plant food, 

 probably by means of the carbonic acid or other substances which 

 they excrete. 



The juices of plants are commonly distinctly acid, and the roots 

 have some power to exude moisture, which certainly contains 

 carbonic acid and very possibly contains no other solvents, although 

 this question is not fully settled. Where growing plant roots lie in 

 contact with the polished surface of marble (calcium carbonate) 

 and some other materials (as prepared slabs of calcium phosphate), 

 distinct etching occurs, as was early shown by Sachs and Czapek. 



Kossowitsch conducted an experiment in which he grew plants 

 (peas, flax, and mustard) in two pots of sand which differed only 

 by the addition of fine-ground rock phosphate to one. The plants 

 were watered with the slow and constant application of like 

 amounts of a dilute solution containing all essential plant-food ele- 

 ments, except phosphorus. For the pot which contained no phos- 

 phate this solution was, in this continuous process of watering, 

 passed through a third pot of sand to which the fine-ground rock 

 phosphate had also been added. 



Kossowitsch found that the plants made a much better growth 

 in the pot where the roots were in direct contact with the phos- 

 phate, thus showing that they exert a solvent action in addition 

 to any that may be exerted by the nutrient solution. 



If, for example, the plant roots come in contact with, or exert 

 an influence upon, the equivalent of only one per cent of the sur- 

 face of the soil particles within the root range, this would offer an 

 explanation of the relationship (which is very irregular in different 

 soils and seasons) that exists between the total amount of any 

 plant-food element in a given soil stratum and the amount secured 

 by a given crop during the growing season. 



It is well known that soluble phosphates and soluble potassium 

 salts, when applied to normal soils, are almost immediately con- 

 verted into insoluble forms; and the higher availability of such 

 soluble fertilizers is now believed to be largely due to the fact that 



